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diff --git a/Documentation/git-bisect.txt b/Documentation/git-bisect.txt new file mode 100644 index 0000000..fbb39fb --- /dev/null +++ b/Documentation/git-bisect.txt @@ -0,0 +1,510 @@ +git-bisect(1) +============= + +NAME +---- +git-bisect - Use binary search to find the commit that introduced a bug + + +SYNOPSIS +-------- +[verse] +'git bisect' <subcommand> <options> + +DESCRIPTION +----------- +The command takes various subcommands, and different options depending +on the subcommand: + + git bisect start [--term-{new,bad}=<term> --term-{old,good}=<term>] + [--no-checkout] [--first-parent] [<bad> [<good>...]] [--] [<paths>...] + git bisect (bad|new|<term-new>) [<rev>] + git bisect (good|old|<term-old>) [<rev>...] + git bisect terms [--term-good | --term-bad] + git bisect skip [(<rev>|<range>)...] + git bisect reset [<commit>] + git bisect (visualize|view) + git bisect replay <logfile> + git bisect log + git bisect run <cmd>... + git bisect help + +This command uses a binary search algorithm to find which commit in +your project's history introduced a bug. You use it by first telling +it a "bad" commit that is known to contain the bug, and a "good" +commit that is known to be before the bug was introduced. Then `git +bisect` picks a commit between those two endpoints and asks you +whether the selected commit is "good" or "bad". It continues narrowing +down the range until it finds the exact commit that introduced the +change. + +In fact, `git bisect` can be used to find the commit that changed +*any* property of your project; e.g., the commit that fixed a bug, or +the commit that caused a benchmark's performance to improve. To +support this more general usage, the terms "old" and "new" can be used +in place of "good" and "bad", or you can choose your own terms. See +section "Alternate terms" below for more information. + +Basic bisect commands: start, bad, good +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +As an example, suppose you are trying to find the commit that broke a +feature that was known to work in version `v2.6.13-rc2` of your +project. You start a bisect session as follows: + +------------------------------------------------ +$ git bisect start +$ git bisect bad # Current version is bad +$ git bisect good v2.6.13-rc2 # v2.6.13-rc2 is known to be good +------------------------------------------------ + +Once you have specified at least one bad and one good commit, `git +bisect` selects a commit in the middle of that range of history, +checks it out, and outputs something similar to the following: + +------------------------------------------------ +Bisecting: 675 revisions left to test after this (roughly 10 steps) +------------------------------------------------ + +You should now compile the checked-out version and test it. If that +version works correctly, type + +------------------------------------------------ +$ git bisect good +------------------------------------------------ + +If that version is broken, type + +------------------------------------------------ +$ git bisect bad +------------------------------------------------ + +Then `git bisect` will respond with something like + +------------------------------------------------ +Bisecting: 337 revisions left to test after this (roughly 9 steps) +------------------------------------------------ + +Keep repeating the process: compile the tree, test it, and depending +on whether it is good or bad run `git bisect good` or `git bisect bad` +to ask for the next commit that needs testing. + +Eventually there will be no more revisions left to inspect, and the +command will print out a description of the first bad commit. The +reference `refs/bisect/bad` will be left pointing at that commit. + + +Bisect reset +~~~~~~~~~~~~ + +After a bisect session, to clean up the bisection state and return to +the original HEAD, issue the following command: + +------------------------------------------------ +$ git bisect reset +------------------------------------------------ + +By default, this will return your tree to the commit that was checked +out before `git bisect start`. (A new `git bisect start` will also do +that, as it cleans up the old bisection state.) + +With an optional argument, you can return to a different commit +instead: + +------------------------------------------------ +$ git bisect reset <commit> +------------------------------------------------ + +For example, `git