1 Like other projects, we also have some guidelines to keep to the
2 code. For Git in general, a few rough rules are:
4 - Most importantly, we never say "It's in POSIX; we'll happily
5 ignore your needs should your system not conform to it."
6 We live in the real world.
8 - However, we often say "Let's stay away from that construct,
9 it's not even in POSIX".
11 - In spite of the above two rules, we sometimes say "Although
12 this is not in POSIX, it (is so convenient | makes the code
13 much more readable | has other good characteristics) and
14 practically all the platforms we care about support it, so
17 Again, we live in the real world, and it is sometimes a
18 judgement call, the decision based more on real world
19 constraints people face than what the paper standard says.
21 - Fixing style violations while working on a real change as a
22 preparatory clean-up step is good, but otherwise avoid useless code
23 churn for the sake of conforming to the style.
25 "Once it _is_ in the tree, it's not really worth the patch noise to
27 Cf. http://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html
29 - Log messages to explain your changes are as important as the
30 changes themselves. Clearly written code and in-code comments
31 explain how the code works and what is assumed from the surrounding
32 context. The log messages explain what the changes wanted to
33 achieve and why the changes were necessary (more on this in the
34 accompanying SubmittingPatches document).
36 Make your code readable and sensible, and don't try to be clever.
38 As for more concrete guidelines, just imitate the existing code
39 (this is a good guideline, no matter which project you are
40 contributing to). It is always preferable to match the _local_
41 convention. New code added to Git suite is expected to match
42 the overall style of existing code. Modifications to existing
43 code is expected to match the style the surrounding code already
44 uses (even if it doesn't match the overall style of existing code).
46 But if you must have a list of rules, here are some language
47 specific ones. Note that Documentation/ToolsForGit.txt document
48 has a collection of tips to help you use some external tools
49 to conform to these guidelines.
51 For shell scripts specifically (not exhaustive):
53 - We use tabs for indentation.
55 - Case arms are indented at the same depth as case and esac lines,
67 - Redirection operators should be written with space before, but no
68 space after them. In other words, write 'echo test >"$file"'
69 instead of 'echo test> $file' or 'echo test > $file'. Note that
70 even though it is not required by POSIX to double-quote the
71 redirection target in a variable (as shown above), our code does so
72 because some versions of bash issue a warning without the quotes.
75 cat hello > world < universe
79 cat hello >world <universe
82 - We prefer $( ... ) for command substitution; unlike ``, it
83 properly nests. It should have been the way Bourne spelled
84 it from day one, but unfortunately isn't.
86 - If you want to find out if a command is available on the user's
87 $PATH, you should use 'type <command>', instead of 'which <command>'.
88 The output of 'which' is not machine parsable and its exit code
89 is not reliable across platforms.
91 - We use POSIX compliant parameter substitutions and avoid bashisms;
94 - We use ${parameter-word} and its [-=?+] siblings, and their
95 colon'ed "unset or null" form.
97 - We use ${parameter#word} and its [#%] siblings, and their
98 doubled "longest matching" form.
100 - No "Substring Expansion" ${parameter:offset:length}.
104 - No pattern replacement ${parameter/pattern/string}.
106 - We use Arithmetic Expansion $(( ... )).
108 - We do not use Process Substitution <(list) or >(list).
110 - Do not write control structures on a single line with semicolon.
111 "then" should be on the next line for if statements, and "do"
112 should be on the next line for "while" and "for".
115 if test -f hello; then
125 - If a command sequence joined with && or || or | spans multiple
126 lines, put each command on a separate line and put && and || and |
127 operators at the end of each line, rather than the start. This
128 means you don't need to use \ to join lines, since the above
129 operators imply the sequence isn't finished.
132 grep blob verify_pack_result \
133 | awk -f print_1.awk \
138 grep blob verify_pack_result |
143 - We prefer "test" over "[ ... ]".
145 - We do not write the noiseword "function" in front of shell
148 - We prefer a space between the function name and the parentheses,
149 and no space inside the parentheses. The opening "{" should also
160 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
161 [::], [==], or [..]) for portability.
