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diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt new file mode 100644 index 0000000..d8dbe6b --- /dev/null +++ b/Documentation/user-manual.txt @@ -0,0 +1,4606 @@ += Git User Manual + +[preface] +== Introduction + +Git is a fast distributed revision control system. + +This manual is designed to be readable by someone with basic UNIX +command-line skills, but no previous knowledge of Git. + +<<repositories-and-branches>> and <<exploring-git-history>> explain how +to fetch and study a project using git--read these chapters to learn how +to build and test a particular version of a software project, search for +regressions, and so on. + +People needing to do actual development will also want to read +<<Developing-With-git>> and <<sharing-development>>. + +Further chapters cover more specialized topics. + +Comprehensive reference documentation is available through the man +pages, or linkgit:git-help[1] command. For example, for the command +`git clone <repo>`, you can either use: + +------------------------------------------------ +$ man git-clone +------------------------------------------------ + +or: + +------------------------------------------------ +$ git help clone +------------------------------------------------ + +With the latter, you can use the manual viewer of your choice; see +linkgit:git-help[1] for more information. + +See also <<git-quick-start>> for a brief overview of Git commands, +without any explanation. + +Finally, see <<todo>> for ways that you can help make this manual more +complete. + + +[[repositories-and-branches]] +== Repositories and Branches + +[[how-to-get-a-git-repository]] +=== How to get a Git repository + +It will be useful to have a Git repository to experiment with as you +read this manual. + +The best way to get one is by using the linkgit:git-clone[1] command to +download a copy of an existing repository. If you don't already have a +project in mind, here are some interesting examples: + +------------------------------------------------ + # Git itself (approx. 40MB download): +$ git clone git://git.kernel.org/pub/scm/git/git.git + # the Linux kernel (approx. 640MB download): +$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git +------------------------------------------------ + +The initial clone may be time-consuming for a large project, but you +will only need to clone once. + +The clone command creates a new directory named after the project +(`git` or `linux` in the examples above). After you cd into this +directory, you will see that it contains a copy of the project files, +called the <<def_working_tree,working tree>>, together with a special +top-level directory named `.git`, which contains all the information +about the history of the project. + +[[how-to-check-out]] +=== How to check out a different version of a project + +Git is best thought of as a tool for storing the history of a collection +of files. It stores the history as a compressed collection of +interrelated snapshots of the project's contents. In Git each such +version is called a <<def_commit,commit>>. + +Those snapshots aren't necessarily all arranged in a single line from +oldest to newest; instead, work may simultaneously proceed along +parallel lines of development, called <<def_branch,branches>>, which may +merge and diverge. + +A single Git repository can track development on multiple branches. It +does this by keeping a list of <<def_head,heads>> which reference the +latest commit on each branch; the linkgit:git-branch[1] command shows +you the list of branch heads: + +------------------------------------------------ +$ git branch +* master +------------------------------------------------ + +A freshly cloned repository contains a single branch head, by default +named "master", with the working directory initialized to the state of +the project referred to by that branch head. + +Most projects also use <<def_tag,tags>>. Tags, like heads, are +references into the project's history, and can be listed using the +linkgit:git-tag[1] command: + +------------------------------------------------ +$ git tag -l +v2.6.11 +v2.6.11-tree +v2.6.12 +v2.6.12-rc2 +v2.6.12-rc3 +v2.6.12-rc4 +v2.6.12-rc5 +v2.6.12-rc6 +v2.6.13 +... +------------------------------------------------ + +Tags are expected to always point at the same version of a project, +while heads are expected to advance as development progresses. + +Create a new branch head pointing to one of these versions and check it +out using linkgit:git-switch[1]: + +------------------------------------------------ +$ git switch -c new v2.6.13 +------------------------------------------------ + +The working directory then reflects the contents that the project had +when it was tagged v2.6.13, and linkgit:git-branch[1] shows two +branches, with an asterisk marking the currently checked-out branch: + +------------------------------------------------ +$ git branch + master +* new +------------------------------------------------ + +If you decide that you'd rather see version 2.6.17, you can modify +the current branch to point at v2.6.17 instead, with + +------------------------------------------------ +$ git reset --hard v2.6.17 +------------------------------------------------ + +Note that if the current branch head was your only reference to a +particular point in history, then resetting that branch may leave you +with no way to find the history it used to point to; so use this command +carefully. + +[[understanding-commits]] +=== Understanding History: Commits + +Every change in the history of a project is represented by a commit. +The linkgit:git-show[1] command shows the most recent commit on the +current branch: + +------------------------------------------------ +$ git show +commit 17cf781661e6d38f737f15f53ab552f1e95960d7 +Author: Linus Torvalds <torvalds@ppc970.osdl.org.(none)> +Date: Tue Apr 19 14:11:06 2005 -0700 + + Remove duplicate getenv(DB_ENVIRONMENT) call + + Noted by Tony Luck. + +diff --git a/init-db.c b/init-db.c +index 65898fa..b002dc6 100644 +--- a/init-db.c ++++ b/init-db.c +@@ -7,7 +7,7 @@ + + int main(int argc, char **argv) + { +- char *sha1_dir = getenv(DB_ENVIRONMENT), *path; ++ char *sha1_dir, *path; + int len, i; + + if (mkdir(".git", 0755) < 0) { +------------------------------------------------ + +As you can see, a commit shows who made the latest change, what they +did, and why. + +Every commit has a 40-hexdigit id, sometimes called the "object name" or the +"SHA-1 id", shown on the first line of the `git show` output. You can usually +refer to a commit by a shorter name, such as a tag or a branch name, but this +longer name can also be useful. Most importantly, it is a globally unique +name for this commit: so if you tell somebody else the object name (for +example in email), then you are guaranteed that name will refer to the same +commit in their repository that it does in yours (assuming their repository +has that commit at all). Since the object name is computed as a hash over the +contents of the commit, you are guaranteed that the commit can never change +without its name also changing. + +In fact, in <<git-concepts>> we shall see that everything stored in Git +history, including file data and directory contents, is stored in an object +with a name that is a hash of its contents. + +[[understanding-reachability]] +==== Understanding history: commits, parents, and reachability + +Every commit (except the very first commit in a project) also has a +parent commit which shows what happened before this commit. +Following the chain of parents will eventually take you back to the +beginning of the project. + +However, the commits do not form a simple list; Git allows lines of +development to diverge and then reconverge, and the point where two +lines of development reconverge is called a "merge". The commit +representing a merge can therefore have more than one parent, with +each parent representing the most recent commit on one of the lines +of development leading to that point. + +The best way to see how this works is using the linkgit:gitk[1] +command; running gitk now on a Git repository and looking for merge +commits will help understand how Git organizes history. + +In the following, we say that commit X is "reachable" from commit Y +if commit X is an ancestor of commit Y. Equivalently, you could say +that Y is a descendant of X, or that there is a chain of parents +leading from commit Y to commit X. + +[[history-diagrams]] +==== Understanding history: History diagrams + +We will sometimes represent Git history using diagrams like the one +below. Commits are shown as "o", and the links between them with +lines drawn with - / and \. Time goes left to right: + + +................................................ + o--o--o <-- Branch A + / + o--o--o <-- master + \ + o--o--o <-- Branch B +................................................ + +If we need to talk about a particular commit, the character "o" may +be replaced with another letter or number. + +[[what-is-a-branch]] +==== Understanding history: What is a branch? + +When we need to be precise, we will use the word "branch" to mean a line +of development, and "branch head" (or just "head") to mean a reference +to the most recent commit on a branch. In the example above, the branch +head named "A" is a pointer to one particular commit, but we refer to +the line of three commits leading up to that point as all being part of +"branch A". + +However, when no confusion will result, we often just use the term +"branch" both for branches and for branch heads. + +[[manipulating-branches]] +=== Manipulating branches + +Creating, deleting, and modifying branches is quick and easy; here's +a summary of the commands: + +`git branch`:: + list all branches. +`git branch <branch>`:: + create a new branch named `<branch>`, referencing the same + point in history as the current branch. +`git branch <branch> <start-point>`:: + create a new branch named `<branch>`, referencing + `<start-point>`, which may be specified any way you like, + including using a branch name or a tag name. +`git branch -d <branch>`:: + delete the branch `<branch>`; if the branch is not fully + merged in its upstream branch or contained in the current branch, + this command will fail with a warning. +`git branch -D <branch>`:: + delete the branch `<branch>` irrespective of its merged status. +`git switch <branch>`:: + make the current branch `<branch>`, updating the working + directory to reflect the version referenced by `<branch>`. +`git switch -c <new> <start-point>`:: + create a new branch `<new>` referencing `<start-point>`, and + check it out. + +The special symbol "HEAD" can always be used to refer to the current +branch. In fact, Git uses a file named `HEAD` in the `.git` directory +to remember which branch is current: + +------------------------------------------------ +$ cat .git/HEAD +ref: refs/heads/master +------------------------------------------------ + +[[detached-head]] +=== Examining an old version without creating a new branch + +The `git switch` command normally expects a branch head, but will also +accept an arbitrary commit when invoked with --detach; for example, +you can check out the commit referenced by a tag: + +------------------------------------------------ +$ git switch --detach v2.6.17 +Note: checking out 'v2.6.17'. + +You are in 'detached HEAD' state. You can look around, make experimental +changes and commit them, and you can discard any commits you make in this +state without impacting any branches by performing another switch. + +If you want to create a new branch to retain commits you create, you may +do so (now or later) by using -c with the switch command again. Example: + + git switch -c new_branch_name + +HEAD is now at 427abfa Linux v2.6.17 +------------------------------------------------ + +The HEAD then refers to the SHA-1 of the commit instead of to a branch, +and git branch shows that you are no longer on a branch: + +------------------------------------------------ +$ cat .git/HEAD +427abfa28afedffadfca9dd8b067eb6d36bac53f +$ git branch +* (detached from v2.6.17) + master +------------------------------------------------ + +In this case we say that the HEAD is "detached". + +This is an easy way to check out a particular version without having to +make up a name for the new branch. You can still create a new branch +(or tag) for this version later if you decide to. + +[[examining-remote-branches]] +=== Examining branches from a remote repository + +The "master" branch that was created at the time you cloned is a copy +of the HEAD in the repository that you cloned from. That repository +may also have had other branches, though, and your local repository +keeps branches which track each of those remote branches, called +remote-tracking branches, which you +can view using the `-r` option to linkgit:git-branch[1]: + +------------------------------------------------ +$ git branch -r + origin/HEAD + origin/html + origin/maint + origin/man + origin/master + origin/next + origin/seen + origin/todo +------------------------------------------------ + +In this example, "origin" is called a remote repository, or "remote" +for short. The branches of this repository are called "remote +branches" from our point of view. The remote-tracking branches listed +above were created based on the remote branches at clone time and will +be updated by `git fetch` (hence `git pull`) and `git push`. See +<<Updating-a-repository-With-git-fetch>> for details. + +You might want to build on one of these remote-tracking branches +on a branch of your own, just as you would for a tag: + +------------------------------------------------ +$ git switch -c my-todo-copy origin/todo +------------------------------------------------ + +You can also check out `origin/todo` directly to examine it or +write a one-off patch. See <<detached-head,detached head>>. + +Note that the name "origin" is just the name that Git uses by default +to refer to the repository that you cloned from. + +[[how-git-stores-references]] +=== Naming branches, tags, and other references + +Branches, remote-tracking branches, and tags are all references to +commits. All references are named with a slash-separated path name +starting with `refs`; the names we've been using so far are actually +shorthand: + + - The branch `test` is short for `refs/heads/test`. + - The tag `v2.6.18` is short for `refs/tags/v2.6.18`. + - `origin/master` is short for `refs/remotes/origin/master`. + +The full name is occasionally useful if, for example, there ever +exists a tag and a branch with the same name. + +(Newly created refs are actually stored in the `.git/refs` directory, +under the path given by their name. However, for efficiency reasons +they may also be packed together in a single file; see +linkgit:git-pack-refs[1]). + +As another useful shortcut, the "HEAD" of a repository can be referred +to just using the name of that repository. So, for example, "origin" +is usually a shortcut for the HEAD branch in the repository "origin". + +For the complete list of paths which Git checks for references, and +the order it uses to decide which to choose when there are multiple +references with the same shorthand name, see the "SPECIFYING +REVISIONS" section of linkgit:gitrevisions[7]. + +[[Updating-a-repository-With-git-fetch]] +=== Updating a repository with git fetch + +After you clone a repository and commit a few changes of your own, you +may wish to check the original repository for updates. + +The `git-fetch` command, with no arguments, will update all of the +remote-tracking branches to the latest version found in the original +repository. It will not touch any of your own branches--not even the +"master" branch that was created for you on clone. + +[[fetching-branches]] +=== Fetching branches from other repositories + +You can also track branches from repositories other than the one you +cloned from, using linkgit:git-remote[1]: + +------------------------------------------------- +$ git remote add staging git://git.kernel.org/.../gregkh/staging.git +$ git fetch staging +... +From git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging + * [new branch] master -> staging/master + * [new branch] staging-linus -> staging/staging-linus + * [new branch] staging-next -> staging/staging-next +------------------------------------------------- + +New remote-tracking branches will be stored under the shorthand name +that you gave `git remote add`, in this case `staging`: + +------------------------------------------------- +$ git branch -r + origin/HEAD -> origin/master + origin/master + staging/master + staging/staging-linus + staging/staging-next +------------------------------------------------- + +If you run `git fetch <remote>` later, the remote-tracking branches +for the named `<remote>` will be updated. + +If you examine the file `.git/config`, you will see that Git has added +a new stanza: + +------------------------------------------------- +$ cat .git/config +... +[remote "staging"] + url = git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git + fetch = +refs/heads/*:refs/remotes/staging/* +... +------------------------------------------------- + +This is what causes Git to track the remote's branches; you may modify +or delete these configuration options by editing `.git/config` with a +text editor. (See the "CONFIGURATION FILE" section of +linkgit:git-config[1] for details.) + +[[exploring-git-history]] +== Exploring Git history + +Git is best thought of as a tool for storing the history of a +collection of files. It does this by storing compressed snapshots of +the contents of a file hierarchy, together with "commits" which show +the relationships between these snapshots. + +Git provides extremely flexible and fast tools for exploring the +history of a project. + +We start with one specialized tool that is useful for finding the +commit that introduced a bug into a project. + +[[using-bisect]] +=== How to use bisect to find a regression + +Suppose version 2.6.18 of your project worked, but the version at +"master" crashes. Sometimes the best way to find the cause of such a +regression is to perform a brute-force search through the project's +history to find the particular commit that caused the problem. The +linkgit:git-bisect[1] command can help you do this: + +------------------------------------------------- +$ git bisect start +$ git bisect good v2.6.18 +$ git bisect bad master +Bisecting: 3537 revisions left to test after this +[65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6] +------------------------------------------------- + +If you run `git branch` at this point, you'll see that Git has +temporarily moved you in "(no branch)". HEAD is now detached from any +branch and points directly to a commit (with commit id 65934) that +is reachable from "master" but not from v2.6.18. Compile and test it, +and see whether it crashes. Assume it does crash. Then: + +------------------------------------------------- +$ git bisect bad +Bisecting: 1769 revisions left to test after this +[7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings +------------------------------------------------- + +checks out an older version. Continue like this, telling Git at each +stage whether the version it gives you is good or bad, and notice +that the number of revisions left to test is cut approximately in +half each time. + +After about 13 tests (in this case), it will output the commit id of +the guilty commit. You can then examine the commit with +linkgit:git-show[1], find out who wrote it, and mail them your bug +report with the commit id. Finally, run + +------------------------------------------------- +$ git bisect reset +------------------------------------------------- + +to return you to the branch you were on before. + +Note that the version which `git bisect` checks out for you at each +point is just a suggestion, and you're free to try a different +version if you think it would be a good idea. For example, +occasionally you may land on a commit that broke something unrelated; +run + +------------------------------------------------- +$ git bisect visualize +------------------------------------------------- + +which will run gitk and label the commit it chose with a marker that +says "bisect". Choose a safe-looking commit nearby, note its commit +id, and check it out with: + +------------------------------------------------- +$ git reset --hard fb47ddb2db +------------------------------------------------- + +then test, run `bisect good` or `bisect bad` as appropriate, and +continue. + +Instead of `git bisect visualize` and then `git reset --hard +fb47ddb2db`, you might just want to tell Git that you want to skip +the current commit: + +------------------------------------------------- +$ git bisect skip +------------------------------------------------- + +In this case, though, Git may not eventually be able to tell the first +bad one between some first skipped commits and a later bad commit. + +There are also ways to automate the bisecting process if you have a +test script that can tell a good from a bad commit. See +linkgit:git-bisect[1] for more information about this and other `git +bisect` features. + +[[naming-commits]] +=== Naming commits + +We have seen several ways of naming commits already: + + - 40-hexdigit object name + - branch name: refers to the commit at the head of the given + branch + - tag name: refers to the commit pointed to by the given tag + (we've seen branches and tags are special cases of + <<how-git-stores-references,references>>). + - HEAD: refers to the head of the current branch + +There are many more; see the "SPECIFYING REVISIONS" section of the +linkgit:gitrevisions[7] man page for the complete list of ways to +name revisions. Some examples: + +------------------------------------------------- +$ git show fb47ddb2 # the first few characters of