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+Like other projects, we also have some guidelines to keep to the
+code. For Git in general, a few rough rules are:
+
+ - Most importantly, we never say "It's in POSIX; we'll happily
+ ignore your needs should your system not conform to it."
+ We live in the real world.
+
+ - However, we often say "Let's stay away from that construct,
+ it's not even in POSIX".
+
+ - In spite of the above two rules, we sometimes say "Although
+ this is not in POSIX, it (is so convenient | makes the code
+ much more readable | has other good characteristics) and
+ practically all the platforms we care about support it, so
+ let's use it".
+
+ Again, we live in the real world, and it is sometimes a
+ judgement call, the decision based more on real world
+ constraints people face than what the paper standard says.
+
+ - Fixing style violations while working on a real change as a
+ preparatory clean-up step is good, but otherwise avoid useless code
+ churn for the sake of conforming to the style.
+
+ "Once it _is_ in the tree, it's not really worth the patch noise to
+ go and fix it up."
+ Cf. http://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html
+
+Make your code readable and sensible, and don't try to be clever.
+
+As for more concrete guidelines, just imitate the existing code
+(this is a good guideline, no matter which project you are
+contributing to). It is always preferable to match the _local_
+convention. New code added to Git suite is expected to match
+the overall style of existing code. Modifications to existing
+code is expected to match the style the surrounding code already
+uses (even if it doesn't match the overall style of existing code).
+
+But if you must have a list of rules, here they are.
+
+For shell scripts specifically (not exhaustive):
+
+ - We use tabs for indentation.
+
+ - Case arms are indented at the same depth as case and esac lines,
+ like this:
+
+ case "$variable" in
+ pattern1)
+ do this
+ ;;
+ pattern2)
+ do that
+ ;;
+ esac
+
+ - Redirection operators should be written with space before, but no
+ space after them. In other words, write 'echo test >"$file"'
+ instead of 'echo test> $file' or 'echo test > $file'. Note that
+ even though it is not required by POSIX to double-quote the
+ redirection target in a variable (as shown above), our code does so
+ because some versions of bash issue a warning without the quotes.
+
+ (incorrect)
+ cat hello > world < universe
+ echo hello >$world
+
+ (correct)
+ cat hello >world <universe
+ echo hello >"$world"
+
+ - We prefer $( ... ) for command substitution; unlike ``, it
+ properly nests. It should have been the way Bourne spelled
+ it from day one, but unfortunately isn't.
+
+ - If you want to find out if a command is available on the user's
+ $PATH, you should use 'type <command>', instead of 'which <command>'.
+ The output of 'which' is not machine parsable and its exit code
+ is not reliable across platforms.
+
+ - We use POSIX compliant parameter substitutions and avoid bashisms;
+ namely:
+
+ - We use ${parameter-word} and its [-=?+] siblings, and their
+ colon'ed "unset or null" form.
+
+ - We use ${parameter#word} and its [#%] siblings, and their
+ doubled "longest matching" form.
+
+ - No "Substring Expansion" ${parameter:offset:length}.
+
+ - No shell arrays.
+
+ - No pattern replacement ${parameter/pattern/string}.
+
+ - We use Arithmetic Expansion $(( ... )).
+
+ - We do not use Process Substitution <(list) or >(list).
+
+ - Do not write control structures on a single line with semicolon.
+ "then" should be on the next line for if statements, and "do"
+ should be on the next line for "while" and "for".
+
+ (incorrect)
+ if test -f hello; then
+ do this
+ fi
+
+ (correct)
+ if test -f hello
+ then
+ do this
+ fi
+
+ - If a command sequence joined with && or || or | spans multiple
+ lines, put each command on a separate line and put && and || and |
+ operators at the end of each line, rather than the start. This
+ means you don't need to use \ to join lines, since the above
+ operators imply the sequence isn't finished.
+
+ (incorrect)
+ grep blob verify_pack_result \
+ | awk -f print_1.awk \
+ | sort >actual &&
+ ...
+
+ (correct)
+ grep blob verify_pack_result |
+ awk -f print_1.awk |
+ sort >actual &&
+ ...