163 - We do not use \{m,n\};
167 - We do not use ? or + (which are \{0,1\} and \{1,\}
168 respectively in BRE) but that goes without saying as these
169 are ERE elements not BRE (note that \? and \+ are not even part
170 of BRE -- making them accessible from BRE is a GNU extension).
172 - Use Git's gettext wrappers in git-sh-i18n to make the user
173 interface translatable. See "Marking strings for translation" in
176 - We do not write our "test" command with "-a" and "-o" and use "&&"
177 or "||" to concatenate multiple "test" commands instead, because
178 the use of "-a/-o" is often error-prone. E.g.
180 test -n "$x" -a "$a" = "$b"
182 is buggy and breaks when $x is "=", but
184 test -n "$x" && test "$a" = "$b"
186 does not have such a problem.
188 - Even though "local" is not part of POSIX, we make heavy use of it
189 in our test suite. We do not use it in scripted Porcelains, and
190 hopefully nobody starts using "local" before they are reimplemented
196 - We use tabs to indent, and interpret tabs as taking up to
199 - We try to keep to at most 80 characters per line.
201 - As a Git developer we assume you have a reasonably modern compiler
202 and we recommend you to enable the DEVELOPER makefile knob to
203 ensure your patch is clear of all compiler warnings we care about,
204 by e.g. "echo DEVELOPER=1 >>config.mak".
206 - We try to support a wide range of C compilers to compile Git with,
207 including old ones. You should not use features from newer C
208 standard, even if your compiler groks them.
210 There are a few exceptions to this guideline:
212 . since early 2012 with e1327023ea, we have been using an enum
213 definition whose last element is followed by a comma. This, like
214 an array initializer that ends with a trailing comma, can be used
215 to reduce the patch noise when adding a new identifier at the end.
217 . since mid 2017 with cbc0f81d, we have been using designated
218 initializers for struct (e.g. "struct t v = { .val = 'a' };").
220 . since mid 2017 with 512f41cf, we have been using designated
221 initializers for array (e.g. "int array[10] = { [5] = 2 }").
223 . since early 2021 with 765dc168882, we have been using variadic
224 macros, mostly for printf-like trace and debug macros.
226 These used to be forbidden, but we have not heard any breakage
227 report, and they are assumed to be safe.
229 - Variables have to be declared at the beginning of the block, before
230 the first statement (i.e. -Wdeclaration-after-statement).
232 - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
233 is still not allowed in this codebase. We are in the process of
234 allowing it by waiting to see that 44ba10d6 (revision: use C99
235 declaration of variable in for() loop, 2021-11-14) does not get
236 complaints. Let's revisit this around November 2022.
238 - NULL pointers shall be written as NULL, not as 0.
240 - When declaring pointers, the star sides with the variable
241 name, i.e. "char *string", not "char* string" or
242 "char * string". This makes it easier to understand code
243 like "char *string, c;".
245 - Use whitespace around operators and keywords, but not inside
246 parentheses and not around functions. So:
256 - Do not explicitly compare an integral value with constant 0 or '\0',
257 or a pointer value with constant NULL. For instance, to validate that
258 counted array <ptr, cnt> is initialized but has no elements, write:
261 BUG("empty array expected");
265 if (ptr == NULL || cnt != 0);
266 BUG("empty array expected");
268 - We avoid using braces unnecessarily. I.e.
274 is frowned upon. But there are a few exceptions:
276 - When the statement extends over a few lines (e.g., a while loop
277 with an embedded conditional, or a comment). E.g.:
288 * This one requires some explanation,
289 * so we're better off with braces to make
290 * it obvious that the indentation is correct.
295 - When there are multiple arms to a conditional and some of them
296 require braces, enclose even a single line block in braces for
307 - We try to avoid assignments in the condition of an "if" statement.
309 - Try to make your code understandable. You may put comments
310 in, but comments invariably tend to stale out when the code
311 they were describing changes. Often splitting a function
312 into two makes the intention of the code much clearer.
