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 - Redirection operators should be written with space before, but no
39 space after them. In other words, write 'echo test >"$file"'
40 instead of 'echo test> $file' or 'echo test > $file'. Note that
41 even though it is not required by POSIX to double-quote the
42 redirection target in a variable (as shown above), our code does so
43 because some versions of bash issue a warning without the quotes.
45 - We prefer $( ... ) for command substitution; unlike ``, it
46 properly nests. It should have been the way Bourne spelled
47 it from day one, but unfortunately isn't.
49 - If you want to find out if a command is available on the user's
50 $PATH, you should use 'type <command>', instead of 'which <command>'.
51 The output of 'which' is not machine parseable and its exit code
52 is not reliable across platforms.
54 - We use POSIX compliant parameter substitutions and avoid bashisms;
57 - We use ${parameter-word} and its [-=?+] siblings, and their
58 colon'ed "unset or null" form.
60 - We use ${parameter#word} and its [#%] siblings, and their
61 doubled "longest matching" form.
63 - No "Substring Expansion" ${parameter:offset:length}.
67 - No strlen ${#parameter}.
69 - No pattern replacement ${parameter/pattern/string}.
71 - We use Arithmetic Expansion $(( ... )).
73 - Inside Arithmetic Expansion, spell shell variables with $ in front
74 of them, as some shells do not grok $((x)) while accepting $(($x))
75 just fine (e.g. dash older than 0.5.4).
77 - We do not use Process Substitution <(list) or >(list).
79 - Do not write control structures on a single line with semicolon.
80 "then" should be on the next line for if statements, and "do"
81 should be on the next line for "while" and "for".
83 - We prefer "test" over "[ ... ]".
85 - We do not write the noiseword "function" in front of shell
88 - We prefer a space between the function name and the parentheses. The
89 opening "{" should also be on the same line.
90 E.g.: my_function () {
92 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
93 [::], [==], nor [..]) for portability.
95 - We do not use \{m,n\};
99 - We do not use ? nor + (which are \{0,1\} and \{1,\}
100 respectively in BRE) but that goes without saying as these
101 are ERE elements not BRE (note that \? and \+ are not even part
102 of BRE -- making them accessible from BRE is a GNU extension).
104 - Use Git's gettext wrappers in git-sh-i18n to make the user
105 interface translatable. See "Marking strings for translation" in
110 - We use tabs to indent, and interpret tabs as taking up to
113 - We try to keep to at most 80 characters per line.
115 - When declaring pointers, the star sides with the variable
116 name, i.e. "char *string", not "char* string" or
117 "char * string". This makes it easier to understand code
118 like "char *string, c;".
120 - We avoid using braces unnecessarily. I.e.
126 is frowned upon. A gray area is when the statement extends
127 over a few lines, and/or you have a lengthy comment atop of
128 it. Also, like in the Linux kernel, if there is a long list
129 of "else if" statements, it can make sense to add braces to
132 - We try to avoid assignments inside if().
134 - Try to make your code understandable. You may put comments
135 in, but comments invariably tend to stale out when the code
136 they were describing changes. Often splitting a function
137 into two makes the intention of the code much clearer.
139 - Double negation is often harder to understand than no negation
142 - Some clever tricks, like using the !! operator with arithmetic
143 constructs, can be extremely confusing to others. Avoid them,
144 unless there is a compelling reason to use them.
146 - Use the API. No, really. We have a strbuf (variable length
147 string), several arrays with the ALLOC_GROW() macro, a
148 string_list for sorted string lists, a hash map (mapping struct
149 objects) named "struct decorate", amongst other things.
151 - When you come up with an API, document it.
153 - The first #include in C files, except in platform specific
154 compat/ implementations, should be git-compat-util.h or another
155 header file that includes it, such as cache.h or builtin.h.
157 - If you are planning a new command, consider writing it in shell
158 or perl first, so that changes in semantics can be easily
159 changed and discussed. Many git commands started out like
160 that, and a few are still scripts.
162 - Avoid introducing a new dependency into git. This means you
163 usually should stay away from scripting languages not already
164 used in the git core command set (unless your command is clearly
165 separate from it, such as an importer to convert random-scm-X
166 repositories to git).
168 - When we pass <string, length> pair to functions, we should try to
169 pass them in that order.
171 - Use Git's gettext wrappers to make the user interface
172 translatable. See "Marking strings for translation" in po/README.
174 Writing Documentation:
176 Every user-visible change should be reflected in the documentation.
177 The same general rule as for code applies -- imitate the existing
178 conventions. A few commented examples follow to provide reference
179 when writing or modifying command usage strings and synopsis sections
182 Placeholders are spelled in lowercase and enclosed in angle brackets:
187 Possibility of multiple occurrences is indicated by three dots:
189 (One or more of <file>.)
191 Optional parts are enclosed in square brackets:
193 (Zero or one <extra>.)
196 (Option with an optional argument. Note that the "=" is inside the
200 (Zero or more of <patch>. Note that the dots are inside, not
201 outside the brackets.)
203 Multiple alternatives are indicated with vertical bar:
207 Parentheses are used for grouping:
209 (Any number of either <rev> or <range>. Parens are needed to make
210 it clear that "..." pertains to both <rev> and <range>.)
213 (Any number of option -p, each with one <parent> argument.)
215 git remote set-head <name> (-a | -d | <branch>)
216 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
217 brackets) be provided.)
219 And a somewhat more contrived example:
220 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
221 Here "=" is outside the brackets, because "--diff-filter=" is a
222 valid usage. "*" has its own pair of brackets, because it can
223 (optionally) be specified only when one or more of the letters is
projects / git / blob