+
+ - We prefer "test" over "[ ... ]".
+
+ - We do not write the noiseword "function" in front of shell
+ functions.
+
+ - We prefer a space between the function name and the parentheses,
+ and no space inside the parentheses. The opening "{" should also
+ be on the same line.
+
+ (incorrect)
+ my_function(){
+ ...
+
+ (correct)
+ my_function () {
+ ...
+
+ - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
+ [::], [==], or [..]) for portability.
+
+ - We do not use \{m,n\};
+
+ - We do not use -E;
+
+ - We do not use ? or + (which are \{0,1\} and \{1,\}
+ respectively in BRE) but that goes without saying as these
+ are ERE elements not BRE (note that \? and \+ are not even part
+ of BRE -- making them accessible from BRE is a GNU extension).
+
+ - Use Git's gettext wrappers in git-sh-i18n to make the user
+ interface translatable. See "Marking strings for translation" in
+ po/README.
+
+ - We do not write our "test" command with "-a" and "-o" and use "&&"
+ or "||" to concatenate multiple "test" commands instead, because
+ the use of "-a/-o" is often error-prone. E.g.
+
+ test -n "$x" -a "$a" = "$b"
+
+ is buggy and breaks when $x is "=", but
+
+ test -n "$x" && test "$a" = "$b"
+
+ does not have such a problem.
+
+
+For C programs:
+
+ - We use tabs to indent, and interpret tabs as taking up to
+ 8 spaces.
+
+ - We try to keep to at most 80 characters per line.
+
+ - As a Git developer we assume you have a reasonably modern compiler
+ and we recommend you to enable the DEVELOPER makefile knob to
+ ensure your patch is clear of all compiler warnings we care about,
+ by e.g. "echo DEVELOPER=1 >>config.mak".
+
+ - We try to support a wide range of C compilers to compile Git with,
+ including old ones. You should not use features from newer C
+ standard, even if your compiler groks them.
+
+ There are a few exceptions to this guideline:
+
+ . since early 2012 with e1327023ea, we have been using an enum
+ definition whose last element is followed by a comma. This, like
+ an array initializer that ends with a trailing comma, can be used
+ to reduce the patch noise when adding a new identifier at the end.
+
+ . since mid 2017 with cbc0f81d, we have been using designated
+ initializers for struct (e.g. "struct t v = { .val = 'a' };").
+
+ . since mid 2017 with 512f41cf, we have been using designated
+ initializers for array (e.g. "int array[10] = { [5] = 2 }").
+
+ These used to be forbidden, but we have not heard any breakage
+ report, and they are assumed to be safe.
+
+ - Variables have to be declared at the beginning of the block, before
+ the first statement (i.e. -Wdeclaration-after-statement).
+
+ - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
+ is still not allowed in this codebase.
+
+ - NULL pointers shall be written as NULL, not as 0.
+
+ - When declaring pointers, the star sides with the variable
+ name, i.e. "char *string", not "char* string" or
+ "char * string". This makes it easier to understand code
+ like "char *string, c;".
+
+ - Use whitespace around operators and keywords, but not inside
+ parentheses and not around functions. So:
+
+ while (condition)
+ func(bar + 1);
+
+ and not:
+
+ while( condition )
+ func (bar+1);
+
+ - Do not explicitly compare an integral value with constant 0 or '\0',
+ or a pointer value with constant NULL. For instance, to validate that
+ counted array <ptr, cnt> is initialized but has no elements, write:
+
+ if (!ptr || cnt)
+ BUG("empty array expected");
+
+ and not:
+
+ if (ptr == NULL || cnt != 0);
+ BUG("empty array expected");
+
+ - We avoid using braces unnecessarily. I.e.
+
+ if (bla) {
+ x = 1;
+ }
+
+ is frowned upon. But there are a few exceptions:
+
+ - When the statement extends over a few lines (e.g., a while loop
+ with an embedded conditional, or a comment). E.g.:
+
+ while (foo) {
+ if (x)
+ one();
+ else
+ two();
+ }
+
+ if (foo) {
+ /*
+ * This one requires some explanation,
+ * so we're better off with braces to make
+ * it obvious that the indentation is correct.
