blob: 1d92b2da03e8ca4f6f562ed55e2a6d9199b592c3 [file] [log] [blame]
Like other projects, we also have some guidelines for our 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."
- Log messages to explain your changes are as important as the
changes themselves. Clearly written code and in-code comments
explain how the code works and what is assumed from the surrounding
context. The log messages explain what the changes wanted to
achieve and why the changes were necessary (more on this in the
accompanying SubmittingPatches document).
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 are 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 are some language
specific ones. Note that Documentation/ToolsForGit.txt document
has a collection of tips to help you use some external tools
to conform to these guidelines.
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 "$variable" in
do this
do that
- Redirection operators should be written with space before, but no
space after them. In other words, write 'echo test >"$file"'
instead of 'echo test> $file' or 'echo test > $file'. Note that
even though it is not required by POSIX to double-quote the
redirection target in a variable (as shown above), our code does so
because some versions of bash issue a warning without the quotes.
cat hello > world < universe
echo hello >$world
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;
- 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 $(( ... )).
- We do not use Process Substitution <(list) or >(list).
- Do not write control structures on a single line with semicolon.
"then" should be on the next line for if statements, and "do"
should be on the next line for "while" and "for".
if test -f hello; then
do this
if test -f hello
do this
- If a command sequence joined with && or || or | spans multiple
lines, put each command on a separate line and put && and || and |
operators at the end of each line, rather than the start. This
means you don't need to use \ to join lines, since the above
operators imply the sequence isn't finished.
grep blob verify_pack_result \
| awk -f print_1.awk \
| sort >actual &&
grep blob verify_pack_result |
awk -f print_1.awk |
sort >actual &&
- We prefer "test" over "[ ... ]".
- We do not write the noiseword "function" in front of shell
- We prefer a space between the function name and the parentheses,
and no space inside the parentheses. The opening "{" should also
be on the same line.
my_function () {
- As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
[::], [==], or [..]) for portability.
- We do not use \{m,n\};
- We do not use ? or + (which are \{0,1\} and \{1,\}
respectively in BRE) but that goes without saying as these
are ERE elements not BRE (note that \? and \+ are not even part
of BRE -- making them accessible from BRE is a GNU extension).
- Use Git's gettext wrappers in git-sh-i18n to make the user
interface translatable. See "Marking strings for translation" in
- We do not write our "test" command with "-a" and "-o" and use "&&"
or "||" to concatenate multiple "test" commands instead, because
the use of "-a/-o" is often error-prone. E.g.
test -n "$x" -a "$a" = "$b"
is buggy and breaks when $x is "=", but
test -n "$x" && test "$a" = "$b"
does not have such a problem.
- Even though "local" is not part of POSIX, we make heavy use of it
in our test suite. We do not use it in scripted Porcelains, and
hopefully nobody starts using "local" before they are reimplemented
in C ;-)
- Some versions of shell do not understand "export variable=value",
so we write "variable=value" and then "export variable" on two
separate lines.
- Some versions of dash have broken variable assignment when prefixed
with "local", "export", and "readonly", in that the value to be
assigned goes through field splitting at $IFS unless quoted.
local variable=$value
local variable=$(command args)
local variable="$value"
local variable="$(command args)"
- Use octal escape sequences (e.g. "\302\242"), not hexadecimal (e.g.
"\xc2\xa2") in printf format strings, since hexadecimal escape
sequences are not portable.
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. As of Git v2.35.0 Git requires C99 (we check
"__STDC_VERSION__"). You should not use features from a newer C
standard, even if your compiler groks them.
New C99 features have been phased in gradually, if something's new
in C99 but not used yet don't assume that it's safe to use, some
compilers we target have only partial support for it. These are
considered safe to use:
. since around 2007 with 2b6854c863a, we have been using
initializer elements which are not computable at load time. E.g.:
const char *args[] = {"constant", variable, NULL};
. 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 }").
. since early 2021 with 765dc168882, we have been using variadic
macros, mostly for printf-like trace and debug macros.
. since late 2021 with 44ba10d6, we have had variables declared in
the for loop "for (int i = 0; i < 10; i++)".
New C99 features that we cannot use yet:
. %z and %zu as a printf() argument for a size_t (the %z being for
the POSIX-specific ssize_t). Instead you should use
printf("%"PRIuMAX, (uintmax_t)v). These days the MSVC version we
rely on supports %z, but the C library used by MinGW does not.
. Shorthand like ".a.b = *c" in struct initializations is known to
trip up an older IBM XLC version, use ".a = { .b = *c }" instead.
See the 33665d98 (reftable: make assignments portable to AIX xlc
v12.01, 2022-03-28).
- Variables have to be declared at the beginning of the block, before
the first statement (i.e. -Wdeclaration-after-statement).
- 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)
if (foo) {
* This one requires some explanation,
* so we're better off with braces to make
* it obvious that the indentation is correct.
- 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) {
} else {
- 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: 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;
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;
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
- 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
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) {
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
- The first #include in C files, except in platform specific compat/
implementations and sha1dc/, must be <git-compat-util.h>. This
header file insulates other header files and source files from
platform differences, like which system header files must be
included in what order, and what C preprocessor feature macros must
be defined to trigger certain features we expect out of the system.
A collorary to this is that C files should not directly include
system header files themselves.
