1 Like other projects, we also have some guidelines to keep to the
2 code. For git in general, three 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.
22 As for more concrete guidelines, just imitate the existing code
23 (this is a good guideline, no matter which project you are
24 contributing to). It is always preferable to match the _local_
25 convention. New code added to git suite is expected to match
26 the overall style of existing code. Modifications to existing
27 code is expected to match the style the surrounding code already
28 uses (even if it doesn't match the overall style of existing code).
30 But if you must have a list of rules, here they are.
32 For shell scripts specifically (not exhaustive):
34 - We use tabs for indentation.
36 - Case arms are indented at the same depth as case and esac lines.
38 - We prefer $( ... ) for command substitution; unlike ``, it
39 properly nests. It should have been the way Bourne spelled
40 it from day one, but unfortunately isn't.
42 - We use POSIX compliant parameter substitutions and avoid bashisms;
45 - We use ${parameter-word} and its [-=?+] siblings, and their
46 colon'ed "unset or null" form.
48 - We use ${parameter#word} and its [#%] siblings, and their
49 doubled "longest matching" form.
51 - No "Substring Expansion" ${parameter:offset:length}.
55 - No strlen ${#parameter}.
57 - No pattern replacement ${parameter/pattern/string}.
59 - We use Arithmetic Expansion $(( ... )).
61 - Inside Arithmetic Expansion, spell shell variables with $ in front
62 of them, as some shells do not grok $((x)) while accepting $(($x))
63 just fine (e.g. dash older than 0.5.4).
65 - We do not use Process Substitution <(list) or >(list).
67 - We prefer "test" over "[ ... ]".
69 - We do not write the noiseword "function" in front of shell
72 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
73 [::], [==], nor [..]) for portability.
75 - We do not use \{m,n\};
79 - We do not use ? nor + (which are \{0,1\} and \{1,\}
80 respectively in BRE) but that goes without saying as these
81 are ERE elements not BRE (note that \? and \+ are not even part
82 of BRE -- making them accessible from BRE is a GNU extension).
84 - Use Git's gettext wrappers in git-sh-i18n to make the user
85 interface translatable. See "Marking strings for translation" in
90 - We use tabs to indent, and interpret tabs as taking up to
93 - We try to keep to at most 80 characters per line.
95 - When declaring pointers, the star sides with the variable
96 name, i.e. "char *string", not "char* string" or
97 "char * string". This makes it easier to understand code
98 like "char *string, c;".
100 - We avoid using braces unnecessarily. I.e.
106 is frowned upon. A gray area is when the statement extends
107 over a few lines, and/or you have a lengthy comment atop of
108 it. Also, like in the Linux kernel, if there is a long list
109 of "else if" statements, it can make sense to add braces to
112 - We try to avoid assignments inside if().
114 - Try to make your code understandable. You may put comments
115 in, but comments invariably tend to stale out when the code
116 they were describing changes. Often splitting a function
117 into two makes the intention of the code much clearer.
119 - Double negation is often harder to understand than no negation
122 - Some clever tricks, like using the !! operator with arithmetic
123 constructs, can be extremely confusing to others. Avoid them,
124 unless there is a compelling reason to use them.
126 - Use the API. No, really. We have a strbuf (variable length
127 string), several arrays with the ALLOC_GROW() macro, a
128 string_list for sorted string lists, a hash map (mapping struct
129 objects) named "struct decorate", amongst other things.
131 - When you come up with an API, document it.
133 - The first #include in C files, except in platform specific
134 compat/ implementations, should be git-compat-util.h or another
135 header file that includes it, such as cache.h or builtin.h.
137 - If you are planning a new command, consider writing it in shell
138 or perl first, so that changes in semantics can be easily
139 changed and discussed. Many git commands started out like
140 that, and a few are still scripts.
142 - Avoid introducing a new dependency into git. This means you
143 usually should stay away from scripting languages not already
144 used in the git core command set (unless your command is clearly
145 separate from it, such as an importer to convert random-scm-X
146 repositories to git).
148 - When we pass <string, length> pair to functions, we should try to
149 pass them in that order.
151 - Use Git's gettext wrappers to make the user interface
152 translatable. See "Marking strings for translation" in po/README.
154 Writing Documentation:
156 Every user-visible change should be reflected in the documentation.
157 The same general rule as for code applies -- imitate the existing
158 conventions. A few commented examples follow to provide reference
159 when writing or modifying command usage strings and synopsis sections
162 Placeholders are spelled in lowercase and enclosed in angle brackets:
167 Possibility of multiple occurrences is indicated by three dots:
169 (One or more of <file>.)
171 Optional parts are enclosed in square brackets:
173 (Zero or one <extra>.)
176 (Option with an optional argument. Note that the "=" is inside the
180 (Zero or more of <patch>. Note that the dots are inside, not
181 outside the brackets.)
183 Multiple alternatives are indicated with vertical bar:
187 Parentheses are used for grouping:
189 (Any number of either <rev> or <range>. Parens are needed to make
190 it clear that "..." pertains to both <rev> and <range>.)
193 (Any number of option -p, each with one <parent> argument.)
195 git remote set-head <name> (-a | -d | <branch>)
196 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
197 brackets) be provided.)
199 And a somewhat more contrived example:
200 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
201 Here "=" is outside the brackets, because "--diff-filter=" is a
202 valid usage. "*" has its own pair of brackets, because it can
203 (optionally) be specified only when one or more of the letters is
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