+ */
+ doit();
+ }
+
+ - When there are multiple arms to a conditional and some of them
+ require braces, enclose even a single line block in braces for
+ consistency. E.g.:
+
+ if (foo) {
+ doit();
+ } else {
+ one();
+ two();
+ three();
+ }
+
+ - We try to avoid assignments in the condition of an "if" statement.
+
+ - Try to make your code understandable. You may put comments
+ in, but comments invariably tend to stale out when the code
+ they were describing changes. Often splitting a function
+ into two makes the intention of the code much clearer.
+
+ - Multi-line comments include their delimiters on separate lines from
+ the text. E.g.
+
+ /*
+ * A very long
+ * multi-line comment.
+ */
+
+ Note however that a comment that explains a translatable string to
+ translators uses a convention of starting with a magic token
+ "TRANSLATORS: ", e.g.
+
+ /*
+ * TRANSLATORS: here is a comment that explains the string to
+ * be translated, that follows immediately after it.
+ */
+ _("Here is a translatable string explained by the above.");
+
+ - Double negation is often harder to understand than no negation
+ at all.
+
+ - There are two schools of thought when it comes to comparison,
+ especially inside a loop. Some people prefer to have the less stable
+ value on the left hand side and the more stable value on the right hand
+ side, e.g. if you have a loop that counts variable i down to the
+ lower bound,
+
+ while (i > lower_bound) {
+ do something;
+ i--;
+ }
+
+ Other people prefer to have the textual order of values match the
+ actual order of values in their comparison, so that they can
+ mentally draw a number line from left to right and place these
+ values in order, i.e.
+
+ while (lower_bound < i) {
+ do something;
+ i--;
+ }
+
+ Both are valid, and we use both. However, the more "stable" the
+ stable side becomes, the more we tend to prefer the former
+ (comparison with a constant, "i > 0", is an extreme example).
+ Just do not mix styles in the same part of the code and mimic
+ existing styles in the neighbourhood.
+
+ - There are two schools of thought when it comes to splitting a long
+ logical line into multiple lines. Some people push the second and
+ subsequent lines far enough to the right with tabs and align them:
+
+ if (the_beginning_of_a_very_long_expression_that_has_to ||
+ span_more_than_a_single_line_of ||
+ the_source_text) {
+ ...
+
+ while other people prefer to align the second and the subsequent
+ lines with the column immediately inside the opening parenthesis,
+ with tabs and spaces, following our "tabstop is always a multiple
+ of 8" convention:
+
+ if (the_beginning_of_a_very_long_expression_that_has_to ||
+ span_more_than_a_single_line_of ||
+ the_source_text) {
+ ...
+
+ Both are valid, and we use both. Again, just do not mix styles in
+ the same part of the code and mimic existing styles in the
+ neighbourhood.
+
+ - When splitting a long logical line, some people change line before
+ a binary operator, so that the result looks like a parse tree when
+ you turn your head 90-degrees counterclockwise:
+
+ if (the_beginning_of_a_very_long_expression_that_has_to
+ || span_more_than_a_single_line_of_the_source_text) {
+
+ while other people prefer to leave the operator at the end of the
+ line:
+
+ if (the_beginning_of_a_very_long_expression_that_has_to ||
+ span_more_than_a_single_line_of_the_source_text) {
+
+ Both are valid, but we tend to use the latter more, unless the
+ expression gets fairly complex, in which case the former tends to
+ be easier to read. Again, just do not mix styles in the same part
+ of the code and mimic existing styles in the neighbourhood.
+
+ - When splitting a long logical line, with everything else being
+ equal, it is preferable to split after the operator at higher
+ level in the parse tree. That is, this is more preferable:
+
+ if (a_very_long_variable * that_is_used_in +
+ a_very_long_expression) {
+ ...
+
+ than
+
+ if (a_very_long_variable *
+ that_is_used_in + a_very_long_expression) {
+ ...
+
+ - Some clever tricks, like using the !! operator with arithmetic
+ constructs, can be extremely confusing to others. Avoid them,
+ unless there is a compelling reason to use them.
