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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 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).
 
 - We prefer "test" over "[ ... ]".
 
 - We do not write the noiseword "function" in front of shell
   functions.
 
 - 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.