314 - Multi-line comments include their delimiters on separate lines from
319 * multi-line comment.
322 Note however that a comment that explains a translatable string to
323 translators uses a convention of starting with a magic token
324 "TRANSLATORS: ", e.g.
327 * TRANSLATORS: here is a comment that explains the string to
328 * be translated, that follows immediately after it.
330 _("Here is a translatable string explained by the above.");
332 - Double negation is often harder to understand than no negation
335 - There are two schools of thought when it comes to comparison,
336 especially inside a loop. Some people prefer to have the less stable
337 value on the left hand side and the more stable value on the right hand
338 side, e.g. if you have a loop that counts variable i down to the
341 while (i > lower_bound) {
346 Other people prefer to have the textual order of values match the
347 actual order of values in their comparison, so that they can
348 mentally draw a number line from left to right and place these
349 values in order, i.e.
351 while (lower_bound < i) {
356 Both are valid, and we use both. However, the more "stable" the
357 stable side becomes, the more we tend to prefer the former
358 (comparison with a constant, "i > 0", is an extreme example).
359 Just do not mix styles in the same part of the code and mimic
360 existing styles in the neighbourhood.
362 - There are two schools of thought when it comes to splitting a long
363 logical line into multiple lines. Some people push the second and
364 subsequent lines far enough to the right with tabs and align them:
366 if (the_beginning_of_a_very_long_expression_that_has_to ||
367 span_more_than_a_single_line_of ||
371 while other people prefer to align the second and the subsequent
372 lines with the column immediately inside the opening parenthesis,
373 with tabs and spaces, following our "tabstop is always a multiple
376 if (the_beginning_of_a_very_long_expression_that_has_to ||
377 span_more_than_a_single_line_of ||
381 Both are valid, and we use both. Again, just do not mix styles in
382 the same part of the code and mimic existing styles in the
385 - When splitting a long logical line, some people change line before
386 a binary operator, so that the result looks like a parse tree when
387 you turn your head 90-degrees counterclockwise:
389 if (the_beginning_of_a_very_long_expression_that_has_to
390 || span_more_than_a_single_line_of_the_source_text) {
392 while other people prefer to leave the operator at the end of the
395 if (the_beginning_of_a_very_long_expression_that_has_to ||
396 span_more_than_a_single_line_of_the_source_text) {
398 Both are valid, but we tend to use the latter more, unless the
399 expression gets fairly complex, in which case the former tends to
400 be easier to read. Again, just do not mix styles in the same part
401 of the code and mimic existing styles in the neighbourhood.
403 - When splitting a long logical line, with everything else being
404 equal, it is preferable to split after the operator at higher
405 level in the parse tree. That is, this is more preferable:
407 if (a_very_long_variable * that_is_used_in +
408 a_very_long_expression) {
413 if (a_very_long_variable *
414 that_is_used_in + a_very_long_expression) {
417 - Some clever tricks, like using the !! operator with arithmetic
418 constructs, can be extremely confusing to others. Avoid them,
419 unless there is a compelling reason to use them.
421 - Use the API. No, really. We have a strbuf (variable length
422 string), several arrays with the ALLOC_GROW() macro, a
423 string_list for sorted string lists, a hash map (mapping struct
424 objects) named "struct decorate", amongst other things.
426 - When you come up with an API, document its functions and structures
427 in the header file that exposes the API to its callers. Use what is
428 in "strbuf.h" as a model for the appropriate tone and level of
431 - The first #include in C files, except in platform specific compat/
432 implementations, must be either "git-compat-util.h", "cache.h" or
433 "builtin.h". You do not have to include more than one of these.
435 - A C file must directly include the header files that declare the
436 functions and the types it uses, except for the functions and types
437 that are made available to it by including one of the header files
438 it must include by the previous rule.
440 - If you are planning a new command, consider writing it in shell
441 or perl first, so that changes in semantics can be easily
442 changed and discussed. Many Git commands started out like
443 that, and a few are still scripts.