There are some exceptions, because certain group of files that
implement an API all have to include the same header file that
defines the API and it is convenient to include <git-compat-util.h>
there. Namely:
- the implementation of the built-in commands in the "builtin/"
directory that include "builtin.h" for the cmd_foo() prototype
- the test helper programs in the "t/helper/" directory that include
"t/helper/test-tool.h" for the cmd__foo() prototype definition,
- the xdiff implementation in the "xdiff/" directory that includes
"xdiff/xinclude.h" for the xdiff machinery internals,
- the unit test programs in "t/unit-tests/" directory that include
"t/unit-tests/test-lib.h" that gives them the unit-tests
framework, and
- the source files that implement reftable in the "reftable/"
directory that include "reftable/system.h" for the reftable
are allowed to assume that they do not have to include
<git-compat-util.h> themselves, as it is included as the first
'#include' in these header files. These headers must be the first
header file to be "#include"d in them, though.
- 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 ``.)
For Perl programs:
- Most of the C guidelines above apply.
- We try to support Perl 5.8.1 and later ("use Perl 5.008001").
- 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 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 if you need that functionality.
For Python scripts:
- We follow PEP-8 (
- 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.
Program Output
We make a distinction between a Git command's primary output and
output which is merely chatty feedback (for instance, status
messages, running transcript, or progress display), as well as error
messages. Roughly speaking, a Git command's primary output is that
which one might want to capture to a file or send down a pipe; its
chatty output should not interfere with these use-cases.
As such, primary output should be sent to the standard output stream
(stdout), and chatty output should be sent to the standard error
stream (stderr). Examples of commands which produce primary output
include `git log`, `git show`, and `git branch --list` which generate
output on the stdout stream.
Not all Git commands have primary output; this is often true of
commands whose main function is to perform an action. Some action
commands are silent, whereas others are chatty. An example of a
chatty action commands is `git clone` with its "Cloning into
'<path>'..." and "Checking connectivity..." status messages which it
sends to the stderr stream.
Error messages from Git commands should always be sent to the stderr
Error Messages
- Do not end error messages with a full stop.
- Do not capitalize the first word, only because it is the first word
in the message ("unable to open %s", not "Unable to open %s"). But
"SHA-3 not supported" is fine, because the reason the first word is
capitalized is not because it is at the beginning of the sentence,
but because the word would be spelled in capital letters even when
it appeared in the middle of the sentence.
- 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 punctuation marks (e.g. `-`),
and are broken using bumpyCaps in documentation as a hint to the
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).
In order to ensure the documentation is inclusive, avoid assuming
that an unspecified example person is male or female, and think
twice before using "he", "him", "she", or "her". Here are some
tips to avoid use of gendered pronouns:
- Prefer succinctness and matter-of-factly describing functionality
in the abstract. E.g.
`--short`:: Emit output in the short-format.
and avoid something like these overly verbose alternatives:
`--short`:: Use this to emit output in the short-format.
`--short`:: You can use this to get output in the short-format.
`--short`:: A user who prefers shorter output could....
`--short`:: Should a person and/or program want shorter output, he
she/they/it can...
This practice often eliminates the need to involve human actors in
your description, but it is a good practice regardless of the
avoidance of gendered pronouns.
- When it becomes awkward to stick to this style, prefer "you" when
addressing the hypothetical user, and possibly "we" when
discussing how the program might react to the user. E.g.
You can use this option instead of `--xyz`, but we might remove
support for it in future versions.
while keeping in mind that you can probably be less verbose, e.g.
Use this instead of `--xyz`. This option might be removed in future
- If you still need to refer to an example person that is
third-person singular, you may resort to "singular they" to avoid
"he/she/him/her", e.g.
A contributor asks their upstream to pull from them.
Note that this sounds ungrammatical and unnatural to those who
learned that "they" is only used for third-person plural, e.g.
those who learn English as a second language in some parts of the
Every user-visible change should be reflected in the documentation.
The same general rule as for code applies -- imitate the existing
Literal parts (e.g. use of command-line options, command names,
branch names, URLs, pathnames (files and directories), configuration and
environment variables) must be typeset as verbatim (i.e. wrapped with
`git rev-list`
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
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
Placeholders are spelled in lowercase and enclosed in
angle brackets surrounded by underscores:
If a placeholder has multiple words, they are separated by dashes:
A placeholder is not enclosed in backticks, as it is not a literal.
When needed, use a distinctive identifier for placeholders, usually
made of a qualification and a type:
When literal and placeholders are mixed, each markup is applied for
each sub-entity. If they are stuck, a special markup, called
unconstrained formatting is required.
Unconstrained formating for placeholders is __<like-this>__
Unconstrained formatting for literal formatting is ++like this++
`--jobs` _<n>_
caveat: ++ unconstrained format is not verbatim and may expand
content. Use Asciidoc escapes inside them.
Synopsis Syntax
Syntax grammar is formatted neither as literal nor as placeholder.
A few commented examples follow to provide reference when writing or
modifying command usage strings and synopsis sections in the manual
Possibility of multiple occurrences is indicated by three dots:
(One or more of <file>.)
Optional parts are enclosed in square brackets:
(Zero or more of <file>.)
(Option with an optional argument. Note that the "=" is inside the
(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`]
Use spacing around "|" token(s), but not immediately after opening or
before closing a [] or () pair:
Do: [`-q` | `--quiet`]
Don't: [`-q`|`--quiet`]
Don't use spacing around "|" tokens when they're used to separate the
alternate arguments of an option:
Do: ++--track++[++=++(`direct`|`inherit`)]`
Don't: ++--track++[++=++(`direct` | `inherit`)]
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:
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.
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