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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 prefer $( ... ) for command substitution; unlike ``, it
properly nests. It should have been the way Bourne spelled
it from day one, but unfortunately isn't.
- 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.
- We use Arithmetic Expansion $(( ... )).
- No "Substring Expansion" ${parameter:offset:length}.
- No shell arrays.
- No strlen ${#parameter}.
- No regexp ${parameter/pattern/string}.
- We do not use Process Substitution <(list) or >(list).
- We prefer "test" over "[ ... ]".
- We do not write the noiseword "function" in front of shell
- 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).
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.
- 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.