445 - Avoid introducing a new dependency into Git. This means you
446 usually should stay away from scripting languages not already
447 used in the Git core command set (unless your command is clearly
448 separate from it, such as an importer to convert random-scm-X
449 repositories to Git).
451 - When we pass <string, length> pair to functions, we should try to
452 pass them in that order.
454 - Use Git's gettext wrappers to make the user interface
455 translatable. See "Marking strings for translation" in po/README.
457 - Variables and functions local to a given source file should be marked
458 with "static". Variables that are visible to other source files
459 must be declared with "extern" in header files. However, function
460 declarations should not use "extern", as that is already the default.
462 - You can launch gdb around your program using the shorthand GIT_DEBUGGER.
463 Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or
464 run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to
465 use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
466 ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
470 - Most of the C guidelines above apply.
472 - We try to support Perl 5.8 and later ("use Perl 5.008").
474 - use strict and use warnings are strongly preferred.
476 - Don't overuse statement modifiers unless using them makes the
477 result easier to follow.
480 do_this() unless (condition);
481 ... do something else ...
483 is more readable than:
489 ... do something else ...
491 *only* when the condition is so rare that do_this() will be almost
494 - We try to avoid assignments inside "if ()" conditions.
496 - Learn and use Git.pm if you need that functionality.
500 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
502 - As a minimum, we aim to be compatible with Python 2.7.
504 - Where required libraries do not restrict us to Python 2, we try to
505 also be compatible with Python 3.1 and later.
510 We make a distinction between a Git command's primary output and
511 output which is merely chatty feedback (for instance, status
512 messages, running transcript, or progress display), as well as error
513 messages. Roughly speaking, a Git command's primary output is that
514 which one might want to capture to a file or send down a pipe; its
515 chatty output should not interfere with these use-cases.
517 As such, primary output should be sent to the standard output stream
518 (stdout), and chatty output should be sent to the standard error
519 stream (stderr). Examples of commands which produce primary output
520 include `git log`, `git show`, and `git branch --list` which generate
521 output on the stdout stream.
523 Not all Git commands have primary output; this is often true of
524 commands whose main function is to perform an action. Some action
525 commands are silent, whereas others are chatty. An example of a
526 chatty action commands is `git clone` with its "Cloning into
527 '<path>'..." and "Checking connectivity..." status messages which it
528 sends to the stderr stream.
530 Error messages from Git commands should always be sent to the stderr
536 - Do not end error messages with a full stop.
538 - Do not capitalize the first word, only because it is the first word
539 in the message ("unable to open %s", not "Unable to open %s"). But
540 "SHA-3 not supported" is fine, because the reason the first word is
541 capitalized is not because it is at the beginning of the sentence,
542 but because the word would be spelled in capital letters even when
543 it appeared in the middle of the sentence.
545 - Say what the error is first ("cannot open %s", not "%s: cannot open")
548 Externally Visible Names
550 - For configuration variable names, follow the existing convention:
552 . The section name indicates the affected subsystem.
554 . The subsection name, if any, indicates which of an unbounded set
555 of things to set the value for.
557 . The variable name describes the effect of tweaking this knob.
559 The section and variable names that consist of multiple words are
560 formed by concatenating the words without punctuations (e.g. `-`),
561 and are broken using bumpyCaps in documentation as a hint to the
564 When choosing the variable namespace, do not use variable name for
565 specifying possibly unbounded set of things, most notably anything
566 an end user can freely come up with (e.g. branch names). Instead,
567 use subsection names or variable values, like the existing variable
568 branch.<name>.description does.
571 Writing Documentation:
573 Most (if not all) of the documentation pages are written in the
574 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
575 processed into HTML and manpages (e.g. git.html and git.1 in the
578 The documentation liberally mixes US and UK English (en_US/UK)
579 norms for spelling and grammar, which is somewhat unfortunate.