+
+ - Use the API. No, really. We have a strbuf (variable length
+ string), several arrays with the ALLOC_GROW() macro, a
+ string_list for sorted string lists, a hash map (mapping struct
+ objects) named "struct decorate", amongst other things.
+
+ - When you come up with an API, document its functions and structures
+ in the header file that exposes the API to its callers. Use what is
+ in "strbuf.h" as a model for the appropriate tone and level of
+ detail.
+
+ - The first #include in C files, except in platform specific compat/
+ implementations, must be either "git-compat-util.h", "cache.h" or
+ "builtin.h". You do not have to include more than one of these.
+
+ - A C file must directly include the header files that declare the
+ functions and the types it uses, except for the functions and types
+ that are made available to it by including one of the header files
+ it must include by the previous rule.
+
+ - If you are planning a new command, consider writing it in shell
+ or perl first, so that changes in semantics can be easily
+ changed and discussed. Many Git commands started out like
+ that, and a few are still scripts.
+
+ - Avoid introducing a new dependency into Git. This means you
+ usually should stay away from scripting languages not already
+ used in the Git core command set (unless your command is clearly
+ separate from it, such as an importer to convert random-scm-X
+ repositories to Git).
+
+ - When we pass <string, length> pair to functions, we should try to
+ pass them in that order.
+
+ - Use Git's gettext wrappers to make the user interface
+ translatable. See "Marking strings for translation" in po/README.
+
+ - Variables and functions local to a given source file should be marked
+ with "static". Variables that are visible to other source files
+ must be declared with "extern" in header files. However, function
+ declarations should not use "extern", as that is already the default.
+
+ - You can launch gdb around your program using the shorthand GIT_DEBUGGER.
+ Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or
+ run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to
+ use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
+ ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
+
+For Perl programs:
+
+ - Most of the C guidelines above apply.
+
+ - We try to support Perl 5.8 and later ("use Perl 5.008").
+
+ - use strict and use warnings are strongly preferred.
+
+ - Don't overuse statement modifiers unless using them makes the
+ result easier to follow.
+
+ ... do something ...
+ do_this() unless (condition);
+ ... do something else ...
+
+ is more readable than:
+
+ ... do something ...
+ unless (condition) {
+ do_this();
+ }
+ ... do something else ...
+
+ *only* when the condition is so rare that do_this() will be almost
+ always called.
+
+ - We try to avoid assignments inside "if ()" conditions.
+
+ - Learn and use Git.pm if you need that functionality.
+
+ - For Emacs, it's useful to put the following in
+ GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
+
+ ;; note the first part is useful for C editing, too
+ ((nil . ((indent-tabs-mode . t)
+ (tab-width . 8)
+ (fill-column . 80)))
+ (cperl-mode . ((cperl-indent-level . 8)
+ (cperl-extra-newline-before-brace . nil)
+ (cperl-merge-trailing-else . t))))
+
+For Python scripts:
+
+ - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
+
+ - As a minimum, we aim to be compatible with Python 2.7.
+
+ - Where required libraries do not restrict us to Python 2, we try to
+ also be compatible with Python 3.1 and later.
+
+Error Messages
+
+ - Do not end error messages with a full stop.
+
+ - Do not capitalize ("unable to open %s", not "Unable to open %s")
+
+ - Say what the error is first ("cannot open %s", not "%s: cannot open")
+
+
+Externally Visible Names
+
+ - For configuration variable names, follow the existing convention:
+
+ . The section name indicates the affected subsystem.
+
+ . The subsection name, if any, indicates which of an unbounded set
+ of things to set the value for.
+
+ . The variable name describes the effect of tweaking this knob.
+
+ The section and variable names that consist of multiple words are
+ formed by concatenating the words without punctuations (e.g. `-`),
+ and are broken using bumpyCaps in documentation as a hint to the
+ reader.
+
+ When choosing the variable namespace, do not use variable name for
+ specifying possibly unbounded set of things, most notably anything
+ an end user can freely come up with (e.g. branch names). Instead,
+ use subsection names or variable values, like the existing variable
+ branch.<name>.description does.