bisect reset bisect/bad` will check out the first +bad revision, while `git bisect reset HEAD` will leave you on the +current bisection commit and avoid switching commits at all. + + +Alternate terms +~~~~~~~~~~~~~~~ + +Sometimes you are not looking for the commit that introduced a +breakage, but rather for a commit that caused a change between some +other "old" state and "new" state. For example, you might be looking +for the commit that introduced a particular fix. Or you might be +looking for the first commit in which the source-code filenames were +finally all converted to your company's naming standard. Or whatever. + +In such cases it can be very confusing to use the terms "good" and +"bad" to refer to "the state before the change" and "the state after +the change". So instead, you can use the terms "old" and "new", +respectively, in place of "good" and "bad". (But note that you cannot +mix "good" and "bad" with "old" and "new" in a single session.) + +In this more general usage, you provide `git bisect` with a "new" +commit that has some property and an "old" commit that doesn't have that +property. Each time `git bisect` checks out a commit, you test if that +commit has the property. If it does, mark the commit as "new"; +otherwise, mark it as "old". When the bisection is done, `git bisect` +will report which commit introduced the property. + +To use "old" and "new" instead of "good" and bad, you must run `git +bisect start` without commits as argument and then run the following +commands to add the commits: + +------------------------------------------------ +git bisect old [<rev>] +------------------------------------------------ + +to indicate that a commit was before the sought change, or + +------------------------------------------------ +git bisect new [<rev>...] +------------------------------------------------ + +to indicate that it was after. + +To get a reminder of the currently used terms, use + +------------------------------------------------ +git bisect terms +------------------------------------------------ + +You can get just the old (respectively new) term with `git bisect terms +--term-old` or `git bisect terms --term-good`. + +If you would like to use your own terms instead of "bad"/"good" or +"new"/"old", you can choose any names you like (except existing bisect +subcommands like `reset`, `start`, ...) by starting the +bisection using + +------------------------------------------------ +git bisect start --term-old <term-old> --term-new <term-new> +------------------------------------------------ + +For example, if you are looking for a commit that introduced a +performance regression, you might use + +------------------------------------------------ +git bisect start --term-old fast --term-new slow +------------------------------------------------ + +Or if you are looking for the commit that fixed a bug, you might use + +------------------------------------------------ +git bisect start --term-new fixed --term-old broken +------------------------------------------------ + +Then, use `git bisect <term-old>` and `git bisect <term-new>` instead +of `git bisect good` and `git bisect bad` to mark commits. + +Bisect visualize/view +~~~~~~~~~~~~~~~~~~~~~ + +To see the currently remaining suspects in 'gitk', issue the following +command during the bisection process (the subcommand `view` can be used +as an alternative to `visualize`): + +------------ +$ git bisect visualize +------------ + +If the `DISPLAY` environment variable is not set, 'git log' is used +instead. You can also give command-line options such as `-p` and +`--stat`. + +------------ +$ git bisect visualize --stat +------------ + +Bisect log and bisect replay +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +After having marked revisions as good or bad, issue the following +command to show what has been done so far: + +------------ +$ git bisect log +------------ + +If you discover that you made a mistake in specifying the status of a +revision, you can save the output of this command to a file, edit it to +remove the incorrect entries, and then issue the following commands to +return to a corrected state: + +------------ +$ git bisect reset +$ git bisect replay that-file +------------ + +Avoiding testing a commit +~~~~~~~~~~~~~~~~~~~~~~~~~ + +If, in the middle of a bisect session, you know that the suggested +revision is not a good one to test (e.g. it fails to build and you +know that the failure