the object name + # are usually enough to specify it uniquely +$ git show HEAD^ # the parent of the HEAD commit +$ git show HEAD^^ # the grandparent +$ git show HEAD~4 # the great-great-grandparent +------------------------------------------------- + +Recall that merge commits may have more than one parent; by default, +`^` and `~` follow the first parent listed in the commit, but you can +also choose: + +------------------------------------------------- +$ git show HEAD^1 # show the first parent of HEAD +$ git show HEAD^2 # show the second parent of HEAD +------------------------------------------------- + +In addition to HEAD, there are several other special names for +commits: + +Merges (to be discussed later), as well as operations such as +`git reset`, which change the currently checked-out commit, generally +set ORIG_HEAD to the value HEAD had before the current operation. + +The `git fetch` operation always stores the head of the last fetched +branch in FETCH_HEAD. For example, if you run `git fetch` without +specifying a local branch as the target of the operation + +------------------------------------------------- +$ git fetch git://example.com/proj.git theirbranch +------------------------------------------------- + +the fetched commits will still be available from FETCH_HEAD. + +When we discuss merges we'll also see the special name MERGE_HEAD, +which refers to the other branch that we're merging in to the current +branch. + +The linkgit:git-rev-parse[1] command is a low-level command that is +occasionally useful for translating some name for a commit to the object +name for that commit: + +------------------------------------------------- +$ git rev-parse origin +e05db0fd4f31dde7005f075a84f96b360d05984b +------------------------------------------------- + +[[creating-tags]] +=== Creating tags + +We can also create a tag to refer to a particular commit; after +running + +------------------------------------------------- +$ git tag stable-1 1b2e1d63ff +------------------------------------------------- + +You can use `stable-1` to refer to the commit 1b2e1d63ff. + +This creates a "lightweight" tag. If you would also like to include a +comment with the tag, and possibly sign it cryptographically, then you +should create a tag object instead; see the linkgit:git-tag[1] man page +for details. + +[[browsing-revisions]] +=== Browsing revisions + +The linkgit:git-log[1] command can show lists of commits. On its +own, it shows all commits reachable from the parent commit; but you +can also make more specific requests: + +------------------------------------------------- +$ git log v2.5.. # commits since (not reachable from) v2.5 +$ git log test..master # commits reachable from master but not test +$ git log master..test # ...reachable from test but not master +$ git log master...test # ...reachable from either test or master, + # but not both +$ git log --since="2 weeks ago" # commits from the last 2 weeks +$ git log Makefile # commits which modify Makefile +$ git log fs/ # ... which modify any file under fs/ +$ git log -S'foo()' # commits which add or remove any file data + # matching the string 'foo()' +------------------------------------------------- + +And of course you can combine all of these; the following finds +commits since v2.5 which touch the `Makefile` or any file under `fs`: + +------------------------------------------------- +$ git log v2.5.. Makefile fs/ +------------------------------------------------- + +You can also ask git log to show patches: + +------------------------------------------------- +$ git log -p +------------------------------------------------- + +See the `--pretty` option in the linkgit:git-log[1] man page for more +display options. + +Note that git log starts with the most recent commit and works +backwards through the parents; however, since Git history can contain +multiple independent lines of development, the particular order that +commits are listed in may be somewhat arbitrary. + +[[generating-diffs]] +=== Generating diffs + +You can generate diffs between any two versions using +linkgit:git-diff[1]: + +------------------------------------------------- +$ git diff master..test +------------------------------------------------- + +That will produce the diff between the tips of the two branches. If +you'd prefer to find the diff from their common ancestor to test, you +can use three dots instead of two: + +------------------------------------------------- +$ git diff master...test +------------------------------------------------- + +Sometimes what you want instead is a set of patches; for this you can +use linkgit:git-format-patch[1]: + +------------------------------------------------- +$ git format-patch master..test +------------------------------------------------- + +will generate a file with a patch for each commit reachable from test +but not from master. + +[[viewing-old-file-versions]] +=== Viewing old file versions + +You can always view an old version of a file by just checking out the +correct revision first. But sometimes it is more convenient to be +able to view an old version of a single file without checking +anything out; this command does that: + +------------------------------------------------- +$ git show v2.5:fs/locks.c +------------------------------------------------- + +Before the colon may be anything that names a commit, and after it +may be any path to a file tracked by Git. + +[[history-examples]] +=== Examples + +[[counting-commits-on-a-branch]] +==== Counting the number of commits on a branch + +Suppose you want to know how many commits you've made on `mybranch` +since it diverged from `origin`: + +------------------------------------------------- +$ git log --pretty=oneline origin..mybranch | wc -l +------------------------------------------------- + +Alternatively, you may often see this sort of thing done with the +lower-level command linkgit:git-rev-list[1], which just lists the SHA-1's +of all the given commits: + +------------------------------------------------- +$ git rev-list origin..mybranch | wc -l +------------------------------------------------- + +[[checking-for-equal-branches]] +==== Check whether two branches point at the same history + +Suppose you want to check whether two branches point at the same point +in history. + +------------------------------------------------- +$ git diff origin..master +------------------------------------------------- + +will tell you whether the contents of the project are the same at the +two branches; in theory, however, it's possible that the same project +contents could have been arrived at by two different historical +routes. You could compare the object names: + +------------------------------------------------- +$ git rev-list origin +e05db0fd4f31dde7005f075a84f96b360d05984b +$ git rev-list master +e05db0fd4f31dde7005f075a84f96b360d05984b +------------------------------------------------- + +Or you could recall that the `...` operator selects all commits +reachable from either one reference or the other but not +both; so + +------------------------------------------------- +$ git log origin...master +------------------------------------------------- + +will return no commits when the two branches are equal. + +[[finding-tagged-descendants]] +==== Find first tagged version including a given fix + +Suppose you know that the commit e05db0fd fixed a certain problem. +You'd like to find the earliest tagged release that contains that +fix. + +Of course, there may be more than one answer--if the history branched +after commit e05db0fd, then there could be multiple "earliest" tagged +releases. + +You could just visually inspect the commits since e05db0fd: + +------------------------------------------------- +$ gitk e05db0fd.. +------------------------------------------------- + +or you can use linkgit:git-name-rev[1], which will give the commit a +name based on any tag it finds pointing to one of the commit's +descendants: + +------------------------------------------------- +$ git name-rev --tags e05db0fd +e05db0fd tags/v1.5.0-rc1^0~23 +------------------------------------------------- + +The linkgit:git-describe[1] command does the opposite, naming the +revision using a tag on which the given commit is based: + +------------------------------------------------- +$ git describe e05db0fd +v1.5.0-rc0-260-ge05db0f +------------------------------------------------- + +but that may sometimes help you guess which tags might come after the +given commit. + +If you just want to verify whether a given tagged version contains a +given commit, you could use linkgit:git-merge-base[1]: + +------------------------------------------------- +$ git merge-base e05db0fd v1.5.0-rc1 +e05db0fd4f31dde7005f075a84f96b360d05984b +------------------------------------------------- + +The merge-base command finds a common ancestor of the given commits, +and always returns one or the other in the case where one is a +descendant of the other; so the above output shows that e05db0fd +actually is an ancestor of v1.5.0-rc1. + +Alternatively, note that + +------------------------------------------------- +$ git log v1.5.0-rc1..e05db0fd +------------------------------------------------- + +will produce empty output if and only if v1.5.0-rc1 includes e05db0fd, +because it outputs only commits that are not reachable from v1.5.0-rc1. + +As yet another alternative, the linkgit:git-show-branch[1] command lists +the commits reachable from its arguments with a display on the left-hand +side that indicates which arguments that commit is reachable from. +So, if you run something like + +------------------------------------------------- +$ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2 +! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if +available + ! [v1.5.0-rc0] GIT v1.5.0 preview + ! [v1.5.0-rc1] GIT v1.5.0-rc1 + ! [v1.5.0-rc2] GIT v1.5.0-rc2 +... +------------------------------------------------- + +then a line like + +------------------------------------------------- ++ ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if +available +------------------------------------------------- + +shows that e05db0fd is reachable from itself, from v1.5.0-rc1, +and from v1.5.0-rc2, and not from v1.5.0-rc0. + +[[showing-commits-unique-to-a-branch]] +==== Showing commits unique to a given branch + +Suppose you would like to see all the commits reachable from the branch +head named `master` but not from any other head in your repository. + +We can list all the heads in this repository with +linkgit:git-show-ref[1]: + +------------------------------------------------- +$ git show-ref --heads +bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial +db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint +a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master +24dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2 +1e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes +------------------------------------------------- + +We can get just the branch-head names, and remove `master`, with +the help of the standard utilities cut and grep: + +------------------------------------------------- +$ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master' +refs/heads/core-tutorial +refs/heads/maint +refs/heads/tutorial-2 +refs/heads/tutorial-fixes +------------------------------------------------- + +And then we can ask to see all the commits reachable from master +but not from these other heads: + +------------------------------------------------- +$ gitk master --not $( git show-ref --heads | cut -d' ' -f2 | + grep -v '^refs/heads/master' ) +------------------------------------------------- + +Obviously, endless variations are possible; for example, to see all +commits reachable from some head but not from any tag in the repository: + +------------------------------------------------- +$ gitk $( git show-ref --heads ) --not $( git show-ref --tags ) +------------------------------------------------- + +(See linkgit:gitrevisions[7] for explanations of commit-selecting +syntax such as `--not`.) + +[[making-a-release]] +==== Creating a changelog and tarball for a software release + +The linkgit:git-archive[1] command can create a tar or zip archive from +any version of a project; for example: + +------------------------------------------------- +$ git archive -o latest.tar.gz --prefix=project/ HEAD +------------------------------------------------- + +will use HEAD to produce a gzipped tar archive in which each filename +is preceded by `project/`. The output file format is inferred from +the output file extension if possible, see linkgit:git-archive[1] for +details. + +Versions of Git older than 1.7.7 don't know about the `tar.gz` format, +you'll need to use gzip explicitly: + +------------------------------------------------- +$ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz +------------------------------------------------- + +If you're releasing a new version of a software project, you may want +to simultaneously make a changelog to include in the release +announcement. + +Linus Torvalds, for example, makes new kernel releases by tagging them, +then running: + +------------------------------------------------- +$ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7 +------------------------------------------------- + +where release-script is a shell script that looks like: + +------------------------------------------------- +#!/bin/sh +stable="$1" +last="$2" +new="$3" +echo "# git tag v$new" +echo "git archive --prefix=linux-$new/ v$new | gzip -9 > ../linux-$new.tar.gz" +echo "git diff v$stable v$new | gzip -9 > ../patch-$new.gz" +echo "git log --no-merges v$new ^v$last > ../ChangeLog-$new" +echo "git shortlog --no-merges v$new ^v$last > ../ShortLog" +echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new" +------------------------------------------------- + +and then he just cut-and-pastes the output commands after verifying that +they look OK. + +[[Finding-commits-With-given-Content]] +==== Finding commits referencing a file with given content + +Somebody hands you a copy of a file, and asks which commits modified a +file such that it contained the given content either before or after the +commit. You can find out with this: + +------------------------------------------------- +$ git log --raw --abbrev=40 --pretty=oneline | + grep -B 1 `git hash-object filename` +------------------------------------------------- + +Figuring out why this works is left as an exercise to the (advanced) +student. The linkgit:git-log[1], linkgit:git-diff-tree[1], and +linkgit:git-hash-object[1] man pages may prove helpful. + +[[Developing-With-git]] +== Developing with Git + +[[telling-git-your-name]] +=== Telling Git your name + +Before creating any commits, you should introduce yourself to Git. +The easiest way to do so is to use linkgit:git-config[1]: + +------------------------------------------------ +$ git config --global user.name 'Your Name Comes Here' +$ git config --global user.email 'you@yourdomain.example.com' +------------------------------------------------ + +Which will add the following to a file named `.gitconfig` in your +home directory: + +------------------------------------------------ +[user] + name = Your Name Comes Here + email = you@yourdomain.example.com +------------------------------------------------ + +See the "CONFIGURATION FILE" section of linkgit:git-config[1] for +details on the configuration file. The file is plain text, so you can +also edit it with your favorite editor. + + +[[creating-a-new-repository]] +=== Creating a new repository + +Creating a new repository from scratch is very easy: + +------------------------------------------------- +$ mkdir project +$ cd project +$ git init +------------------------------------------------- + +If you have some initial content (say, a tarball): + +------------------------------------------------- +$ tar xzvf project.tar.gz +$ cd project +$ git init +$ git add . # include everything below ./ in the first commit: +$ git commit +------------------------------------------------- + +[[how-to-make-a-commit]] +=== How to make a commit + +Creating a new commit takes three steps: + + 1. Making some changes to the working directory using your + favorite editor. + 2. Telling Git about your changes. + 3. Creating the commit using the content you told Git about + in step 2. + +In practice, you can interleave and repeat steps 1 and 2 as many +times as you want: in order to keep track of what you want committed +at step 3, Git maintains a snapshot of the tree's contents in a +special staging area called "the index." + +At the beginning, the content of the index will be identical to +that of the HEAD. The command `git diff --cached`, which shows +the difference between the HEAD and the index, should therefore +produce no output at that point. + +Modifying the index is easy: + +To update the index with the contents of a new or modified file, use + +------------------------------------------------- +$ git add path/to/file +------------------------------------------------- + +To remove a file from the index and from the working tree, use + +------------------------------------------------- +$ git rm path/to/file +------------------------------------------------- + +After each step you can verify that + +------------------------------------------------- +$ git diff --cached +------------------------------------------------- + +always shows the difference between the HEAD and the index file--this +is what you'd commit if you created the commit now--and that + +------------------------------------------------- +$ git diff +------------------------------------------------- + +shows the difference between the working tree and the index file. + +Note that `git add` always adds just the current contents of a file +to the index; further changes to the same file will be ignored unless +you run `git add` on the file again. + +When you're ready, just run + +------------------------------------------------- +$ git commit +------------------------------------------------- + +and Git will prompt you for a commit message and then create the new +commit. Check to make sure it looks like what you expected with + +------------------------------------------------- +$ git show +------------------------------------------------- + +As a special shortcut, + +------------------------------------------------- +$ git commit -a +------------------------------------------------- + +will update the index with any files that you've modified or removed +and create a commit, all in one step. + +A number of commands are useful for keeping track of what you're +about to commit: + +------------------------------------------------- +$ git diff --cached # difference between HEAD and the index; what + # would be committed if you ran "commit" now. +$ git diff # difference between the index file and your + # working directory; changes that would not + # be included if you ran "commit" now. +$ git diff HEAD # difference between HEAD and working tree; what + # would be committed if you ran "commit -a" now. +$ git status # a brief per-file summary of the above. +------------------------------------------------- + +You can also use linkgit:git-gui[1] to create commits, view changes in +the index and the working tree files, and individually select diff hunks +for inclusion in the index (by right-clicking on the diff hunk and +choosing "Stage Hunk For Commit"). + +[[creating-good-commit-messages]] +=== Creating good commit messages + +Though not required, it's a good idea to begin the commit message +with a single short (no more than 50 characters) line summarizing the +change, followed by a blank line and then a more thorough +description. The text up to the first blank line in a commit +message is treated as the commit title, and that title is used +throughout Git. For example, linkgit:git-format-patch[1] turns a +commit into email, and it uses the title on the Subject line and the +rest of the commit in the body. + + +[[ignoring-files]] +=== Ignoring files + +A project will often generate files that you do 'not' want to track with Git. +This typically includes files generated by a build process or temporary +backup files made by your editor. Of course, 'not' tracking files with Git +is just a matter of 'not' calling `git add` on them. But it quickly becomes +annoying to have these untracked files lying around; e.g. they make +`git add .` practically useless, and they keep showing up in the output of +`git status`. + +You can tell Git to ignore certain files by creating a file called +`.gitignore` in the top level of your working directory, with contents +such as: + +------------------------------------------------- +# Lines starting with '#' are considered comments. +# Ignore any file named foo.txt. +foo.txt +# Ignore (generated) html files, +*.html +# except foo.html which is maintained by hand. +!foo.html +# Ignore objects and archives. +*.