580 In an ideal world, it would have been better if it consistently
581 used only one and not the other, and we would have picked en_US
582 (if you wish to correct the English of some of the existing
583 documentation, please see the documentation-related advice in the
584 Documentation/SubmittingPatches file).
586 In order to ensure the documentation is inclusive, avoid assuming
587 that an unspecified example person is male or female, and think
588 twice before using "he", "him", "she", or "her". Here are some
589 tips to avoid use of gendered pronouns:
591 - Prefer succinctness and matter-of-factly describing functionality
592 in the abstract. E.g.
594 --short:: Emit output in the short-format.
596 and avoid something like these overly verbose alternatives:
598 --short:: Use this to emit output in the short-format.
599 --short:: You can use this to get output in the short-format.
600 --short:: A user who prefers shorter output could....
601 --short:: Should a person and/or program want shorter output, he
604 This practice often eliminates the need to involve human actors in
605 your description, but it is a good practice regardless of the
606 avoidance of gendered pronouns.
608 - When it becomes awkward to stick to this style, prefer "you" when
609 addressing the the hypothetical user, and possibly "we" when
610 discussing how the program might react to the user. E.g.
612 You can use this option instead of --xyz, but we might remove
613 support for it in future versions.
615 while keeping in mind that you can probably be less verbose, e.g.
617 Use this instead of --xyz. This option might be removed in future
620 - If you still need to refer to an example person that is
621 third-person singular, you may resort to "singular they" to avoid
622 "he/she/him/her", e.g.
624 A contributor asks their upstream to pull from them.
626 Note that this sounds ungrammatical and unnatural to those who
627 learned that "they" is only used for third-person plural, e.g.
628 those who learn English as a second language in some parts of the
631 Every user-visible change should be reflected in the documentation.
632 The same general rule as for code applies -- imitate the existing
635 A few commented examples follow to provide reference when writing or
636 modifying command usage strings and synopsis sections in the manual
639 Placeholders are spelled in lowercase and enclosed in angle brackets:
644 If a placeholder has multiple words, they are separated by dashes:
646 --template=<template-directory>
648 Possibility of multiple occurrences is indicated by three dots:
650 (One or more of <file>.)
652 Optional parts are enclosed in square brackets:
654 (Zero or one <extra>.)
657 (Option with an optional argument. Note that the "=" is inside the
661 (Zero or more of <patch>. Note that the dots are inside, not
662 outside the brackets.)
664 Multiple alternatives are indicated with vertical bars:
668 Parentheses are used for grouping:
669 [(<rev> | <range>)...]
670 (Any number of either <rev> or <range>. Parens are needed to make
671 it clear that "..." pertains to both <rev> and <range>.)
674 (Any number of option -p, each with one <parent> argument.)
676 git remote set-head <name> (-a | -d | <branch>)
677 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
678 brackets) be provided.)
680 And a somewhat more contrived example:
681 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
682 Here "=" is outside the brackets, because "--diff-filter=" is a
683 valid usage. "*" has its own pair of brackets, because it can
684 (optionally) be specified only when one or more of the letters is
688 Use 'git' (all lowercase) when talking about commands i.e. something
689 the user would type into a shell and use 'Git' (uppercase first letter)
690 when talking about the version control system and its properties.
692 A few commented examples follow to provide reference when writing or
693 modifying paragraphs or option/command explanations that contain options
696 Literal examples (e.g. use of command-line options, command names,
697 branch names, URLs, pathnames (files and directories), configuration and
698 environment variables) must be typeset in monospace (i.e. wrapped with
703 `http://git.example.com`
708 An environment variable must be prefixed with "$" only when referring to its
709 value and not when referring to the variable itself, in this case there is
710 nothing to add except the backticks:
711 `GIT_DIR` is specified
712 `$GIT_DIR/hooks/pre-receive`
714 Word phrases enclosed in `backtick characters` are rendered literally
715 and will not be further expanded. The use of `backticks` to achieve the
716 previous rule means that literal examples should not use AsciiDoc
723 If some place in the documentation needs to typeset a command usage
724 example with inline substitutions, it is fine to use +monospaced and
725 inline substituted text+ instead of `monospaced literal text`, and with
726 the former, the part that should not get substituted must be