does not have anything to do with the bug you +are chasing), you can manually select a nearby commit and test that +one instead. + +For example: + +------------ +$ git bisect good/bad # previous round was good or bad. +Bisecting: 337 revisions left to test after this (roughly 9 steps) +$ git bisect visualize # oops, that is uninteresting. +$ git reset --hard HEAD~3 # try 3 revisions before what + # was suggested +------------ + +Then compile and test the chosen revision, and afterwards mark +the revision as good or bad in the usual manner. + +Bisect skip +~~~~~~~~~~~ + +Instead of choosing a nearby commit by yourself, you can ask Git to do +it for you by issuing the command: + +------------ +$ git bisect skip # Current version cannot be tested +------------ + +However, if you skip a commit adjacent to the one you are looking for, +Git will be unable to tell exactly which of those commits was the +first bad one. + +You can also skip a range of commits, instead of just one commit, +using range notation. For example: + +------------ +$ git bisect skip v2.5..v2.6 +------------ + +This tells the bisect process that no commit after `v2.5`, up to and +including `v2.6`, should be tested. + +Note that if you also want to skip the first commit of the range you +would issue the command: + +------------ +$ git bisect skip v2.5 v2.5..v2.6 +------------ + +This tells the bisect process that the commits between `v2.5` and +`v2.6` (inclusive) should be skipped. + + +Cutting down bisection by giving more parameters to bisect start +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +You can further cut down the number of trials, if you know what part of +the tree is involved in the problem you are tracking down, by specifying +path parameters when issuing the `bisect start` command: + +------------ +$ git bisect start -- arch/i386 include/asm-i386 +------------ + +If you know beforehand more than one good commit, you can narrow the +bisect space down by specifying all of the good commits immediately after +the bad commit when issuing the `bisect start` command: + +------------ +$ git bisect start v2.6.20-rc6 v2.6.20-rc4 v2.6.20-rc1 -- + # v2.6.20-rc6 is bad + # v2.6.20-rc4 and v2.6.20-rc1 are good +------------ + +Bisect run +~~~~~~~~~~ + +If you have a script that can tell if the current source code is good +or bad, you can bisect by issuing the command: + +------------ +$ git bisect run my_script arguments +------------ + +Note that the script (`my_script` in the above example) should exit +with code 0 if the current source code is good/old, and exit with a +code between 1 and 127 (inclusive), except 125, if the current source +code is bad/new. + +Any other exit code will abort the bisect process. It should be noted +that a program that terminates via `exit(-1)` leaves $? = 255, (see the +exit(3) manual page), as the value is chopped with `& 0377`. + +The special exit code 125 should be used when the current source code +cannot be tested. If the script exits with this code, the current +revision will be skipped (see `git bisect skip` above). 125 was chosen +as the highest sensible value to use for this purpose, because 126 and 127 +are used by POSIX shells to signal specific error status (127 is for +command not found, 126 is for command found but not executable--these +details do not matter, as they are normal errors in the script, as far as +`bisect run` is concerned). + +You may often find that during a bisect session you want to have +temporary modifications (e.g. s/#define DEBUG 0/#define DEBUG 1/ in a +header file, or "revision that does not have this commit needs this +patch applied to work around another problem this bisection is not +interested in") applied to the revision being tested. + +To cope with such a situation, after the inner 'git bisect' finds the +next revision to test, the script can apply the patch +before compiling, run the real test, and afterwards decide if the +revision (possibly with the needed patch) passed the test and then +rewind the tree to the pristine state. Finally the script should exit +with the status of the real test to let the `git bisect run` command loop +determine the eventual outcome of the bisect session. + +OPTIONS +------- +--no-checkout:: ++ +Do not checkout the new working tree at each iteration of the bisection +process. Instead just update a special reference named `BISECT_HEAD` to make +it point to the commit that should be tested. ++ +This option may be useful