[oa] +------------------------------------------------- + +See linkgit:gitignore[5] for a detailed explanation of the syntax. You can +also place .gitignore files in other directories in your working tree, and they +will apply to those directories and their subdirectories. The `.gitignore` +files can be added to your repository like any other files (just run `git add +.gitignore` and `git commit`, as usual), which is convenient when the exclude +patterns (such as patterns matching build output files) would also make sense +for other users who clone your repository. + +If you wish the exclude patterns to affect only certain repositories +(instead of every repository for a given project), you may instead put +them in a file in your repository named `.git/info/exclude`, or in any +file specified by the `core.excludesFile` configuration variable. +Some Git commands can also take exclude patterns directly on the +command line. See linkgit:gitignore[5] for the details. + +[[how-to-merge]] +=== How to merge + +You can rejoin two diverging branches of development using +linkgit:git-merge[1]: + +------------------------------------------------- +$ git merge branchname +------------------------------------------------- + +merges the development in the branch `branchname` into the current +branch. + +A merge is made by combining the changes made in `branchname` and the +changes made up to the latest commit in your current branch since +their histories forked. The work tree is overwritten by the result of +the merge when this combining is done cleanly, or overwritten by a +half-merged results when this combining results in conflicts. +Therefore, if you have uncommitted changes touching the same files as +the ones impacted by the merge, Git will refuse to proceed. Most of +the time, you will want to commit your changes before you can merge, +and if you don't, then linkgit:git-stash[1] can take these changes +away while you're doing the merge, and reapply them afterwards. + +If the changes are independent enough, Git will automatically complete +the merge and commit the result (or reuse an existing commit in case +of <<fast-forwards,fast-forward>>, see below). On the other hand, +if there are conflicts--for example, if the same file is +modified in two different ways in the remote branch and the local +branch--then you are warned; the output may look something like this: + +------------------------------------------------- +$ git merge next + 100% (4/4) done +Auto-merged file.txt +CONFLICT (content): Merge conflict in file.txt +Automatic merge failed; fix conflicts and then commit the result. +------------------------------------------------- + +Conflict markers are left in the problematic files, and after +you resolve the conflicts manually, you can update the index +with the contents and run Git commit, as you normally would when +creating a new file. + +If you examine the resulting commit using gitk, you will see that it +has two parents, one pointing to the top of the current branch, and +one to the top of the other branch. + +[[resolving-a-merge]] +=== Resolving a merge + +When a merge isn't resolved automatically, Git leaves the index and +the working tree in a special state that gives you all the +information you need to help resolve the merge. + +Files with conflicts are marked specially in the index, so until you +resolve the problem and update the index, linkgit:git-commit[1] will +fail: + +------------------------------------------------- +$ git commit +file.txt: needs merge +------------------------------------------------- + +Also, linkgit:git-status[1] will list those files as "unmerged", and the +files with conflicts will have conflict markers added, like this: + +------------------------------------------------- +<<<<<<< HEAD:file.txt +Hello world +======= +Goodbye +>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt +------------------------------------------------- + +All you need to do is edit the files to resolve the conflicts, and then + +------------------------------------------------- +$ git add file.txt +$ git commit +------------------------------------------------- + +Note that the commit message will already be filled in for you with +some information about the merge. Normally you can just use this +default message unchanged, but you may add additional commentary of +your own if desired. + +The above is all you need to know to resolve a simple merge. But Git +also provides more information to help resolve conflicts: + +[[conflict-resolution]] +==== Getting conflict-resolution help during a merge + +All of the changes that Git was able to merge automatically are +already added to the index file, so linkgit:git-diff[1] shows only +the conflicts. It uses an unusual syntax: + +------------------------------------------------- +$ git diff +diff --cc file.txt +index 802992c,2b60207..0000000 +--- a/file.txt ++++ b/file.txt +@@@ -1,1 -1,1 +1,5 @@@ +++<<<<<<< HEAD:file.txt + +Hello world +++======= ++ Goodbye +++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt +------------------------------------------------- + +Recall that the commit which will be committed after we resolve this +conflict will have two parents instead of the usual one: one parent +will be HEAD, the tip of the current branch; the other will be the +tip of the other branch, which is stored temporarily in MERGE_HEAD. + +During the merge, the index holds three versions of each file. Each of +these three "file stages" represents a different version of the file: + +------------------------------------------------- +$ git show :1:file.txt # the file in a common ancestor of both branches +$ git show :2:file.txt # the version from HEAD. +$ git show :3:file.txt # the version from MERGE_HEAD. +------------------------------------------------- + +When you ask linkgit:git-diff[1] to show the conflicts, it runs a +three-way diff between the conflicted merge results in the work tree with +stages 2 and 3 to show only hunks whose contents come from both sides, +mixed (in other words, when a hunk's merge results come only from stage 2, +that part is not conflicting and is not shown. Same for stage 3). + +The diff above shows the differences between the working-tree version of +file.txt and the stage 2 and stage 3 versions. So instead of preceding +each line by a single `+` or `-`, it now uses two columns: the first +column is used for differences between the first parent and the working +directory copy, and the second for differences between the second parent +and the working directory copy. (See the "COMBINED DIFF FORMAT" section +of linkgit:git-diff-files[1] for a details of the format.) + +After resolving the conflict in the obvious way (but before updating the +index), the diff will look like: + +------------------------------------------------- +$ git diff +diff --cc file.txt +index 802992c,2b60207..0000000 +--- a/file.txt ++++ b/file.txt +@@@ -1,1 -1,1 +1,1 @@@ +- Hello world + -Goodbye +++Goodbye world +------------------------------------------------- + +This shows that our resolved version deleted "Hello world" from the +first parent, deleted "Goodbye" from the second parent, and added +"Goodbye world", which was previously absent from both. + +Some special diff options allow diffing the working directory against +any of these stages: + +------------------------------------------------- +$ git diff -1 file.txt # diff against stage 1 +$ git diff --base file.txt # same as the above +$ git diff -2 file.txt # diff against stage 2 +$ git diff --ours file.txt # same as the above +$ git diff -3 file.txt # diff against stage 3 +$ git diff --theirs file.txt # same as the above. +------------------------------------------------- + +When using the 'ort' merge strategy (the default), before updating the working +tree with the result of the merge, Git writes a special ref named AUTO_MERGE +reflecting the state of the tree it is about to write. Conflicted paths with +textual conflicts that could not be automatically merged are written to this +tree with conflict markers, just as in the working tree. AUTO_MERGE can thus be +used with linkgit:git-diff[1] to show the changes you've made so far to resolve +conflicts. Using the same example as above, after resolving the conflict we +get: + +------------------------------------------------- +$ git diff AUTO_MERGE +diff --git a/file.txt b/file.txt +index cd10406..8bf5ae7 100644 +--- a/file.txt ++++ b/file.txt +@@ -1,5 +1 @@ +-<<<<<<< HEAD:file.txt +-Hello world +-======= +-Goodbye +->>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt ++Goodbye world +------------------------------------------------- + +Notice that the diff shows we deleted the conflict markers and both versions of +the content line, and wrote "Goodbye world" instead. + +The linkgit:git-log[1] and linkgit:gitk[1] commands also provide special help +for merges: + +------------------------------------------------- +$ git log --merge +$ gitk --merge +------------------------------------------------- + +These will display all commits which exist only on HEAD or on +MERGE_HEAD, and which touch an unmerged file. + +You may also use linkgit:git-mergetool[1], which lets you merge the +unmerged files using external tools such as Emacs or kdiff3. + +Each time you resolve the conflicts in a file and update the index: + +------------------------------------------------- +$ git add file.txt +------------------------------------------------- + +the different stages of that file will be "collapsed", after which +`git diff` will (by default) no longer show diffs for that file. + +[[undoing-a-merge]] +=== Undoing a merge + +If you get stuck and decide to just give up and throw the whole mess +away, you can always return to the pre-merge state with + +------------------------------------------------- +$ git merge --abort +------------------------------------------------- + +Or, if you've already committed the merge that you want to throw away, + +------------------------------------------------- +$ git reset --hard ORIG_HEAD +------------------------------------------------- + +However, this last command can be dangerous in some cases--never +throw away a commit you have already committed if that commit may +itself have been merged into another branch, as doing so may confuse +further merges. + +[[fast-forwards]] +=== Fast-forward merges + +There is one special case not mentioned above, which is treated +differently. Normally, a merge results in a merge commit, with two +parents, one pointing at each of the two lines of development that +were merged. + +However, if the current branch is an ancestor of the other--so every commit +present in the current branch is already contained in the other branch--then Git +just performs a "fast-forward"; the head of the current branch is moved forward +to point at the head of the merged-in branch, without any new commits being +created. + +[[fixing-mistakes]] +=== Fixing mistakes + +If you've messed up the working tree, but haven't yet committed your +mistake, you can return the entire working tree to the last committed +state with + +------------------------------------------------- +$ git restore --staged --worktree :/ +------------------------------------------------- + +If you make a commit that you later wish you hadn't, there are two +fundamentally different ways to fix the problem: + + 1. You can create a new commit that undoes whatever was done + by the old commit. This is the correct thing if your + mistake has already been made public. + + 2. You can go back and modify the old commit. You should + never do this if you have already made the history public; + Git does not normally expect the "history" of a project to + change, and cannot correctly perform repeated merges from + a branch that has had its history changed. + +[[reverting-a-commit]] +==== Fixing a mistake with a new commit + +Creating a new commit that reverts an earlier change is very easy; +just pass the linkgit:git-revert[1] command a reference to the bad +commit; for example, to revert the most recent commit: + +------------------------------------------------- +$ git revert HEAD +------------------------------------------------- + +This will create a new commit which undoes the change in HEAD. You +will be given a chance to edit the commit message for the new commit. + +You can also revert an earlier change, for example, the next-to-last: + +------------------------------------------------- +$ git revert HEAD^ +------------------------------------------------- + +In this case Git will attempt to undo the old change while leaving +intact any changes made since then. If more recent changes overlap +with the changes to be reverted, then you will be asked to fix +conflicts manually, just as in the case of <<resolving-a-merge, +resolving a merge>>. + +[[fixing-a-mistake-by-rewriting-history]] +==== Fixing a mistake by rewriting history + +If the problematic commit is the most recent commit, and you have not +yet made that commit public, then you may just +<<undoing-a-merge,destroy it using `git reset`>>. + +Alternatively, you +can edit the working directory and update the index to fix your +mistake, just as if you were going to <<how-to-make-a-commit,create a +new commit>>, then run + +------------------------------------------------- +$ git commit --amend +------------------------------------------------- + +which will replace the old commit by a new commit incorporating your +changes, giving you a chance to edit the old commit message first. + +Again, you should never do this to a commit that may already have +been merged into another branch; use linkgit:git-revert[1] instead in +that case. + +It is also possible to replace commits further back in the history, but +this is an advanced topic to be left for +<<cleaning-up-history,another chapter>>. + +[[checkout-of-path]] +==== Checking out an old version of a file + +In the process of undoing a previous bad change, you may find it +useful to check out an older version of a particular file using +linkgit:git-restore[1]. The command + +------------------------------------------------- +$ git restore --source=HEAD^ path/to/file +------------------------------------------------- + +replaces path/to/file by the contents it had in the commit HEAD^, and +also updates the index to match. It does not change branches. + +If you just want to look at an old version of the file, without +modifying the working directory, you can do that with +linkgit:git-show[1]: + +------------------------------------------------- +$ git show HEAD^:path/to/file +------------------------------------------------- + +which will display the given version of the file. + +[[interrupted-work]] +==== Temporarily setting aside work in progress + +While you are in the middle of working on something complicated, you +find an unrelated but obvious and trivial bug. You would like to fix it +before continuing. You can use linkgit:git-stash[1] to save the current +state of your work, and after fixing the bug (or, optionally after doing +so on a different branch and then coming back), unstash the +work-in-progress changes. + +------------------------------------------------ +$ git stash push -m "work in progress for foo feature" +------------------------------------------------ + +This command will save your changes away to the `stash`, and +reset your working tree and the index to match the tip of your +current branch. Then you can make your fix as usual. + +------------------------------------------------ +... edit and test ... +$ git commit -a -m "blorpl: typofix" +------------------------------------------------ + +After that, you can go back to what you were working on with +`git stash pop`: + +------------------------------------------------ +$ git stash pop +------------------------------------------------ + + +[[ensuring-good-performance]] +=== Ensuring good performance + +On large repositories, Git depends on compression to keep the history +information from taking up too much space on disk or in memory. Some +Git commands may automatically run linkgit:git-gc[1], so you don't +have to worry about running it manually. However, compressing a large +repository may take a while, so you may want to call `gc` explicitly +to avoid automatic compression kicking in when it is not convenient. + + +[[ensuring-reliability]] +=== Ensuring reliability + +[[checking-for-corruption]] +==== Checking the repository for corruption + +The linkgit:git-fsck[1] command runs a number of self-consistency checks +on the repository, and reports on any problems. This may take some +time. + +------------------------------------------------- +$ git fsck +dangling commit 7281251ddd2a61e38657c827739c57015671a6b3 +dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63 +dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5 +dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb +dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f +dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e +dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085 +dangling tree b24c2473f1fd3d91352a624795be026d64c8841f +... +------------------------------------------------- + +You will see informational messages on dangling objects. They are objects +that still exist in the repository but are no longer referenced by any of +your branches, and can (and will) be removed after a while with `gc`. +You can run `git fsck --no-dangling` to suppress these messages, and still +view real errors. + +[[recovering-lost-changes]] +==== Recovering lost changes + +[[reflogs]] +===== Reflogs + +Say you modify a branch with <<fixing-mistakes,`git reset --hard`>>, +and then realize that the branch was the only reference you had to +that point in history. + +Fortunately, Git also keeps a log, called a "reflog", of all the +previous values of each branch. So in this case you can still find the +old history using, for example, + +------------------------------------------------- +$ git log master@{1} +------------------------------------------------- + +This lists the commits reachable from the previous version of the +`master` branch head. This syntax can be used with any Git command +that accepts a commit, not just with `git log`. Some other examples: + +------------------------------------------------- +$ git show master@{2} # See where the branch pointed 2, +$ git show master@{3} # 3, ... changes ago. +$ gitk master@{yesterday} # See where it pointed yesterday, +$ gitk master@{"1 week ago"} # ... or last week +$ git log --walk-reflogs master # show reflog entries for master +------------------------------------------------- + +A separate reflog is kept for the HEAD, so + +------------------------------------------------- +$ git show HEAD@{"1 week ago"} +------------------------------------------------- + +will show what HEAD pointed to one week ago, not what the current branch +pointed to one week ago. This allows you to see the history of what +you've checked out. + +The reflogs are kept by default for 30 days, after which they may be +pruned. See linkgit:git-reflog[1] and linkgit:git-gc[1] to learn +how to control this pruning, and see the "SPECIFYING REVISIONS" +section of linkgit:gitrevisions[7] for details. + +Note that the reflog history is very different from normal Git history. +While normal history is shared by every repository that works on the +same project, the reflog history is not shared: it tells you only about +how the branches in your local repository have changed over time. + +[[dangling-object-recovery]] +===== Examining dangling objects + +In some situations the reflog may not be able to save you. For example, +suppose you delete a branch, then realize you need the history it +contained. The reflog is also deleted; however, if you have not yet +pruned the repository, then you may still be able to find the lost +commits in the dangling objects that `git fsck` reports. See +<<dangling-objects>> for the details. + +------------------------------------------------- +$ git fsck +dangling commit 7281251ddd2a61e38657c827739c57015671a6b3 +dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63 +dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5 +... +------------------------------------------------- + +You can examine +one of those dangling commits with, for example, + +------------------------------------------------ +$ gitk 7281251ddd --not --all +------------------------------------------------ + +which does what it sounds like: it says that you want to see the commit +history that is described by the dangling commit(s), but not the +history that is described by all your existing branches and tags. Thus +you get exactly the history reachable from that commit that is lost. +(And notice that it might not be just one commit: we only report the +"tip of the line" as being dangling, but there might be a whole deep +and complex commit history that was dropped.) + +If you decide you want the history back, you can always create a new +reference pointing to it, for example, a new branch: + +------------------------------------------------ +$ git branch recovered-branch 7281251ddd +------------------------------------------------ + +Other types of dangling objects (blobs and trees) are also possible, and +dangling objects can arise in other situations. + + +[[sharing-development]] +== Sharing development with others + +[[getting-updates-With-git-pull]] +=== Getting updates with git pull + +After you clone a repository and commit a few changes of your own, you +may wish to check the original repository for updates and merge them +into your own work. + +We have already seen <<Updating-a-repository-With-git-fetch,how to +keep remote-tracking branches up to date>> with linkgit:git-fetch[1], +and how to merge two branches. So you can merge in changes from the +original repository's master branch with: + +------------------------------------------------- +$ git fetch +$ git merge origin/master +------------------------------------------------- + +However, the linkgit:git-pull[1] command provides a way to do this in +one step: + +------------------------------------------------- +$ git pull origin master +------------------------------------------------- + +In fact, if you have `master` checked out, then this branch has been +configured by `git clone` to get changes from the HEAD branch of the +origin repository. So often you can +accomplish the above with just a simple + +------------------------------------------------- +$ git pull +------------------------------------------------- + +This command will fetch changes from the remote branches to your +remote-tracking branches `origin/*`, and merge the default branch into +the current branch. + +More generally, a branch that is created from a remote-tracking branch +will pull +by default from that branch. See the descriptions of the +`branch.<name>.remote` and `branch.