+
+
+Writing Documentation:
+
+ Most (if not all) of the documentation pages are written in the
+ AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
+ processed into HTML and manpages (e.g. git.html and git.1 in the
+ same directory).
+
+ The documentation liberally mixes US and UK English (en_US/UK)
+ norms for spelling and grammar, which is somewhat unfortunate.
+ In an ideal world, it would have been better if it consistently
+ used only one and not the other, and we would have picked en_US
+ (if you wish to correct the English of some of the existing
+ documentation, please see the documentation-related advice in the
+ Documentation/SubmittingPatches file).
+
+ Every user-visible change should be reflected in the documentation.
+ The same general rule as for code applies -- imitate the existing
+ conventions.
+
+ A few commented examples follow to provide reference when writing or
+ modifying command usage strings and synopsis sections in the manual
+ pages:
+
+ Placeholders are spelled in lowercase and enclosed in angle brackets:
+ <file>
+ --sort=<key>
+ --abbrev[=<n>]
+
+ If a placeholder has multiple words, they are separated by dashes:
+ <new-branch-name>
+ --template=<template-directory>
+
+ Possibility of multiple occurrences is indicated by three dots:
+ <file>...
+ (One or more of <file>.)
+
+ Optional parts are enclosed in square brackets:
+ [<extra>]
+ (Zero or one <extra>.)
+
+ --exec-path[=<path>]
+ (Option with an optional argument. Note that the "=" is inside the
+ brackets.)
+
+ [<patch>...]
+ (Zero or more of <patch>. Note that the dots are inside, not
+ outside the brackets.)
+
+ Multiple alternatives are indicated with vertical bars:
+ [-q | --quiet]
+ [--utf8 | --no-utf8]
+
+ Parentheses are used for grouping:
+ [(<rev> | <range>)...]
+ (Any number of either <rev> or <range>. Parens are needed to make
+ it clear that "..." pertains to both <rev> and <range>.)
+
+ [(-p <parent>)...]
+ (Any number of option -p, each with one <parent> argument.)
+
+ git remote set-head <name> (-a | -d | <branch>)
+ (One and only one of "-a", "-d" or "<branch>" _must_ (no square
+ brackets) be provided.)
+
+ And a somewhat more contrived example:
+ --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
+ Here "=" is outside the brackets, because "--diff-filter=" is a
+ valid usage. "*" has its own pair of brackets, because it can
+ (optionally) be specified only when one or more of the letters is
+ also provided.
+
+ A note on notation:
+ Use 'git' (all lowercase) when talking about commands i.e. something
+ the user would type into a shell and use 'Git' (uppercase first letter)
+ when talking about the version control system and its properties.
+
+ A few commented examples follow to provide reference when writing or
+ modifying paragraphs or option/command explanations that contain options
+ or commands:
+
+ Literal examples (e.g. use of command-line options, command names,
+ branch names, URLs, pathnames (files and directories), configuration and
+ environment variables) must be typeset in monospace (i.e. wrapped with
+ backticks):
+ `--pretty=oneline`
+ `git rev-list`
+ `remote.pushDefault`
+ `http://git.example.com`
+ `.git/config`
+ `GIT_DIR`
+ `HEAD`
+
+ An environment variable must be prefixed with "$" only when referring to its
+ value and not when referring to the variable itself, in this case there is
+ nothing to add except the backticks:
+ `GIT_DIR` is specified
+ `$GIT_DIR/hooks/pre-receive`
+
+ Word phrases enclosed in `backtick characters` are rendered literally
+ and will not be further expanded. The use of `backticks` to achieve the
+ previous rule means that literal examples should not use AsciiDoc
+ escapes.
+ Correct:
+ `--pretty=oneline`
+ Incorrect:
+ `\--pretty=oneline`
+
+ If some place in the documentation needs to typeset a command usage
+ example with inline substitutions, it is fine to use +monospaced and
+ inline substituted text+ instead of `monospaced literal text`, and with
+ the former, the part that should not get substituted must be
+ quoted/escaped.