when the test you would perform in each step +does not require a checked out tree. ++ +If the repository is bare, `--no-checkout` is assumed. + +--first-parent:: ++ +Follow only the first parent commit upon seeing a merge commit. ++ +In detecting regressions introduced through the merging of a branch, the merge +commit will be identified as introduction of the bug and its ancestors will be +ignored. ++ +This option is particularly useful in avoiding false positives when a merged +branch contained broken or non-buildable commits, but the merge itself was OK. + +EXAMPLES +-------- + +* Automatically bisect a broken build between v1.2 and HEAD: ++ +------------ +$ git bisect start HEAD v1.2 -- # HEAD is bad, v1.2 is good +$ git bisect run make # "make" builds the app +$ git bisect reset # quit the bisect session +------------ + +* Automatically bisect a test failure between origin and HEAD: ++ +------------ +$ git bisect start HEAD origin -- # HEAD is bad, origin is good +$ git bisect run make test # "make test" builds and tests +$ git bisect reset # quit the bisect session +------------ + +* Automatically bisect a broken test case: ++ +------------ +$ cat ~/test.sh +#!/bin/sh +make || exit 125 # this skips broken builds +~/check_test_case.sh # does the test case pass? +$ git bisect start HEAD HEAD~10 -- # culprit is among the last 10 +$ git bisect run ~/test.sh +$ git bisect reset # quit the bisect session +------------ ++ +Here we use a `test.sh` custom script. In this script, if `make` +fails, we skip the current commit. +`check_test_case.sh` should `exit 0` if the test case passes, +and `exit 1` otherwise. ++ +It is safer if both `test.sh` and `check_test_case.sh` are +outside the repository to prevent interactions between the bisect, +make and test processes and the scripts. + +* Automatically bisect with temporary modifications (hot-fix): ++ +------------ +$ cat ~/test.sh +#!/bin/sh + +# tweak the working tree by merging the hot-fix branch +# and then attempt a build +if git merge --no-commit --no-ff hot-fix && + make +then + # run project specific test and report its status + ~/check_test_case.sh + status=$? +else + # tell the caller this is untestable + status=125 +fi + +# undo the tweak to allow clean flipping to the next commit +git reset --hard + +# return control +exit $status +------------ ++ +This applies modifications from a hot-fix branch before each test run, +e.g. in case your build or test environment changed so that older +revisions may need a fix which newer ones have already. (Make sure the +hot-fix branch is based off a commit which is contained in all revisions +which you are bisecting, so that the merge does not pull in too much, or +use `git cherry-pick` instead of `git merge`.) + +* Automatically bisect a broken test case: ++ +------------ +$ git bisect start HEAD HEAD~10 -- # culprit is among the last 10 +$ git bisect run sh -c "make || exit 125; ~/check_test_case.sh" +$ git bisect reset # quit the bisect session +------------ ++ +This shows that you can do without a run script if you write the test +on a single line. + +* Locate a good region of the object graph in a damaged repository ++ +------------ +$ git bisect start HEAD <known-good-commit> [ <boundary-commit> ... ] --no-checkout +$ git bisect run sh -c ' + GOOD=$(git for-each-ref "--format=%(objectname)" refs/bisect/good-*) && + git rev-list --objects BISECT_HEAD --not $GOOD >tmp.$$ && + git pack-objects --stdout >/dev/null <tmp.$$ + rc=$? + rm -f tmp.$$ + test $rc = 0' + +$ git bisect reset # quit the bisect session +------------ ++ +In this case, when 'git bisect run' finishes, bisect/bad will refer to a commit that +has at least one parent whose reachable graph is fully traversable in the sense +required by 'git pack objects'. + +* Look for a fix instead of a regression in the code ++ +------------ +$ git bisect start +$ git bisect new HEAD # current commit is marked as new +$ git bisect old HEAD~10 # the tenth commit from now is marked as old +------------ ++ +or: +------------ +$ git bisect start --term-old broken --term-new fixed +$ git bisect fixed +$ git bisect broken HEAD~10 +------------ + +Getting help +~~~~~~~~~~~~ + +Use `git bisect` to get a short usage description, and `git bisect +help` or `git bisect -h` to get a long usage description. + +SEE ALSO +-------- +link:git-bisect-lk2009.html[Fighting regressions with git bisect], +linkgit:git-blame[1]. + +GIT +--- +Part of the linkgit:git[1] suite |