<name>.merge` options in +linkgit:git-config[1], and the discussion of the `--track` option in +linkgit:git-checkout[1], to learn how to control these defaults. + +In addition to saving you keystrokes, `git pull` also helps you by +producing a default commit message documenting the branch and +repository that you pulled from. + +(But note that no such commit will be created in the case of a +<<fast-forwards,fast-forward>>; instead, your branch will just be +updated to point to the latest commit from the upstream branch.) + +The `git pull` command can also be given `.` as the "remote" repository, +in which case it just merges in a branch from the current repository; so +the commands + +------------------------------------------------- +$ git pull . branch +$ git merge branch +------------------------------------------------- + +are roughly equivalent. + +[[submitting-patches]] +=== Submitting patches to a project + +If you just have a few changes, the simplest way to submit them may +just be to send them as patches in email: + +First, use linkgit:git-format-patch[1]; for example: + +------------------------------------------------- +$ git format-patch origin +------------------------------------------------- + +will produce a numbered series of files in the current directory, one +for each patch in the current branch but not in `origin/HEAD`. + +`git format-patch` can include an initial "cover letter". You can insert +commentary on individual patches after the three dash line which +`format-patch` places after the commit message but before the patch +itself. If you use `git notes` to track your cover letter material, +`git format-patch --notes` will include the commit's notes in a similar +manner. + +You can then import these into your mail client and send them by +hand. However, if you have a lot to send at once, you may prefer to +use the linkgit:git-send-email[1] script to automate the process. +Consult the mailing list for your project first to determine +their requirements for submitting patches. + +[[importing-patches]] +=== Importing patches to a project + +Git also provides a tool called linkgit:git-am[1] (am stands for +"apply mailbox"), for importing such an emailed series of patches. +Just save all of the patch-containing messages, in order, into a +single mailbox file, say `patches.mbox`, then run + +------------------------------------------------- +$ git am -3 patches.mbox +------------------------------------------------- + +Git will apply each patch in order; if any conflicts are found, it +will stop, and you can fix the conflicts as described in +"<<resolving-a-merge,Resolving a merge>>". (The `-3` option tells +Git to perform a merge; if you would prefer it just to abort and +leave your tree and index untouched, you may omit that option.) + +Once the index is updated with the results of the conflict +resolution, instead of creating a new commit, just run + +------------------------------------------------- +$ git am --continue +------------------------------------------------- + +and Git will create the commit for you and continue applying the +remaining patches from the mailbox. + +The final result will be a series of commits, one for each patch in +the original mailbox, with authorship and commit log message each +taken from the message containing each patch. + +[[public-repositories]] +=== Public Git repositories + +Another way to submit changes to a project is to tell the maintainer +of that project to pull the changes from your repository using +linkgit:git-pull[1]. In the section "<<getting-updates-With-git-pull, +Getting updates with `git pull`>>" we described this as a way to get +updates from the "main" repository, but it works just as well in the +other direction. + +If you and the maintainer both have accounts on the same machine, then +you can just pull changes from each other's repositories directly; +commands that accept repository URLs as arguments will also accept a +local directory name: + +------------------------------------------------- +$ git clone /path/to/repository +$ git pull /path/to/other/repository +------------------------------------------------- + +or an ssh URL: + +------------------------------------------------- +$ git clone ssh://yourhost/~you/repository +------------------------------------------------- + +For projects with few developers, or for synchronizing a few private +repositories, this may be all you need. + +However, the more common way to do this is to maintain a separate public +repository (usually on a different host) for others to pull changes +from. This is usually more convenient, and allows you to cleanly +separate private work in progress from publicly visible work. + +You will continue to do your day-to-day work in your personal +repository, but periodically "push" changes from your personal +repository into your public repository, allowing other developers to +pull from that repository. So the flow of changes, in a situation +where there is one other developer with a public repository, looks +like this: + +.... + you push +your personal repo ------------------> your public repo + ^ | + | | + | you pull | they pull + | | + | | + | they push V +their public repo <------------------- their repo +.... + +We explain how to do this in the following sections. + +[[setting-up-a-public-repository]] +==== Setting up a public repository + +Assume your personal repository is in the directory `~/proj`. We +first create a new clone of the repository and tell `git daemon` that it +is meant to be public: + +------------------------------------------------- +$ git clone --bare ~/proj proj.git +$ touch proj.git/git-daemon-export-ok +------------------------------------------------- + +The resulting directory proj.git contains a "bare" git repository--it is +just the contents of the `.git` directory, without any files checked out +around it. + +Next, copy `proj.git` to the server where you plan to host the +public repository. You can use scp, rsync, or whatever is most +convenient. + +[[exporting-via-git]] +==== Exporting a Git repository via the Git protocol + +This is the preferred method. + +If someone else administers the server, they should tell you what +directory to put the repository in, and what `git://` URL it will +appear at. You can then skip to the section +"<<pushing-changes-to-a-public-repository,Pushing changes to a public +repository>>", below. + +Otherwise, all you need to do is start linkgit:git-daemon[1]; it will +listen on port 9418. By default, it will allow access to any directory +that looks like a Git directory and contains the magic file +git-daemon-export-ok. Passing some directory paths as `git daemon` +arguments will further restrict the exports to those paths. + +You can also run `git daemon` as an inetd service; see the +linkgit:git-daemon[1] man page for details. (See especially the +examples section.) + +[[exporting-via-http]] +==== Exporting a git repository via HTTP + +The Git protocol gives better performance and reliability, but on a +host with a web server set up, HTTP exports may be simpler to set up. + +All you need to do is place the newly created bare Git repository in +a directory that is exported by the web server, and make some +adjustments to give web clients some extra information they need: + +------------------------------------------------- +$ mv proj.git /home/you/public_html/proj.git +$ cd proj.git +$ git --bare update-server-info +$ mv hooks/post-update.sample hooks/post-update +------------------------------------------------- + +(For an explanation of the last two lines, see +linkgit:git-update-server-info[1] and linkgit:githooks[5].) + +Advertise the URL of `proj.git`. Anybody else should then be able to +clone or pull from that URL, for example with a command line like: + +------------------------------------------------- +$ git clone http://yourserver.com/~you/proj.git +------------------------------------------------- + +(See also +link:howto/setup-git-server-over-http.html[setup-git-server-over-http] +for a slightly more sophisticated setup using WebDAV which also +allows pushing over HTTP.) + +[[pushing-changes-to-a-public-repository]] +==== Pushing changes to a public repository + +Note that the two techniques outlined above (exporting via +<<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other +maintainers to fetch your latest changes, but they do not allow write +access, which you will need to update the public repository with the +latest changes created in your private repository. + +The simplest way to do this is using linkgit:git-push[1] and ssh; to +update the remote branch named `master` with the latest state of your +branch named `master`, run + +------------------------------------------------- +$ git push ssh://yourserver.com/~you/proj.git master:master +------------------------------------------------- + +or just + +------------------------------------------------- +$ git push ssh://yourserver.com/~you/proj.git master +------------------------------------------------- + +As with `git fetch`, `git push` will complain if this does not result in a +<<fast-forwards,fast-forward>>; see the following section for details on +handling this case. + +Note that the target of a `push` is normally a +<<def_bare_repository,bare>> repository. You can also push to a +repository that has a checked-out working tree, but a push to update the +currently checked-out branch is denied by default to prevent confusion. +See the description of the receive.denyCurrentBranch option +in linkgit:git-config[1] for details. + +As with `git fetch`, you may also set up configuration options to +save typing; so, for example: + +------------------------------------------------- +$ git remote add public-repo ssh://yourserver.com/~you/proj.git +------------------------------------------------- + +adds the following to `.git/config`: + +------------------------------------------------- +[remote "public-repo"] + url = yourserver.com:proj.git + fetch = +refs/heads/*:refs/remotes/example/* +------------------------------------------------- + +which lets you do the same push with just + +------------------------------------------------- +$ git push public-repo master +------------------------------------------------- + +See the explanations of the `remote.<name>.url`, +`branch.<name>.remote`, and `remote.<name>.push` options in +linkgit:git-config[1] for details. + +[[forcing-push]] +==== What to do when a push fails + +If a push would not result in a <<fast-forwards,fast-forward>> of the +remote branch, then it will fail with an error like: + +------------------------------------------------- + ! [rejected] master -> master (non-fast-forward) +error: failed to push some refs to '...' +hint: Updates were rejected because the tip of your current branch is behind +hint: its remote counterpart. Integrate the remote changes (e.g. +hint: 'git pull ...') before pushing again. +hint: See the 'Note about fast-forwards' in 'git push --help' for details. +------------------------------------------------- + +This can happen, for example, if you: + + - use `git reset --hard` to remove already-published commits, or + - use `git commit --amend` to replace already-published commits + (as in <<fixing-a-mistake-by-rewriting-history>>), or + - use `git rebase` to rebase any already-published commits (as + in <<using-git-rebase>>). + +You may force `git push` to perform the update anyway by preceding the +branch name with a plus sign: + +------------------------------------------------- +$ git push ssh://yourserver.com/~you/proj.git +master +------------------------------------------------- + +Note the addition of the `+` sign. Alternatively, you can use the +`-f` flag to force the remote update, as in: + +------------------------------------------------- +$ git push -f ssh://yourserver.com/~you/proj.git master +------------------------------------------------- + +Normally whenever a branch head in a public repository is modified, it +is modified to point to a descendant of the commit that it pointed to +before. By forcing a push in this situation, you break that convention. +(See <<problems-With-rewriting-history>>.) + +Nevertheless, this is a common practice for people that need a simple +way to publish a work-in-progress patch series, and it is an acceptable +compromise as long as you warn other developers that this is how you +intend to manage the branch. + +It's also possible for a push to fail in this way when other people have +the right to push to the same repository. In that case, the correct +solution is to retry the push after first updating your work: either by a +pull, or by a fetch followed by a rebase; see the +<<setting-up-a-shared-repository,next section>> and +linkgit:gitcvs-migration[7] for more. + +[[setting-up-a-shared-repository]] +==== Setting up a shared repository + +Another way to collaborate is by using a model similar to that +commonly used in CVS, where several developers with special rights +all push to and pull from a single shared repository. See +linkgit:gitcvs-migration[7] for instructions on how to +set this up. + +However, while there is nothing wrong with Git's support for shared +repositories, this mode of operation is not generally recommended, +simply because the mode of collaboration that Git supports--by +exchanging patches and pulling from public repositories--has so many +advantages over the central shared repository: + + - Git's ability to quickly import and merge patches allows a + single maintainer to process incoming changes even at very + high rates. And when that becomes too much, `git pull` provides + an easy way for that maintainer to delegate this job to other + maintainers while still allowing optional review of incoming + changes. + - Since every developer's repository has the same complete copy + of the project history, no repository is special, and it is + trivial for another developer to take over maintenance of a + project, either by mutual agreement, or because a maintainer + becomes unresponsive or difficult to work with. + - The lack of a central group of "committers" means there is + less need for formal decisions about who is "in" and who is + "out". + +[[setting-up-gitweb]] +==== Allowing web browsing of a repository + +The gitweb cgi script provides users an easy way to browse your +project's revisions, file contents and logs without having to install +Git. Features like RSS/Atom feeds and blame/annotation details may +optionally be enabled. + +The linkgit:git-instaweb[1] command provides a simple way to start +browsing the repository using gitweb. The default server when using +instaweb is lighttpd. + +See the file gitweb/INSTALL in the Git source tree and +linkgit:gitweb[1] for instructions on details setting up a permanent +installation with a CGI or Perl capable server. + +[[how-to-get-a-git-repository-with-minimal-history]] +=== How to get a Git repository with minimal history + +A <<def_shallow_clone,shallow clone>>, with its truncated +history, is useful when one is interested only in recent history +of a project and getting full history from the upstream is +expensive. + +A <<def_shallow_clone,shallow clone>> is created by specifying +the linkgit:git-clone[1] `--depth` switch. The depth can later be +changed with the linkgit:git-fetch[1] `--depth` switch, or full +history restored with `--unshallow`. + +Merging inside a <<def_shallow_clone,shallow clone>> will work as long +as a merge base is in the recent history. +Otherwise, it will be like merging unrelated histories and may +have to result in huge conflicts. This limitation may make such +a repository unsuitable to be used in merge based workflows. + +[[sharing-development-examples]] +=== Examples + +[[maintaining-topic-branches]] +==== Maintaining topic branches for a Linux subsystem maintainer + +This describes how Tony Luck uses Git in his role as maintainer of the +IA64 architecture for the Linux kernel. + +He uses two public branches: + + - A "test" tree into which patches are initially placed so that they + can get some exposure when integrated with other ongoing development. + This tree is available to Andrew for pulling into -mm whenever he + wants. + + - A "release" tree into which tested patches are moved for final sanity + checking, and as a vehicle to send them upstream to Linus (by sending + him a "please pull" request.) + +He also uses a set of temporary branches ("topic branches"), each +containing a logical grouping of patches. + +To set this up, first create your work tree by cloning Linus's public +tree: + +------------------------------------------------- +$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git work +$ cd work +------------------------------------------------- + +Linus's tree will be stored in the remote-tracking branch named origin/master, +and can be updated using linkgit:git-fetch[1]; you can track other +public trees using linkgit:git-remote[1] to set up a "remote" and +linkgit:git-fetch[1] to keep them up to date; see +<<repositories-and-branches>>. + +Now create the branches in which you are going to work; these start out +at the current tip of origin/master branch, and should be set up (using +the `--track` option to linkgit:git-branch[1]) to merge changes in from +Linus by default. + +------------------------------------------------- +$ git branch --track test origin/master +$ git branch --track release origin/master +------------------------------------------------- + +These can be easily kept up to date using linkgit:git-pull[1]. + +------------------------------------------------- +$ git switch test && git pull +$ git switch release && git pull +------------------------------------------------- + +Important note! If you have any local changes in these branches, then +this merge will create a commit object in the history (with no local +changes Git will simply do a "fast-forward" merge). Many people dislike +the "noise" that this creates in the Linux history, so you should avoid +doing this capriciously in the `release` branch, as these noisy commits +will become part of the permanent history when you ask Linus to pull +from the release branch. + +A few configuration variables (see linkgit:git-config[1]) can +make it easy to push both branches to your public tree. (See +<<setting-up-a-public-repository>>.) + +------------------------------------------------- +$ cat >> .git/config <<EOF +[remote "mytree"] + url = master.kernel.org:/pub/scm/linux/kernel/git/aegl/linux.git + push = release + push = test +EOF +------------------------------------------------- + +Then you can push both the test and release trees using +linkgit:git-push[1]: + +------------------------------------------------- +$ git push mytree +------------------------------------------------- + +or push just one of the test and release branches using: + +------------------------------------------------- +$ git push mytree test +------------------------------------------------- + +or + +------------------------------------------------- +$ git push mytree release +------------------------------------------------- + +Now to apply some patches from the community. Think of a short +snappy name for a branch to hold this patch (or related group of +patches), and create a new branch from a recent stable tag of +Linus's branch. Picking a stable base for your branch will: +1) help you: by avoiding inclusion of unrelated and perhaps lightly +tested changes +2) help future bug hunters that use `git bisect` to find problems + +------------------------------------------------- +$ git switch -c speed-up-spinlocks v2.6.35 +------------------------------------------------- + +Now you apply the patch(es), run some tests, and commit the change(s). If +the patch is a multi-part series, then you should apply each as a separate +commit to this branch. + +------------------------------------------------- +$ ... patch ... test ... commit [ ... patch ... test ... commit ]* +------------------------------------------------- + +When you are happy with the state of this change, you can merge it into the +"test" branch in preparation to make it public: + +------------------------------------------------- +$ git switch test && git merge speed-up-spinlocks +------------------------------------------------- + +It is unlikely that you would have any conflicts here ... but you might if you +spent a while on this step and had also pulled new versions from upstream. + +Sometime later when enough time has passed and testing done, you can pull the +same branch into the `release` tree ready to go upstream. This is where you +see the value of keeping each patch (or patch series) in its own branch. It +means that the patches can be moved into the `release` tree in any order. + +------------------------------------------------- +$ git switch release && git merge speed-up-spinlocks +------------------------------------------------- + +After a while, you will have a number of branches, and despite the +well chosen names you picked for each of them, you may forget what +they are for, or what status they are in. To get a reminder of what +changes are in a specific branch, use: + +------------------------------------------------- +$ git log linux..branchname | git shortlog +------------------------------------------------- + +To see whether it has already been merged into the test or release branches, +use: + +------------------------------------------------- +$ git log test..branchname +------------------------------------------------- + +or + +------------------------------------------------- +$ git log release..branchname +------------------------------------------------- + +(If this branch has not yet been merged, you will see some log entries. +If it has been merged, then there will be no output.) + +Once a patch completes the great cycle (moving from test to release, +then pulled by Linus, and finally coming back into your local +`origin/master` branch), the branch for this change is no longer needed. +You detect this when the output from: + +------------------------------------------------- +$ git log origin..branchname +------------------------------------------------- + +is empty. At this point the branch can be deleted: + +------------------------------------------------- +$ git branch -d branchname +------------------------------------------------- + +Some changes are so trivial that it is not necessary to create a separate +branch and then merge into each of the test and release branches. For +these changes, just apply directly to the `release` branch, and then +merge that into the `test` branch. + +After pushing your work to `mytree`, you can use +linkgit:git-request-pull[1] to prepare a "please pull" request message +to send to Linus: + +------------------------------------------------- +$ git push mytree +$ git request-pull origin mytree release +------------------------------------------------- + +Here are some of the scripts that simplify all this even further. + +------------------------------------------------- +==== update script ==== +# Update a branch in my Git tree. If the branch to be updated +# is origin, then pull from kernel.org. Otherwise merge +# origin/master branch into test|release branch + +case "$1" in +test|release) + git checkout $1 && git pull . origin + ;; +origin) + before=$(git rev-parse refs/remotes/origin/master) + git fetch origin + after=$(git rev-parse refs/remotes/origin/master) + if [ $before != $after ] + then + git log $before..