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Like other projects, we also have some guidelines to keep to the
code. For Git in general, three 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.
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.
- 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.
- 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 parseable 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 strlen ${#parameter}.
- No pattern replacement ${parameter/pattern/string}.
- We use Arithmetic Expansion $(( ... )).
- Inside Arithmetic Expansion, spell shell variables with $ in front
of them, as some shells do not grok $((x)) while accepting $(($x))
just fine (e.g. dash older than 0.5.4).
- 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".
- 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. The
opening "{" should also be on the same line.
E.g.: my_function () {
- As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
[::], [==], nor [..]) for portability.
- We do not use \{m,n\};
- We do not use -E;
- We do not use ? nor + (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.
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.
- We try to support a wide range of C compilers to compile Git with,
including old ones. That means that you should not use C99
initializers, even if a lot of compilers grok it.
- Variables have to be declared at the beginning of the block.
- 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;".
- We avoid using braces unnecessarily. I.e.
if (bla) {
x = 1;
}
is frowned upon. A gray area is when the statement extends
over a few lines, and/or you have a lengthy comment atop of
it. Also, like in the Linux kernel, if there is a long list
of "else if" statements, it can make sense to add braces to
single line blocks.
- We try to avoid assignments inside if().
- 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.
- Double negation is often harder to understand than no negation
at all.
- 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 it.
- The first #include in C files, except in platform specific
compat/ implementations, should be git-compat-util.h or another
header file that includes it, such as cache.h or builtin.h.
- 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.
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 2.6 and 2.7.
- Where required libraries do not restrict us to Python 2, we try to
also be compatible with Python 3.1 and later.
- When you must differentiate between Unicode literals and byte string
literals, it is OK to use the 'b' prefix. Even though the Python
documentation for version 2.6 does not mention this prefix, it has
been supported since version 2.6.0.
Writing Documentation:
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>]
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 bar:
[-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.
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