$after | git shortlog + fi + ;; +*) + echo "usage: $0 origin|test|release" 1>&2 + exit 1 + ;; +esac +------------------------------------------------- + +------------------------------------------------- +==== merge script ==== +# Merge a branch into either the test or release branch + +pname=$0 + +usage() +{ + echo "usage: $pname branch test|release" 1>&2 + exit 1 +} + +git show-ref -q --verify -- refs/heads/"$1" || { + echo "Can't see branch <$1>" 1>&2 + usage +} + +case "$2" in +test|release) + if [ $(git log $2..$1 | wc -c) -eq 0 ] + then + echo $1 already merged into $2 1>&2 + exit 1 + fi + git checkout $2 && git pull . $1 + ;; +*) + usage + ;; +esac +------------------------------------------------- + +------------------------------------------------- +==== status script ==== +# report on status of my ia64 Git tree + +gb=$(tput setab 2) +rb=$(tput setab 1) +restore=$(tput setab 9) + +if [ `git rev-list test..release | wc -c` -gt 0 ] +then + echo $rb Warning: commits in release that are not in test $restore + git log test..release +fi + +for branch in `git show-ref --heads | sed 's|^.*/||'` +do + if [ $branch = test -o $branch = release ] + then + continue + fi + + echo -n $gb ======= $branch ====== $restore " " + status= + for ref in test release origin/master + do + if [ `git rev-list $ref..$branch | wc -c` -gt 0 ] + then + status=$status${ref:0:1} + fi + done + case $status in + trl) + echo $rb Need to pull into test $restore + ;; + rl) + echo "In test" + ;; + l) + echo "Waiting for linus" + ;; + "") + echo $rb All done $restore + ;; + *) + echo $rb "<$status>" $restore + ;; + esac + git log origin/master..$branch | git shortlog +done +------------------------------------------------- + + +[[cleaning-up-history]] +== Rewriting history and maintaining patch series + +Normally commits are only added to a project, never taken away or +replaced. Git is designed with this assumption, and violating it will +cause Git's merge machinery (for example) to do the wrong thing. + +However, there is a situation in which it can be useful to violate this +assumption. + +[[patch-series]] +=== Creating the perfect patch series + +Suppose you are a contributor to a large project, and you want to add a +complicated feature, and to present it to the other developers in a way +that makes it easy for them to read your changes, verify that they are +correct, and understand why you made each change. + +If you present all of your changes as a single patch (or commit), they +may find that it is too much to digest all at once. + +If you present them with the entire history of your work, complete with +mistakes, corrections, and dead ends, they may be overwhelmed. + +So the ideal is usually to produce a series of patches such that: + + 1. Each patch can be applied in order. + + 2. Each patch includes a single logical change, together with a + message explaining the change. + + 3. No patch introduces a regression: after applying any initial + part of the series, the resulting project still compiles and + works, and has no bugs that it didn't have before. + + 4. The complete series produces the same end result as your own + (probably much messier!) development process did. + +We will introduce some tools that can help you do this, explain how to +use them, and then explain some of the problems that can arise because +you are rewriting history. + +[[using-git-rebase]] +=== Keeping a patch series up to date using git rebase + +Suppose that you create a branch `mywork` on a remote-tracking branch +`origin`, and create some commits on top of it: + +------------------------------------------------- +$ git switch -c mywork origin +$ vi file.txt +$ git commit +$ vi otherfile.txt +$ git commit +... +------------------------------------------------- + +You have performed no merges into mywork, so it is just a simple linear +sequence of patches on top of `origin`: + +................................................ + o--o--O <-- origin + \ + a--b--c <-- mywork +................................................ + +Some more interesting work has been done in the upstream project, and +`origin` has advanced: + +................................................ + o--o--O--o--o--o <-- origin + \ + a--b--c <-- mywork +................................................ + +At this point, you could use `pull` to merge your changes back in; +the result would create a new merge commit, like this: + +................................................ + o--o--O--o--o--o <-- origin + \ \ + a--b--c--m <-- mywork +................................................ + +However, if you prefer to keep the history in mywork a simple series of +commits without any merges, you may instead choose to use +linkgit:git-rebase[1]: + +------------------------------------------------- +$ git switch mywork +$ git rebase origin +------------------------------------------------- + +This will remove each of your commits from mywork, temporarily saving +them as patches (in a directory named `.git/rebase-apply`), update mywork to +point at the latest version of origin, then apply each of the saved +patches to the new mywork. The result will look like: + + +................................................ + o--o--O--o--o--o <-- origin + \ + a'--b'--c' <-- mywork +................................................ + +In the process, it may discover conflicts. In that case it will stop +and allow you to fix the conflicts; after fixing conflicts, use `git add` +to update the index with those contents, and then, instead of +running `git commit`, just run + +------------------------------------------------- +$ git rebase --continue +------------------------------------------------- + +and Git will continue applying the rest of the patches. + +At any point you may use the `--abort` option to abort this process and +return mywork to the state it had before you started the rebase: + +------------------------------------------------- +$ git rebase --abort +------------------------------------------------- + +If you need to reorder or edit a number of commits in a branch, it may +be easier to use `git rebase -i`, which allows you to reorder and +squash commits, as well as marking them for individual editing during +the rebase. See <<interactive-rebase>> for details, and +<<reordering-patch-series>> for alternatives. + +[[rewriting-one-commit]] +=== Rewriting a single commit + +We saw in <<fixing-a-mistake-by-rewriting-history>> that you can replace the +most recent commit using + +------------------------------------------------- +$ git commit --amend +------------------------------------------------- + +which will replace the old commit by a new commit incorporating your +changes, giving you a chance to edit the old commit message first. +This is useful for fixing typos in your last commit, or for adjusting +the patch contents of a poorly staged commit. + +If you need to amend commits from deeper in your history, you can +use <<interactive-rebase,interactive rebase's `edit` instruction>>. + +[[reordering-patch-series]] +=== Reordering or selecting from a patch series + +Sometimes you want to edit a commit deeper in your history. One +approach is to use `git format-patch` to create a series of patches +and then reset the state to before the patches: + +------------------------------------------------- +$ git format-patch origin +$ git reset --hard origin +------------------------------------------------- + +Then modify, reorder, or eliminate patches as needed before applying +them again with linkgit:git-am[1]: + +------------------------------------------------- +$ git am *.patch +------------------------------------------------- + +[[interactive-rebase]] +=== Using interactive rebases + +You can also edit a patch series with an interactive rebase. This is +the same as <<reordering-patch-series,reordering a patch series using +`format-patch`>>, so use whichever interface you like best. + +Rebase your current HEAD on the last commit you want to retain as-is. +For example, if you want to reorder the last 5 commits, use: + +------------------------------------------------- +$ git rebase -i HEAD~5 +------------------------------------------------- + +This will open your editor with a list of steps to be taken to perform +your rebase. + +------------------------------------------------- +pick deadbee The oneline of this commit +pick fa1afe1 The oneline of the next commit +... + +# Rebase c0ffeee..deadbee onto c0ffeee +# +# Commands: +# p, pick = use commit +# r, reword = use commit, but edit the commit message +# e, edit = use commit, but stop for amending +# s, squash = use commit, but meld into previous commit +# f, fixup = like "squash", but discard this commit's log message +# x, exec = run command (the rest of the line) using shell +# +# These lines can be re-ordered; they are executed from top to bottom. +# +# If you remove a line here THAT COMMIT WILL BE LOST. +# +# However, if you remove everything, the rebase will be aborted. +# +# Note that empty commits are commented out +------------------------------------------------- + +As explained in the comments, you can reorder commits, squash them +together, edit commit messages, etc. by editing the list. Once you +are satisfied, save the list and close your editor, and the rebase +will begin. + +The rebase will stop where `pick` has been replaced with `edit` or +when a step in the list fails to mechanically resolve conflicts and +needs your help. When you are done editing and/or resolving conflicts +you can continue with `git rebase --continue`. If you decide that +things are getting too hairy, you can always bail out with `git rebase +--abort`. Even after the rebase is complete, you can still recover +the original branch by using the <<reflogs,reflog>>. + +For a more detailed discussion of the procedure and additional tips, +see the "INTERACTIVE MODE" section of linkgit:git-rebase[1]. + +[[patch-series-tools]] +=== Other tools + +There are numerous other tools, such as StGit, which exist for the +purpose of maintaining a patch series. These are outside of the scope of +this manual. + +[[problems-With-rewriting-history]] +=== Problems with rewriting history + +The primary problem with rewriting the history of a branch has to do +with merging. Suppose somebody fetches your branch and merges it into +their branch, with a result something like this: + +................................................ + o--o--O--o--o--o <-- origin + \ \ + t--t--t--m <-- their branch: +................................................ + +Then suppose you modify the last three commits: + +................................................ + o--o--o <-- new head of origin + / + o--o--O--o--o--o <-- old head of origin +................................................ + +If we examined all this history together in one repository, it will +look like: + +................................................ + o--o--o <-- new head of origin + / + o--o--O--o--o--o <-- old head of origin + \ \ + t--t--t--m <-- their branch: +................................................ + +Git has no way of knowing that the new head is an updated version of +the old head; it treats this situation exactly the same as it would if +two developers had independently done the work on the old and new heads +in parallel. At this point, if someone attempts to merge the new head +in to their branch, Git will attempt to merge together the two (old and +new) lines of development, instead of trying to replace the old by the +new. The results are likely to be unexpected. + +You may still choose to publish branches whose history is rewritten, +and it may be useful for others to be able to fetch those branches in +order to examine or test them, but they should not attempt to pull such +branches into their own work. + +For true distributed development that supports proper merging, +published branches should never be rewritten. + +[[bisect-merges]] +=== Why bisecting merge commits can be harder than bisecting linear history + +The linkgit:git-bisect[1] command correctly handles history that +includes merge commits. However, when the commit that it finds is a +merge commit, the user may need to work harder than usual to figure out +why that commit introduced a problem. + +Imagine this history: + +................................................ + ---Z---o---X---...---o---A---C---D + \ / + o---o---Y---...---o---B +................................................ + +Suppose that on the upper line of development, the meaning of one +of the functions that exists at Z is changed at commit X. The +commits from Z leading to A change both the function's +implementation and all calling sites that exist at Z, as well +as new calling sites they add, to be consistent. There is no +bug at A. + +Suppose that in the meantime on the lower line of development somebody +adds a new calling site for that function at commit Y. The +commits from Z leading to B all assume the old semantics of that +function and the callers and the callee are consistent with each +other. There is no bug at B, either. + +Suppose further that the two development lines merge cleanly at C, +so no conflict resolution is required. + +Nevertheless, the code at C is broken, because the callers added +on the lower line of development have not been converted to the new +semantics introduced on the upper line of development. So if all +you know is that D is bad, that Z is good, and that +linkgit:git-bisect[1] identifies C as the culprit, how will you +figure out that the problem is due to this change in semantics? + +When the result of a `git bisect` is a non-merge commit, you should +normally be able to discover the problem by examining just that commit. +Developers can make this easy by breaking their changes into small +self-contained commits. That won't help in the case above, however, +because the problem isn't obvious from examination of any single +commit; instead, a global view of the development is required. To +make matters worse, the change in semantics in the problematic +function may be just one small part of the changes in the upper +line of development. + +On the other hand, if instead of merging at C you had rebased the +history between Z to B on top of A, you would have gotten this +linear history: + +................................................................ + ---Z---o---X--...---o---A---o---o---Y*--...---o---B*--D* +................................................................ + +Bisecting between Z and D* would hit a single culprit commit Y*, +and understanding why Y* was broken would probably be easier. + +Partly for this reason, many experienced Git users, even when +working on an otherwise merge-heavy project, keep the history +linear by rebasing against the latest upstream version before +publishing. + +[[advanced-branch-management]] +== Advanced branch management + +[[fetching-individual-branches]] +=== Fetching individual branches + +Instead of using linkgit:git-remote[1], you can also choose just +to update one branch at a time, and to store it locally under an +arbitrary name: + +------------------------------------------------- +$ git fetch origin todo:my-todo-work +------------------------------------------------- + +The first argument, `origin`, just tells Git to fetch from the +repository you originally cloned from. The second argument tells Git +to fetch the branch named `todo` from the remote repository, and to +store it locally under the name `refs/heads/my-todo-work`. + +You can also fetch branches from other repositories; so + +------------------------------------------------- +$ git fetch git://example.com/proj.git master:example-master +------------------------------------------------- + +will create a new branch named `example-master` and store in it the +branch named `master` from the repository at the given URL. If you +already have a branch named example-master, it will attempt to +<<fast-forwards,fast-forward>> to the commit given by example.com's +master branch. In more detail: + +[[fetch-fast-forwards]] +=== git fetch and fast-forwards + +In the previous example, when updating an existing branch, `git fetch` +checks to make sure that the most recent commit on the remote +branch is a descendant of the most recent commit on your copy of the +branch before updating your copy of the branch to point at the new +commit. Git calls this process a <<fast-forwards,fast-forward>>. + +A fast-forward looks something like this: + +................................................ + o--o--o--o <-- old head of the branch + \ + o--o--o <-- new head of the branch +................................................ + + +In some cases it is possible that the new head will *not* actually be +a descendant of the old head. For example, the developer may have +realized a serious mistake was made and decided to backtrack, +resulting in a situation like: + +................................................ + o--o--o--o--a--b <-- old head of the branch + \ + o--o--o <-- new head of the branch +................................................ + +In this case, `git fetch` will fail, and print out a warning. + +In that case, you can still force Git to update to the new head, as +described in the following section. However, note that in the +situation above this may mean losing the commits labeled `a` and `b`, +unless you've already created a reference of your own pointing to +them. + +[[forcing-fetch]] +=== Forcing git fetch to do non-fast-forward updates + +If git fetch fails because the new head of a branch is not a +descendant of the old head, you may force the update with: + +------------------------------------------------- +$ git fetch git://example.com/proj.git +master:refs/remotes/example/master +------------------------------------------------- + +Note the addition of the `+` sign. Alternatively, you can use the `-f` +flag to force updates of all the fetched branches, as in: + +------------------------------------------------- +$ git fetch -f origin +------------------------------------------------- + +Be aware that commits that the old version of example/master pointed at +may be lost, as we saw in the previous section. + +[[remote-branch-configuration]] +=== Configuring remote-tracking branches + +We saw above that `origin` is just a shortcut to refer to the +repository that you originally cloned from. This information is +stored in Git configuration variables, which you can see using +linkgit:git-config[1]: + +------------------------------------------------- +$ git config -l +core.repositoryformatversion=0 +core.filemode=true +core.logallrefupdates=true +remote.origin.url=git://git.kernel.org/pub/scm/git/git.git +remote.origin.fetch=+refs/heads/*:refs/remotes/origin/* +branch.master.remote=origin +branch.master.merge=refs/heads/master +------------------------------------------------- + +If there are other repositories that you also use frequently, you can +create similar configuration options to save typing; for example, + +------------------------------------------------- +$ git remote add example git://example.com/proj.git +------------------------------------------------- + +adds the following to `.git/config`: + +------------------------------------------------- +[remote "example"] + url = git://example.com/proj.git + fetch = +refs/heads/*:refs/remotes/example/* +------------------------------------------------- + +Also note that the above configuration can be performed by directly +editing the file `.git/config` instead of using linkgit:git-remote[1]. + +After configuring the remote, the following three commands will do the +same thing: + +------------------------------------------------- +$ git fetch git://example.com/proj.git +refs/heads/*:refs/remotes/example/* +$ git fetch example +refs/heads/*:refs/remotes/example/* +$ git fetch example +------------------------------------------------- + +See linkgit:git-config[1] for more details on the configuration +options mentioned above and linkgit:git-fetch[1] for more details on +the refspec syntax. + + +[[git-concepts]] +== Git concepts + +Git is built on a small number of simple but powerful ideas. While it +is possible to get things done without understanding them, you will find +Git much more intuitive if you do. + +We start with the most important, the <<def_object_database,object +database>> and the <<def_index,index>>. + +[[the-object-database]] +=== The Object Database + + +We already saw in <<understanding-commits>> that all commits are stored +under a 40-digit "object name". In fact, all the information needed to +represent the history of a project is stored in objects with such names. +In each case the name is calculated by taking the SHA-1 hash of the +contents of the object. The SHA-1 hash is a cryptographic hash function. +What that means to us is that it is impossible to find two different +objects with the same name. This has a number of advantages; among +others: + +- Git can quickly determine whether two objects are identical or not, + just by comparing names. +- Since object names are computed the same way in every repository, the + same content stored in two repositories will always be stored under + the same name. +- Git can detect errors when it reads an object, by checking that the + object's name is still the SHA-1 hash of its contents. + +(See <<object-details>> for the details of the object formatting and +SHA-1 calculation.) + +There are four different types of objects: "blob", "tree", "commit", and +"tag". + +- A <<def_blob_object,"blob" object>> is used to store file data. +- A <<def_tree_object,"tree" object>> ties one or more + "blob" objects into a directory structure. In addition, a tree object + can refer to other tree objects, thus creating a directory hierarchy. +- A <<def_commit_object,"commit" object>> ties such directory hierarchies + together into a <<def_DAG,directed acyclic graph>> of revisions--each + commit contains the object name of exactly one tree designating the + directory hierarchy at the time of the commit. In addition, a commit + refers to "parent" commit objects that describe the history of how we + arrived at that directory hierarchy. +- A <<def_tag_object,"tag" object>> symbolically identifies and can be + used to sign other objects. It contains the object name and type of + another object, a symbolic name (of course!) and, optionally, a + signature. + +The object types in some more detail: + +[[commit-object]] +==== Commit Object + +The "commit" object links a physical state of a tree with a description +of how we got there and why. Use the `--pretty=raw` option to +linkgit:git-show[1] or linkgit:git-log[1] to examine your favorite +commit: + +------------------------------------------------ +$ git show -s --pretty=raw 2be7fcb476 +commit 2be7fcb4764f2dbcee52635b91fedb1b3dcf7ab4 +tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bf +parent 257a84d9d02e90447b149af58b271c19405edb6a +author Dave Watson <dwatson@mimvista.com> 1187576872 -0400 +committer Junio C Hamano <gitster@pobox.com> 1187591163 -0700 + + Fix misspelling of 'suppress' in docs + + Signed-off-by: Junio C Hamano <gitster@pobox.com> +------------------------------------------------ + +As you can see, a commit is defined by: + +- a tree: The SHA-1 name of a tree object (as defined below), representing + the contents of a directory at a certain point in time. +- parent(s): The SHA-1 name(s) of some number of commits which represent the + immediately previous step(s) in the history of the project. The + example above has one parent; merge commits may have more than + one. A commit with no parents is called a "root" commit, and + represents the initial revision of a project. Each project must have + at least one root. A project can also have multiple roots, though + that isn't common (or necessarily a good idea). +- an author: The name of the person responsible for this change, together + with its date. +- a committer: The name of the person who actually created the commit, + with the date it was done. This may be different from the author, for + example, if the author was someone who wrote a patch and emailed it + to the person who used it to create the commit. +- a comment describing this commit. + +Note that a commit does not itself contain any information about what +actually changed; all changes are calculated by comparing the contents +of the tree referred to by this commit with the trees associated with +its parents. In particular, Git does not attempt to record file renames +explicitly, though it can identify cases where the existence of the same +file data at changing paths suggests a rename. (See, for example, the +`-M` option to linkgit:git-diff[1]). + +A commit is usually created by linkgit:git-commit[1], which creates a +commit whose parent is normally the current HEAD, and whose tree is +taken from the content currently stored in the index. + +[[tree-object]] +==== Tree Object + +The ever-versatile linkgit:git-show[1] command can also be used to +examine tree objects, but linkgit:git-ls-tree[1] will give you more +details: + +------------------------------------------------ +$ git ls-tree fb3a8bdd0ce +100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore +100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap +100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING +040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation +100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN +100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL +100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile +100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README +... +------------------------------------------------ + +As you can see, a tree object contains a list of entries, each with a +mode, object type, SHA-1 name, and name, sorted by name. It represents +the contents of a single directory tree. + +The object type may be a blob, representing the contents of a file, or +another tree, representing the contents of a subdirectory. Since trees +and blobs, like all other objects, are named by the SHA-1 hash of their +contents, two trees have the same SHA-1 name if and only if their +contents (including, recursively, the contents of all subdirectories) +are identical. This allows Git to quickly determine the differences +between two related tree objects, since it can ignore any entries with +identical object names. + +(Note: in the presence of submodules, trees may also have commits as +entries. See <<submodules>> for documentation.) + +Note that the files all have mode 644 or 755: Git actually only pays +attention to the executable bit. + +[[blob-object]] +==== Blob Object + +You can use linkgit:git-show[1] to examine the contents of a blob; take, +for example, the blob in the entry for `COPYING` from the tree above: + +------------------------------------------------ +$ git show 6ff87c4664 + + Note that the only valid version of the GPL as far as this project + is concerned is _this_ particular version of the license (ie v2, not + v2.2 or v3.x or whatever), unless explicitly otherwise stated. +... +------------------------------------------------ + +A "blob" object is nothing but a binary blob of data. It doesn't refer +to anything else or have attributes of any kind. + +Since the blob is entirely defined by its data, if two files in a +directory tree (or in multiple different versions of the repository) +have the same contents, they will share the same blob object. The object +is totally independent of its location in the directory tree, and +renaming a file does not change the object that file is associated with. + +Note that any tree or blob object can be examined using +linkgit:git-show[1] with the <revision>:<path> syntax. This can +sometimes be useful for browsing the contents of a tree that is not +currently checked out. + +[[trust]] +==== Trust + +If you receive the SHA-1 name of a blob from one source, and its contents +from another (possibly untrusted) source, you can still trust that those +contents are correct as long as the SHA-1 name agrees. This is because +the SHA-1 is designed so that it is infeasible to find different contents +that produce the same hash. + +Similarly, you need only trust the SHA-1 name of a top-level tree object +to trust the contents of the entire directory that it refers to, and if +you receive the SHA-1 name of a commit from a trusted source, then you +can easily verify the entire history of commits reachable through +parents of that commit, and all of those contents of the trees referred +to by those commits. + +So to introduce some real trust in the system, the only thing you need +to do is to digitally sign just 'one' special note, which includes the +name of a top-level commit. Your digital signature shows others +that you trust that commit, and the immutability of the history of +commits tells others that they can trust the whole history. + +In other words, you can easily validate a whole archive by just +sending out a single email that tells the people the name (SHA-1 hash) +of the top commit, and digitally sign that email using something +like GPG/PGP. + +To assist in this, Git also provides the tag object... + +[[tag-object]] +==== Tag Object + +A tag object contains an object, object type, tag name, the name of the +person ("tagger") who created the tag, and a message, which may contain +a signature, as can be seen using linkgit:git-cat-file[1]: + +------------------------------------------------ +$ git cat-file tag v1.5.0 +object 437b1b20df4b356c9342dac8d38849f24ef44f27 +type commit +tag v1.5.0 +tagger Junio C Hamano <junkio@cox.net> 1171411200 +0000 + +GIT 1.5.0 +-----BEGIN PGP SIGNATURE----- +Version: GnuPG v1.4.6 (GNU/Linux) + +iD8DBQBF0lGqwMbZpPMRm5oRAuRiAJ9ohBLd7s2kqjkKlq1qqC57SbnmzQCdG4ui +nLE/L9aUXdWeTFPron96DLA= +=2E+0 +-----END PGP SIGNATURE----- +------------------------------------------------ + +See the linkgit:git-tag[1] command to learn how to create and verify tag +objects. (Note that linkgit:git-tag[1] can also be used to create +"lightweight tags", which are not tag objects at all, but just simple +references whose names begin with `refs/tags/`). + +[[pack-files]] +==== How Git stores objects efficiently: pack files + +Newly created objects are initially created in a file named after the +object's SHA-1 hash (stored in `.git/objects`). + +Unfortunately this system becomes inefficient once a project has a +lot of objects. Try this on an old project: + +------------------------------------------------ +$ git count-objects +6930 objects, 47620 kilobytes +------------------------------------------------ + +The first number is the number of objects which are kept in +individual files. The second is the amount of space taken up by +those "loose" objects. + +You can save space and make Git faster by moving these loose objects in +to a "pack file", which stores a group of objects in an efficient +compressed format; the details of how pack files are formatted can be +found in linkgit:gitformat-pack[5]. + +To put the loose objects into a pack, just run git repack: + +------------------------------------------------ +$ git repack +Counting objects: 6020, done. +Delta compression using up to 4 threads. +Compressing objects: 100% (6020/6020), done. +Writing objects: 100% (6020/6020), done. +Total 6020 (delta 4070), reused 0 (delta 0) +------------------------------------------------ + +This creates a single "pack file" in .git/objects/pack/ +containing all currently unpacked objects. You can then run + +------------------------------------------------ +$ git prune +------------------------------------------------ + +to remove any of the "loose" objects that are now contained in the +pack. This will also remove any unreferenced objects (which may be +created when, for example, you use `git reset` to remove a commit). +You can verify that the loose objects are gone by looking at the +`.git/objects` directory or by running + +------------------------------------------------ +$ git count-objects +0 objects, 0 kilobytes +------------------------------------------------ + +Although the object files are gone, any commands that refer to those +objects will work exactly as they did before. + +The linkgit:git-gc[1] command performs packing, pruning, and more for +you, so is normally the only high-level command you need. + +[[dangling-objects]] +==== Dangling objects + +The linkgit:git-fsck[1] command will sometimes complain about dangling +objects. They are not a problem. + +The most common cause of dangling objects is that you've rebased a +branch, or you have pulled from somebody else who rebased a branch--see +<<cleaning-up-history>>. In that case, the old head of the original +branch still exists, as does everything it pointed to. The branch +pointer itself just doesn't, since you replaced it with another one. + +There are also other situations that cause dangling objects. For +example, a "dangling blob" may arise because you did a `git add` of a +file, but then, before you actually committed it and made it part of the +bigger picture, you changed something else in that file and committed +that *updated* thing--the old state that you added originally ends up +not being pointed to by any commit or tree, so it's now a dangling blob +object. + +Similarly, when the "ort" merge strategy runs, and finds that +there are criss-cross merges and thus more than one merge base (which is +fairly unusual, but it does happen), it will generate one temporary +midway tree (or possibly even more, if you had lots of criss-crossing +merges and more than two merge bases) as a temporary internal merge +base, and again, those are real objects, but the end result will not end +up pointing to them, so they end up "dangling" in your repository. + +Generally, dangling objects aren't anything to worry about. They can +even be very useful: if you screw something up, the dangling objects can +be how you recover your old tree (say, you did a rebase, and realized +that you really didn't want to--you can look at what dangling objects +you have, and decide to reset your head to some old dangling state). + +For commits, you can just use: + +------------------------------------------------ +$ gitk <dangling-commit-sha-goes-here> --not --all +------------------------------------------------ + +This asks for all the history reachable from the given commit but not +from any branch, tag, or other reference. If you decide it's something +you want, you can always create a new reference to it, e.g., + +------------------------------------------------ +$ git branch recovered-branch <dangling-commit-sha-goes-here> +------------------------------------------------ + +For blobs and trees, you can't do the same, but you can still examine +them. You can just do + +------------------------------------------------ +$ git show <dangling-blob/tree-sha-goes-here> +------------------------------------------------ + +to show what the contents of the blob were (or, for a tree, basically +what the `ls` for that directory was), and that may give you some idea +of what the operation was that left that dangling object. + +Usually, dangling blobs and trees aren't very interesting. They're +almost always the result of either being a half-way mergebase (the blob +will often even have the conflict markers from a merge in it, if you +have had conflicting merges that you fixed up by hand), or simply +because you interrupted a `git fetch` with ^C or something like that, +leaving _some_ of the new objects in the object database, but just +dangling and useless. + +Anyway, once you are sure that you're not interested in any dangling +state, you can just prune all unreachable objects: + +------------------------------------------------ +$ git prune +------------------------------------------------ + +and they'll be gone. (You should only run `git prune` on a quiescent +repository--it's kind of like doing a filesystem fsck recovery: you +don't want to do that while the filesystem is mounted. +`git prune` is designed not to cause any harm in such cases of concurrent +accesses to a repository but you might receive confusing or scary messages.) + +[[recovering-from-repository-corruption]] +==== Recovering from repository corruption + +By design, Git treats data trusted to it with caution. However, even in +the absence of bugs in Git itself, it is still possible that hardware or +operating system errors could corrupt data. + +The first defense against such problems is backups. You can back up a +Git directory using clone, or just using cp, tar, or any other backup +mechanism. + +As a last resort, you can search for the corrupted objects and attempt +to replace them by hand. Back up your repository before attempting this +in case you corrupt things even more in the process. + +We'll assume that the problem is a single missing or corrupted blob, +which is sometimes a solvable problem. (Recovering missing trees and +especially commits is *much* harder). + +Before starting, verify that there is corruption, and figure out where +it is with linkgit:git-fsck[1]; this may be time-consuming. + +Assume the output looks like this: + +------------------------------------------------ +$ git fsck --full --no-dangling +broken link from tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8 + to blob 4b9458b3786228369c63936db65827de3cc06200 +missing blob 4b9458b3786228369c63936db65827de3cc06200 +------------------------------------------------ + +Now you know that blob 4b9458b3 is missing, and that the tree 2d9263c6 +points to it. If you could find just one copy of that missing blob +object, possibly in some other repository, you could move it into +`.git/objects/4b/9458b3...` and be done. Suppose you can't. You can +still examine the tree that pointed to it with linkgit:git-ls-tree[1], +which might output something like: + +------------------------------------------------ +$ git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8 +100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 .gitignore +100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883 .mailmap +100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING +... +100644 blob 4b9458b3786228369c63936db65827de3cc06200 myfile +... +------------------------------------------------ + +So now you know that the missing blob was the data for a file named +`myfile`. And chances are you can also identify the directory--let's +say it's in `somedirectory`. If you're lucky the missing copy might be +the same as the copy you have checked out in your working tree at +`somedirectory/myfile`; you can test whether that's right with +linkgit:git-hash-object[1]: + +------------------------------------------------ +$ git hash-object -w somedirectory/myfile +------------------------------------------------ + +which will create and store a blob object with the contents of +somedirectory/myfile, and output the SHA-1 of that object. if you're +extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in +which case you've guessed right, and the corruption is fixed! + +Otherwise, you need more information. How do you tell which version of +the file has been lost? + +The easiest way to do this is with: + +------------------------------------------------ +$ git log --raw --all --full-history -- somedirectory/myfile +------------------------------------------------ + +Because you're asking for raw output, you'll now get something like + +------------------------------------------------ +commit abc +Author: +Date: +... +:100644 100644 4b9458b newsha M somedirectory/myfile + + +commit xyz +Author: +Date: + +... +:100644 100644 oldsha 4b9458b M somedirectory/myfile +------------------------------------------------ + +This tells you that the immediately following version of the file was +"newsha", and that the immediately preceding version was "oldsha". +You also know the commit messages that went with the change from oldsha +to 4b9458b and with the change from 4b9458b to newsha. + +If you've been committing small enough changes, you may now have a good +shot at reconstructing the contents of the in-between state 4b9458b. + +If you can do that, you can now recreate the missing object with + +------------------------------------------------ +$ git hash-object -w <recreated-file> +------------------------------------------------ + +and your repository is good again! + +(Btw, you could have ignored the `fsck`, and started with doing a + +------------------------------------------------ +$ git log --raw --all +------------------------------------------------ + +and just looked for the sha of the missing object (4b9458b) in that +whole thing. It's up to you--Git does *have* a lot of information, it is +just missing one particular blob version. + +[[the-index]] +=== The index + +The index is a binary file (generally kept in `.git/index`) containing a +sorted list of path names, each with permissions and the SHA-1 of a blob +object; linkgit:git-ls-files[1] can show you the contents of the index: + +------------------------------------------------- +$ git ls-files --stage +100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0 .gitignore +100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0 .mailmap +100644 6ff87c4664981e4397625791c8ea3bbb5f2279a3 0 COPYING +100644 a37b2152bd26be2c2289e1f57a292534a51a93c7 0 Documentation/.gitignore +100644 fbefe9a45b00a54b58d94d06eca48b03d40a50e0 0 Documentation/Makefile +... +100644 2511aef8d89ab52be5ec6a5e46236b4b6bcd07ea 0 xdiff/xtypes.h +100644 2ade97b2574a9f77e7ae4002a4e07a6a38e46d07 0 xdiff/xutils.c +100644 d5de8292e05e7c36c4b68857c1cf9855e3d2f70a 0 xdiff/xutils.h +------------------------------------------------- + +Note that in older documentation you may see the index called the +"current directory cache" or just the "cache". It has three important +properties: + +1. The index contains all the information necessary to generate a single +(uniquely determined) tree object. ++ +For example, running linkgit:git-commit[1] generates this tree object +from the index, stores it in the object database, and uses it as the +tree object associated with the new commit. + +2. The index enables fast comparisons between the tree object it defines +and the working tree. ++ +It does this by storing some additional data for each entry (such as +the last modified time). This data is not displayed above, and is not +stored in the created tree object, but it can be used to determine +quickly which files in the working directory differ from what was +stored in the index, and thus save Git from having to read all of the +data from such files to look for changes. + +3. It can efficiently represent information about merge conflicts +between different tree objects, allowing each pathname to be +associated with sufficient information about the trees involved that +you can create a three-way merge between them. ++ +We saw in <<conflict-resolution>> that during a merge the index can +store multiple versions of a single file (called "stages"). The third +column in the linkgit:git-ls-files[1] output above is the stage +number, and will take on values other than 0 for files with merge +conflicts. + +The index is thus a sort of temporary staging area, which is filled with +a tree which you are in the process of working on. + +If you blow the index away entirely, you generally haven't lost any +information as long as you have the name of the tree that it described. + +[[submodules]] +== Submodules + +Large projects are often composed of smaller, self-contained modules. For +example, an embedded Linux distribution's source tree would include every +piece of software in the distribution with some local modifications; a movie +player might need to build against a specific, known-working version of a +decompression library; several independent programs might all share the same +build scripts. + +With centralized revision control systems this is often accomplished by +including every module in one single repository. Developers can check out +all modules or only the modules they need to work with. They can even modify +files across several modules in a single commit while moving things around +or updating APIs and translations. + +Git does not allow partial checkouts, so duplicating this approach in Git +would force developers to keep a local copy of modules they are not +interested in touching. Commits in an enormous checkout would be slower +than you'd expect as Git would have to scan every directory for changes. +If modules have a lot of local history, clones would take forever. + +On the plus side, distributed revision control systems can much better +integrate with external sources. In a centralized model, a single arbitrary +snapshot of the external project is exported from its own revision control +and then imported into the local revision control on a vendor branch. All +the history is hidden. With distributed revision control you can clone the +entire external history and much more easily follow development and re-merge +local changes. + +Git's submodule support allows a repository to contain, as a subdirectory, a +checkout of an external project. Submodules maintain their own identity; +the submodule support just stores the submodule repository location and +commit ID, so other developers who clone the containing project +("superproject") can easily clone all the submodules at the same revision. +Partial checkouts of the superproject are possible: you can tell Git to +clone none, some or all of the submodules. + +The linkgit:git-submodule[1] command is available since Git 1.5.3. Users +with Git 1.5.2 can look up the submodule commits in the repository and +manually check them out; earlier versions won't recognize the submodules at +all. + +To see how submodule support works, create four example +repositories that can be used later as a submodule: + +------------------------------------------------- +$ mkdir ~/git +$ cd ~/git +$ for i in a b c d +do + mkdir $i + cd $i + git init + echo "module $i" > $i.txt + git add $i.txt + git commit -m "Initial commit, submodule $i" + cd .. +done +------------------------------------------------- + +Now create the superproject and add all the submodules: + +------------------------------------------------- +$ mkdir super +$ cd super +$ git init +$ for i in a b c d +do + git submodule add ~/git/$i $i +done +------------------------------------------------- + +NOTE: Do not use local URLs here if you plan to publish your superproject! + +See what files `git submodule` created: + +------------------------------------------------- +$ ls -a +. .. .git .gitmodules a b c d +------------------------------------------------- + +The `git submodule add <repo> <path>` command does a couple of things: + +- It clones the submodule from `<repo>` to the given `<path>` under the + current directory and by default checks out the master branch. +- It adds the submodule's clone path to the linkgit:gitmodules[5] file and + adds this file to the index, ready to be committed. +- It adds the submodule's current commit ID to the index, ready to be + committed. + +Commit the superproject: + +------------------------------------------------- +$ git commit -m "Add submodules a, b, c and d." +------------------------------------------------- + +Now clone the superproject: + +------------------------------------------------- +$ cd .. +$ git clone super cloned +$ cd cloned +------------------------------------------------- + +The submodule directories are there, but they're empty: + +------------------------------------------------- +$ ls -a a +. .. +$ git submodule status +-d266b9873ad50488163457f025db7cdd9683d88b a +-e81d457da15309b4fef4249aba9b50187999670d b +-c1536a972b9affea0f16e0680ba87332dc059146 c +-d96249ff5d57de5de093e6baff9e0aafa5276a74 d +------------------------------------------------- + +NOTE: The commit object names shown above would be different for you, but they +should match the HEAD commit object names of your repositories. You can check +it by running `git ls-remote ../a`. + +Pulling down the submodules is a two-step process. First run `git submodule +init` to add the submodule repository URLs to `.git/config`: + +------------------------------------------------- +$ git submodule init +------------------------------------------------- + +Now use `git submodule update` to clone the repositories and check out the +commits specified in the superproject: + +------------------------------------------------- +$ git submodule update +$ cd a +$ ls -a +. .. .git a.txt +------------------------------------------------- + +One major difference between `git submodule update` and `git submodule add` is +that `git submodule update` checks out a specific commit, rather than the tip +of a branch. It's like checking out a tag: the head is detached, so you're not +working on a branch. + +------------------------------------------------- +$ git branch +* (detached from d266b98) + master +------------------------------------------------- + +If you want to make a change within a submodule and you have a detached head, +then you should create or checkout a branch, make your changes, publish the +change within the submodule, and then update the superproject to reference the +new commit: + +------------------------------------------------- +$ git switch master +------------------------------------------------- + +or + +------------------------------------------------- +$ git switch -c fix-up +------------------------------------------------- + +then + +------------------------------------------------- +$ echo "adding a line again" >> a.txt +$ git commit -a -m "Updated the submodule from within the superproject." +$ git push +$ cd .. +$ git diff +diff --git a/a b/a +index d266b98..261dfac 160000 +--- a/a ++++ b/a +@@ -1 +1 @@ +-Subproject commit d266b9873ad50488163457f025db7cdd9683d88b ++Subproject commit 261dfac35cb99d380eb966e102c1197139f7fa24 +$ git add a +$ git commit -m "Updated submodule a." +$ git push +------------------------------------------------- + +You have to run `git submodule update` after `git pull` if you want to update +submodules, too. + +[[pitfalls-with-submodules]] +=== Pitfalls with submodules + +Always publish the submodule change before publishing the change to the +superproject that references it. If you forget to publish the submodule change, +others won't be able to clone the repository: + +------------------------------------------------- +$ cd ~/git/super/a +$ echo i added another line to this file >> a.txt +$ git commit -a -m "doing it wrong this time" +$ cd .. +$ git add a +$ git commit -m "Updated submodule a again." +$ git push +$ cd ~/git/cloned +$ git pull +$ git submodule update +error: pathspec '261dfac35cb99d380eb966e102c1197139f7fa24' did not match any file(s) known to git. +Did you forget to 'git add'? +Unable to checkout '261dfac35cb99d380eb966e102c1197139f7fa24' in submodule path 'a' +------------------------------------------------- + +In older Git versions it could be easily forgotten to commit new or modified +files in a submodule, which silently leads to similar problems as not pushing +the submodule changes. Starting with Git 1.7.0 both `git status` and `git diff` +in the superproject show submodules as modified when they contain new or +modified files to protect against accidentally committing such a state. `git +diff` will also add a `-dirty` to the work tree side when generating patch +output or used with the `--submodule` option: + +------------------------------------------------- +$ git diff +diff --git a/sub b/sub +--- a/sub ++++ b/sub +@@ -1 +1 @@ +-Subproject commit 3f356705649b5d566d97ff843cf193359229a453 ++Subproject commit 3f356705649b5d566d97ff843cf193359229a453-dirty +$ git diff --submodule +Submodule sub 3f35670..3f35670-dirty: +------------------------------------------------- + +You also should not rewind branches in a submodule beyond commits that were +ever recorded in any superproject. + +It's not safe to run `git submodule update` if you've made and committed +changes within a submodule without checking out a branch first. They will be +silently overwritten: + +------------------------------------------------- +$ cat a.txt +module a +$ echo line added from private2 >> a.txt +$ git commit -a -m "line added inside private2" +$ cd .. +$ git submodule update +Submodule path 'a': checked out 'd266b9873ad50488163457f025db7cdd9683d88b' +$ cd a +$ cat a.txt +module a +------------------------------------------------- + +NOTE: The changes are still visible in the submodule's reflog. + +If you have uncommitted changes in your submodule working tree, `git +submodule update` will not overwrite them. Instead, you get the usual +warning about not being able switch from a dirty branch. + +[[low-level-operations]] +== Low-level Git operations + +Many of the higher-level commands were originally implemented as shell +scripts using a smaller core of low-level Git commands. These can still +be useful when doing unusual things with Git, or just as a way to +understand its inner workings. + +[[object-manipulation]] +=== Object access and manipulation + +The linkgit:git-cat-file[1] command can show the contents of any object, +though the higher-level linkgit:git-show[1] is usually more useful. + +The linkgit:git-commit-tree[1] command allows constructing commits with +arbitrary parents and trees. + +A tree can be created with linkgit:git-write-tree[1] and its data can be +accessed by linkgit:git-ls-tree[1]. Two trees can be compared with +linkgit:git-diff-tree[1]. + +A tag is created with linkgit:git-mktag[1], and the signature can be +verified by linkgit:git-verify-tag[1], though it is normally simpler to +use linkgit:git-tag[1] for both. + +[[the-workflow]] +=== The Workflow + +High-level operations such as linkgit:git-commit[1] and +linkgit:git-restore[1] work by moving data +between the working tree, the index, and the object database. Git +provides low-level operations which perform each of these steps +individually. + +Generally, all Git operations work on the index file. Some operations +work *purely* on the index file (showing the current state of the +index), but most operations move data between the index file and either +the database or the working directory. Thus there are four main +combinations: + +[[working-directory-to-index]] +==== working directory -> index + +The linkgit:git-update-index[1] command updates the index with +information from the working directory. You generally update the +index information by just specifying the filename you want to update, +like so: + +------------------------------------------------- +$ git update-index filename +------------------------------------------------- + +but to avoid common mistakes with filename globbing etc., the command +will not normally add totally new entries or remove old entries, +i.e. it will normally just update existing cache entries. + +To tell Git that yes, you really do realize that certain files no +longer exist, or that new files should be added, you +should use the `--remove` and `--add` flags respectively. + +NOTE! A `--remove` flag does 'not' mean that subsequent filenames will +necessarily be removed: if the files still exist in your directory +structure, the index will be updated with their new status, not +removed. The only thing `--remove` means is that update-index will be +considering a removed file to be a valid thing, and if the file really +does not exist any more, it will update the index accordingly. + +As a special case, you can also do `git update-index --refresh`, which +will refresh the "stat" information of each index to match the current +stat information. It will 'not' update the object status itself, and +it will only update the fields that are used to quickly test whether +an object still matches its old backing store object. + +The previously introduced linkgit:git-add[1] is just a wrapper for +linkgit:git-update-index[1]. + +[[index-to-object-database]] +==== index -> object database + +You write your current index file to a "tree" object with the program + +------------------------------------------------- +$ git write-tree +------------------------------------------------- + +that doesn't come with any options--it will just write out the +current index into the set of tree objects that describe that state, +and it will return the name of the resulting top-level tree. You can +use that tree to re-generate the index at any time by going in the +other direction: + +[[object-database-to-index]] +==== object database -> index + +You read a "tree" file from the object database, and use that to +populate (and overwrite--don't do this if your index contains any +unsaved state that you might want to restore later!) your current +index. Normal operation is just + +------------------------------------------------- +$ git read-tree <SHA-1 of tree> +------------------------------------------------- + +and your index file will now be equivalent to the tree that you saved +earlier. However, that is only your 'index' file: your working +directory contents have not been modified. + +[[index-to-working-directory]] +==== index -> working directory + +You update your working directory from the index by "checking out" +files. This is not a very common operation, since normally you'd just +keep your files updated, and rather than write to your working +directory, you'd tell the index files about the changes in your +working directory (i.e. `git update-index`). + +However, if you decide to jump to a new version, or check out somebody +else's version, or just restore a previous tree, you'd populate your +index file with read-tree, and then you need to check out the result +with + +------------------------------------------------- +$ git checkout-index filename +------------------------------------------------- + +or, if you want to check out all of the index, use `-a`. + +NOTE! `git checkout-index` normally refuses to overwrite old files, so +if you have an old version of the tree already checked out, you will +need to use the `-f` flag ('before' the `-a` flag or the filename) to +'force' the checkout. + + +Finally, there are a few odds and ends which are not purely moving +from one representation to the other: + +[[tying-it-all-together]] +==== Tying it all together + +To commit a tree you have instantiated with `git write-tree`, you'd +create a "commit" object that refers to that tree and the history +behind it--most notably the "parent" commits that preceded it in +history. + +Normally a "commit" has one parent: the previous state of the tree +before a certain change was made. However, sometimes it can have two +or more parent commits, in which case we call it a "merge", due to the +fact that such a commit brings together ("merges") two or more +previous states represented by other commits. + +In other words, while a "tree" represents a particular directory state +of a working directory, a "commit" represents that state in time, +and explains how we got there. + +You create a commit object by giving it the tree that describes the +state at the time of the commit, and a list of parents: + +------------------------------------------------- +$ git commit-tree <tree> -p <parent> [(-p <parent2>)...] +------------------------------------------------- + +and then giving the reason for the commit on stdin (either through +redirection from a pipe or file, or by just typing it at the tty). + +`git commit-tree` will return the name of the object that represents +that commit, and you should save it away for later use. Normally, +you'd commit a new `HEAD` state, and while Git doesn't care where you +save the note about that state, in practice we tend to just write the +result to the file pointed at by `.git/HEAD`, so that we can always see +what the last committed state was. + +Here is a picture that illustrates how various pieces fit together: + +------------ + + commit-tree + commit obj + +----+ + | | + | | + V V + +-----------+ + | Object DB | + | Backing | + | Store | + +-----------+ + ^ + write-tree | | + tree obj | | + | | read-tree + | | tree obj + V + +-----------+ + | Index | + | "cache" | + +-----------+ + update-index ^ + blob obj | | + | | + checkout-index -u | | checkout-index + stat | | blob obj + V + +-----------+ + | Working | + | Directory | + +-----------+ + +------------ + + +[[examining-the-data]] +=== Examining the data + +You can examine the data represented in the object database and the +index with various helper tools. For every object, you can use +linkgit:git-cat-file[1] to examine details about the +object: + +------------------------------------------------- +$ git cat-file -t <objectname> +------------------------------------------------- + +shows the type of the object, and once you have the type (which is +usually implicit in where you find the object), you can use + +------------------------------------------------- +$ git cat-file blob|tree|commit|tag <objectname> +------------------------------------------------- + +to show its contents. NOTE! Trees have binary content, and as a result +there is a special helper for showing that content, called +`git ls-tree`, which turns the binary content into a more easily +readable form. + +It's especially instructive to look at "commit" objects, since those +tend to be small and fairly self-explanatory. In particular, if you +follow the convention of having the top commit name in `.git/HEAD`, +you can do + +------------------------------------------------- +$ git cat-file commit HEAD +------------------------------------------------- + +to see what the top commit was. + +[[merging-multiple-trees]] +=== Merging multiple trees + +Git can help you perform a three-way merge, which can in turn be +used for a many-way merge by repeating the merge procedure several +times. The usual situation is that you only do one three-way merge +(reconciling two lines of history) and commit the result, but if +you like to, you can merge several branches in one go. + +To perform a three-way merge, you start with the two commits you +want to merge, find their closest common parent (a third commit), +and compare the trees corresponding to these three commits. + +To get the "base" for the merge, look up the common parent of two +commits: + +------------------------------------------------- +$ git merge-base <commit1> <commit2> +------------------------------------------------- + +This prints the name of a commit they are both based on. You should +now look up the tree objects of those commits, which you can easily +do with + +------------------------------------------------- +$ git cat-file commit <commitname> | head -1 +------------------------------------------------- + +since the tree object information is always the first line in a commit +object. + +Once you know the three trees you are going to merge (the one "original" +tree, aka the common tree, and the two "result" trees, aka the branches +you want to merge), you do a "merge" read into the index. This will +complain if it has to throw away your old index contents, so you should +make sure that you've committed those--in fact you would normally +always do a merge against your last commit (which should thus match what +you have in your current index anyway). + +To do the merge, do + +------------------------------------------------- +$ git read-tree -m -u <origtree> <yourtree> <targettree> +------------------------------------------------- + +which will do all trivial merge operations for you directly in the +index file, and you can just write the result out with +`git write-tree`. + + +[[merging-multiple-trees-2]] +=== Merging multiple trees, continued + +Sadly, many merges aren't trivial. If there are files that have +been added, moved or removed, or if both branches have modified the +same file, you will be left with an index tree that contains "merge +entries" in it. Such an index tree can 'NOT' be written out to a tree +object, and you will have to resolve any such merge clashes using +other tools before you can write out the result. + +You can examine such index state with `git ls-files --unmerged` +command. An example: + +------------------------------------------------ +$ git read-tree -m $orig HEAD $target +$ git ls-files --unmerged +100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c +100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c +100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c +------------------------------------------------ + +Each line of the `git ls-files --unmerged` output begins with +the blob mode bits, blob SHA-1, 'stage number', and the +filename. The 'stage number' is Git's way to say which tree it +came from: stage 1 corresponds to the `$orig` tree, stage 2 to +the `HEAD` tree, and stage 3 to the `$target` tree. + +Earlier we said that trivial merges are done inside +`git read-tree -m`. For example, if the file did not change +from `$orig` to `HEAD` or `$target`, or if the file changed +from `$orig` to `HEAD` and `$orig` to `$target` the same way, +obviously the final outcome is what is in `HEAD`. What the +above example shows is that file `hello.c` was changed from +`$orig` to `HEAD` and `$orig` to `$target` in a different way. +You could resolve this by running your favorite 3-way merge +program, e.g. `diff3`, `merge`, or Git's own merge-file, on +the blob objects from these three stages yourself, like this: + +------------------------------------------------ +$ git cat-file blob 263414f >hello.c~1 +$ git cat-file blob 06fa6a2 >hello.c~2 +$ git cat-file blob cc44c73 >hello.c~3 +$ git merge-file hello.c~2 hello.c~1 hello.c~3 +------------------------------------------------ + +This would leave the merge result in `hello.c~2` file, along +with conflict markers if there are conflicts. After verifying +the merge result makes sense, you can tell Git what the final +merge result for this file is by: + +------------------------------------------------- +$ mv -f hello.c~2 hello.c +$ git update-index hello.c +------------------------------------------------- + +When a path is in the "unmerged" state, running `git update-index` for +that path tells Git to mark the path resolved. + +The above is the description of a Git merge at the lowest level, +to help you understand what conceptually happens under the hood. +In practice, nobody, not even Git itself, runs `git cat-file` three times +for this. There is a `git merge-index` program that extracts the +stages to temporary files and calls a "merge" script on it: + +------------------------------------------------- +$ git merge-index git-merge-one-file hello.c +------------------------------------------------- + +and that is what higher level `git merge -s resolve` is implemented with. + +[[hacking-git]] +== Hacking Git + +This chapter covers internal details of the Git implementation which +probably only Git developers need to understand. + +[[object-details]] +=== Object storage format + +All objects have a statically determined "type" which identifies the +format of the object (i.e. how it is used, and how it can refer to other +objects). There are currently four different object types: "blob", +"tree", "commit", and "tag". + +Regardless of object type, all objects share the following +characteristics: they are all deflated with zlib, and have a header +that not only specifies their type, but also provides size information +about the data in the object. It's worth noting that the SHA-1 hash +that is used to name the object is the hash of the original data +plus this header, so `sha1sum` 'file' does not match the object name +for 'file'. + +As a result, the general consistency of an object can always be tested +independently of the contents or the type of the object: all objects can +be validated by verifying that (a) their hashes match the content of the +file and (b) the object successfully inflates to a stream of bytes that +forms a sequence of +`<ascii type without space> + <space> + <ascii decimal size> + +<byte\0> + <binary object data>`. + +The structured objects can further have their structure and +connectivity to other objects verified. This is generally done with +the `git fsck` program, which generates a full dependency graph +of all objects, and verifies their internal consistency (in addition +to just verifying their superficial consistency through the hash). + +[[birdview-on-the-source-code]] +=== A birds-eye view of Git's source code + +It is not always easy for new developers to find their way through Git's +source code. This section gives you a little guidance to show where to +start. + +A good place to start is with the contents of the initial commit, with: + +---------------------------------------------------- +$ git switch --detach e83c5163 +---------------------------------------------------- + +The initial revision lays the foundation for almost everything Git has +today, but is small enough to read in one sitting. + +Note that terminology has changed since that revision. For example, the +README in that revision uses the word "changeset" to describe what we +now call a <<def_commit_object,commit>>. + +Also, we do not call it "cache" any more, but rather "index"; however, +the file is still called `read-cache.h`. + +If you grasp the ideas in that initial commit, you should check out a +more recent version and skim `read-cache-ll.h`, `object.h` and `commit.h`. + +In the early days, Git (in the tradition of UNIX) was a bunch of programs +which were extremely simple, and which you used in scripts, piping the +output of one into another. This turned out to be good for initial +development, since it was easier to test new things. However, recently +many of these parts have become builtins, and some of the core has been +"libified", i.e. put into libgit.a for performance, portability reasons, +and to avoid code duplication. + +By now, you know what the index is (and find the corresponding data +structures in `read-cache-ll.h`), and that there are just a couple of +object types (blobs, trees, commits and tags) which inherit their +common structure from `struct object`, which is their first member +(and thus, you can cast e.g. `(struct object *)commit` to achieve the +_same_ as `&commit->object`, i.e. get at the object name and flags). + +Now is a good point to take a break to let this information sink in. + +Next step: get familiar with the object naming. Read <<naming-commits>>. +There are quite a few ways to name an object (and not only revisions!). +All of these are handled in `sha1_name.c`. Just have a quick look at +the function `get_sha1()`. A lot of the special handling is done by +functions like `get_sha1_basic()` or the likes. + +This is just to get you into the groove for the most libified part of Git: +the revision walker. + +Basically, the initial version of `git log` was a shell script: + +---------------------------------------------------------------- +$ git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | \ + LESS=-S ${PAGER:-less} +---------------------------------------------------------------- + +What does this mean? + +`git rev-list` is the original version of the revision walker, which +_always_ printed a list of revisions to stdout. It is still functional, +and needs to, since most new Git commands start out as scripts using +`git rev-list`. + +`git rev-parse` is not as important any more; it was only used to filter out +options that were relevant for the different plumbing commands that were +called by the script. + +Most of what `git rev-list` did is contained in `revision.c` and +`revision.h`. It wraps the options in a struct named `rev_info`, which +controls how and what revisions are walked, and more. + +The original job of `git rev-parse` is now taken by the function +`setup_revisions()`, which parses the revisions and the common command-line +options for the revision walker. This information is stored in the struct +`rev_info` for later consumption. You can do your own command-line option +parsing after calling `setup_revisions()`. After that, you have to call +`prepare_revision_walk()` for initialization, and then you can get the +commits one by one with the function `get_revision()`. + +If you are interested in more details of the revision walking process, +just have a look at the first implementation of `cmd_log()`; call +`git show v1.3.0~155^2~4` and scroll down to that function (note that you +no longer need to call `setup_pager()` directly). + +Nowadays, `git log` is a builtin, which means that it is _contained_ in the +command `git`. The source side of a builtin is + +- a function called `cmd_<bla>`, typically defined in `builtin/<bla.c>` + (note that older versions of Git used to have it in `builtin-<bla>.c` + instead), and declared in `builtin.h`. + +- an entry in the `commands[]` array in `git.c`, and + +- an entry in `BUILTIN_OBJECTS` in the `Makefile`. + +Sometimes, more than one builtin is contained in one source file. For +example, `cmd_whatchanged()` and `cmd_log()` both reside in `builtin/log.c`, +since they share quite a bit of code. In that case, the commands which are +_not_ named like the `.c` file in which they live have to be listed in +`BUILT_INS` in the `Makefile`. + +`git log` looks more complicated in C than it does in the original script, +but that allows for a much greater flexibility and performance. + +Here again it is a good point to take a pause. + +Lesson three is: study the code. Really, it is the best way to learn about +the organization of Git (after you know the basic concepts). + +So, think about something which you are interested in, say, "how can I +access a blob just knowing the object name of it?". The first step is to +find a Git command with which you can do it. In this example, it is either +`git show` or `git cat-file`. + +For the sake of clarity, let's stay with `git cat-file`, because it + +- is plumbing, and + +- was around even in the initial commit (it literally went only through + some 20 revisions as `cat-file.c`, was renamed to `builtin/cat-file.c` + when made a builtin, and then saw less than 10 versions). + +So, look into `builtin/cat-file.c`, search for `cmd_cat_file()` and look what +it does. + +------------------------------------------------------------------ + git_config(git_default_config); + if (argc != 3) + usage("git cat-file [-t|-s|-e|-p|<type>] <sha1>"); + if (get_sha1(argv[2], sha1)) + die("Not a valid object name %s", argv[2]); +------------------------------------------------------------------ + +Let's skip over the obvious details; the only really interesting part +here is the call to `get_sha1()`. It tries to interpret `argv[2]` as an +object name, and if it refers to an object which is present in the current +repository, it writes the resulting SHA-1 into the variable `sha1`. + +Two things are interesting here: + +- `get_sha1()` returns 0 on _success_. This might surprise some new + Git hackers, but there is a long tradition in UNIX to return different + negative numbers in case of different errors--and 0 on success. + +- the variable `sha1` in the function signature of `get_sha1()` is `unsigned + char *`, but is actually expected to be a pointer to `unsigned + char[20]`. This variable will contain the 160-bit SHA-1 of the given + commit. Note that whenever a SHA-1 is passed as `unsigned char *`, it + is the binary representation, as opposed to the ASCII representation in + hex characters, which is passed as `char *`. + +You will see both of these things throughout the code. + +Now, for the meat: + +----------------------------------------------------------------------------- + case 0: + buf = read_object_with_reference(sha1, argv[1], &size, NULL); +----------------------------------------------------------------------------- + +This is how you read a blob (actually, not only a blob, but any type of +object). To know how the function `read_object_with_reference()` actually +works, find the source code for it (something like `git grep +read_object_with | grep ":[a-z]"` in the Git repository), and read +the source. + +To find out how the result can be used, just read on in `cmd_cat_file()`: + +----------------------------------- + write_or_die(1, buf, size); +----------------------------------- + +Sometimes, you do not know where to look for a feature. In many such cases, +it helps to search through the output of `git log`, and then `git show` the +corresponding commit. + +Example: If you know that there was some test case for `git bundle`, but +do not remember where it was (yes, you _could_ `git grep bundle t/`, but that +does not illustrate the point!): + +------------------------ +$ git log --no-merges t/ +------------------------ + +In the pager (`less`), just search for "bundle", go a few lines back, +and see that it is in commit 18449ab0. Now just copy this object name, +and paste it into the command line + +------------------- +$ git show 18449ab0 +------------------- + +Voila. + +Another example: Find out what to do in order to make some script a +builtin: + +------------------------------------------------- +$ git log --no-merges --diff-filter=A builtin/*.c +------------------------------------------------- + +You see, Git is actually the best tool to find out about the source of Git +itself! + +[[glossary]] +== Git Glossary + +[[git-explained]] +=== Git explained + +include::glossary-content.txt[] + +[[git-quick-start]] +[appendix] +== Git Quick Reference + +This is a quick summary of the major commands; the previous chapters +explain how these work in more detail. + +[[quick-creating-a-new-repository]] +=== Creating a new repository + +From a tarball: + +----------------------------------------------- +$ tar xzf project.tar.gz +$ cd project +$ git init +Initialized empty Git repository in .git/ +$ git add . +$ git commit +----------------------------------------------- + +From a remote repository: + +----------------------------------------------- +$ git clone git://example.com/pub/project.git +$ cd project +----------------------------------------------- + +[[managing-branches]] +=== Managing branches + +----------------------------------------------- +$ git branch # list all local branches in this repo +$ git switch test # switch working directory to branch "test" +$ git branch new # create branch "new" starting at current HEAD +$ git branch -d new # delete branch "new" +----------------------------------------------- + +Instead of basing a new branch on current HEAD (the default), use: + +----------------------------------------------- +$ git branch new test # branch named "test" +$ git branch new v2.6.15 # tag named v2.6.15 +$ git branch new HEAD^ # commit before the most recent +$ git branch new HEAD^^ # commit before that +$ git branch new test~10 # ten commits before tip of branch "test" +----------------------------------------------- + +Create and switch to a new branch at the same time: + +----------------------------------------------- +$ git switch -c new v2.6.15 +----------------------------------------------- + +Update and examine branches from the repository you cloned from: + +----------------------------------------------- +$ git fetch # update +$ git branch -r # list + origin/master + origin/next + ... +$ git switch -c masterwork origin/master +----------------------------------------------- + +Fetch a branch from a different repository, and give it a new +name in your repository: + +----------------------------------------------- +$ git fetch git://example.com/project.git theirbranch:mybranch +$ git fetch git://example.com/project.git v2.6.15:mybranch +----------------------------------------------- + +Keep a list of repositories you work with regularly: + +----------------------------------------------- +$ git remote add example git://example.com/project.git +$ git remote # list remote repositories +example +origin +$ git remote show example # get details +* remote example + URL: git://example.com/project.git + Tracked remote branches + master + next + ... +$ git fetch example # update branches from example +$ git branch -r # list all remote branches +----------------------------------------------- + + +[[exploring-history]] +=== Exploring history + +----------------------------------------------- +$ gitk # visualize and browse history +$ git log # list all commits +$ git log src/ # ...modifying src/ +$ git log v2.6.15..v2.6.16 # ...in v2.6.16, not in v2.6.15 +$ git log master..test # ...in branch test, not in branch master +$ git log test..master # ...in branch master, but not in test +$ git log test...master # ...in one branch, not in both +$ git log -S'foo()' # ...where difference contain "foo()" +$ git log --since="2 weeks ago" +$ git log -p # show patches as well +$ git show # most recent commit +$ git diff v2.6.15..v2.6.16 # diff between two tagged versions +$ git diff v2.6.15..HEAD # diff with current head +$ git grep "foo()" # search working directory for "foo()" +$ git grep v2.6.15 "foo()" # search old tree for "foo()" +$ git show v2.6.15:a.txt # look at old version of a.txt +----------------------------------------------- + +Search for regressions: + +----------------------------------------------- +$ git bisect start +$ git bisect bad # current version is bad +$ git bisect good v2.6.13-rc2 # last known good revision +Bisecting: 675 revisions left to test after this + # test here, then: +$ git bisect good # if this revision is good, or +$ git bisect bad # if this revision is bad. + # repeat until done. +----------------------------------------------- + +[[making-changes]] +=== Making changes + +Make sure Git knows who to blame: + +------------------------------------------------ +$ cat >>~/.gitconfig <<\EOF +[user] + name = Your Name Comes Here + email = you@yourdomain.example.com +EOF +------------------------------------------------ + +Select file contents to include in the next commit, then make the +commit: + +----------------------------------------------- +$ git add a.txt # updated file +$ git add b.txt # new file +$ git rm c.txt # old file +$ git commit +----------------------------------------------- + +Or, prepare and create the commit in one step: + +----------------------------------------------- +$ git commit d.txt # use latest content only of d.txt +$ git commit -a # use latest content of all tracked files +----------------------------------------------- + +[[merging]] +=== Merging + +----------------------------------------------- +$ git merge test # merge branch "test" into the current branch +$ git pull git://example.com/project.git master + # fetch and merge in remote branch +$ git pull . test # equivalent to git merge test +----------------------------------------------- + +[[sharing-your-changes]] +=== Sharing your changes + +Importing or exporting patches: + +----------------------------------------------- +$ git format-patch origin..HEAD # format a patch for each commit + # in HEAD but not in origin +$ git am mbox # import patches from the mailbox "mbox" +----------------------------------------------- + +Fetch a branch in a different Git repository, then merge into the +current branch: + +----------------------------------------------- +$ git pull git://example.com/project.git theirbranch +----------------------------------------------- + +Store the fetched branch into a local branch before merging into the +current branch: + +----------------------------------------------- +$ git pull git://example.com/project.git theirbranch:mybranch +----------------------------------------------- + +After creating commits on a local branch, update the remote +branch with your commits: + +----------------------------------------------- +$ git push ssh://example.com/project.git mybranch:theirbranch +----------------------------------------------- + +When remote and local branch are both named "test": + +----------------------------------------------- +$ git push ssh://example.com/project.git test +----------------------------------------------- + +Shortcut version for a frequently used remote repository: + +----------------------------------------------- +$ git remote add example ssh://example.com/project.git +$ git push example test +----------------------------------------------- + +[[repository-maintenance]] +=== Repository maintenance + +Check for corruption: + +----------------------------------------------- +$ git fsck +----------------------------------------------- + +Recompress, remove unused cruft: + +----------------------------------------------- +$ git gc +----------------------------------------------- + + +[[todo]] +[appendix] +== Notes and todo list for this manual + +[[todo-list]] +=== Todo list + +This is a work in progress. + +The basic requirements: + +- It must be readable in order, from beginning to end, by someone + intelligent with a basic grasp of the UNIX command line, but without + any special knowledge of Git. If necessary, any other prerequisites + should be specifically mentioned as they arise. +- Whenever possible, section headings should clearly describe the task + they explain how to do, in language that requires no more knowledge + than necessary: for example, "importing patches into a project" rather + than "the `git am` command" + +Think about how to create a clear chapter dependency graph that will +allow people to get to important topics without necessarily reading +everything in between. + +Scan `Documentation/` for other stuff left out; in particular: + +- howto's +- some of `technical/`? +- hooks +- list of commands in linkgit:git[1] + +Scan email archives for other stuff left out + +Scan man pages to see if any assume more background than this manual +provides. + +Add more good examples. Entire sections of just cookbook examples +might be a good idea; maybe make an "advanced examples" section a +standard end-of-chapter section? + +Include cross-references to the glossary, where appropriate. + +Add a section on working with other version control systems, including +CVS, Subversion, and just imports of series of release tarballs. + +Write a chapter on using plumbing and writing scripts. + +Alternates, clone -reference, etc. + +More on recovery from repository corruption. See: + https://lore.kernel.org/git/Pine.LNX.4.64.0702272039540.12485@woody.linux-foundation.org/ + https://lore.kernel.org/git/Pine.LNX.4.64.0702141033400.3604@woody.linux-foundation.org/ |