Fix typos in the documentation
[git/git.git] / Documentation / CodingGuidelines
1 Like other projects, we also have some guidelines to keep to the
2 code. For git in general, three rough rules are:
3
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.
7
8 - However, we often say "Let's stay away from that construct,
9 it's not even in POSIX".
10
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
15 let's use it".
16
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.
20
21
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).
29
30 But if you must have a list of rules, here they are.
31
32 For shell scripts specifically (not exhaustive):
33
34 - We use tabs for indentation.
35
36 - Case arms are indented at the same depth as case and esac lines.
37
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.
41
42 - We use POSIX compliant parameter substitutions and avoid bashisms;
43 namely:
44
45 - We use ${parameter-word} and its [-=?+] siblings, and their
46 colon'ed "unset or null" form.
47
48 - We use ${parameter#word} and its [#%] siblings, and their
49 doubled "longest matching" form.
50
51 - No "Substring Expansion" ${parameter:offset:length}.
52
53 - No shell arrays.
54
55 - No strlen ${#parameter}.
56
57 - No pattern replacement ${parameter/pattern/string}.
58
59 - We use Arithmetic Expansion $(( ... )).
60
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).
64
65 - We do not use Process Substitution <(list) or >(list).
66
67 - We prefer "test" over "[ ... ]".
68
69 - We do not write the noiseword "function" in front of shell
70 functions.
71
72 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
73 [::], [==], nor [..]) for portability.
74
75 - We do not use \{m,n\};
76
77 - We do not use -E;
78
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).
83
84 For C programs:
85
86 - We use tabs to indent, and interpret tabs as taking up to
87 8 spaces.
88
89 - We try to keep to at most 80 characters per line.
90
91 - When declaring pointers, the star sides with the variable
92 name, i.e. "char *string", not "char* string" or
93 "char * string". This makes it easier to understand code
94 like "char *string, c;".
95
96 - We avoid using braces unnecessarily. I.e.
97
98 if (bla) {
99 x = 1;
100 }
101
102 is frowned upon. A gray area is when the statement extends
103 over a few lines, and/or you have a lengthy comment atop of
104 it. Also, like in the Linux kernel, if there is a long list
105 of "else if" statements, it can make sense to add braces to
106 single line blocks.
107
108 - We try to avoid assignments inside if().
109
110 - Try to make your code understandable. You may put comments
111 in, but comments invariably tend to stale out when the code
112 they were describing changes. Often splitting a function
113 into two makes the intention of the code much clearer.
114
115 - Double negation is often harder to understand than no negation
116 at all.
117
118 - Some clever tricks, like using the !! operator with arithmetic
119 constructs, can be extremely confusing to others. Avoid them,
120 unless there is a compelling reason to use them.
121
122 - Use the API. No, really. We have a strbuf (variable length
123 string), several arrays with the ALLOC_GROW() macro, a
124 string_list for sorted string lists, a hash map (mapping struct
125 objects) named "struct decorate", amongst other things.
126
127 - When you come up with an API, document it.
128
129 - The first #include in C files, except in platform specific
130 compat/ implementations, should be git-compat-util.h or another
131 header file that includes it, such as cache.h or builtin.h.
132
133 - If you are planning a new command, consider writing it in shell
134 or perl first, so that changes in semantics can be easily
135 changed and discussed. Many git commands started out like
136 that, and a few are still scripts.
137
138 - Avoid introducing a new dependency into git. This means you
139 usually should stay away from scripting languages not already
140 used in the git core command set (unless your command is clearly
141 separate from it, such as an importer to convert random-scm-X
142 repositories to git).
143
144 - When we pass <string, length> pair to functions, we should try to
145 pass them in that order.
146
147 Writing Documentation:
148
149 Every user-visible change should be reflected in the documentation.
150 The same general rule as for code applies -- imitate the existing
151 conventions. A few commented examples follow to provide reference
152 when writing or modifying command usage strings and synopsis sections
153 in the manual pages:
154
155 Placeholders are enclosed in angle brackets:
156 <file>
157 --sort=<key>
158 --abbrev[=<n>]
159
160 Possibility of multiple occurrences is indicated by three dots:
161 <file>...
162 (One or more of <file>.)
163
164 Optional parts are enclosed in square brackets:
165 [<extra>]
166 (Zero or one <extra>.)
167
168 --exec-path[=<path>]
169 (Option with an optional argument. Note that the "=" is inside the
170 brackets.)
171
172 [<patch>...]
173 (Zero or more of <patch>. Note that the dots are inside, not
174 outside the brackets.)
175
176 Multiple alternatives are indicated with vertical bar:
177 [-q | --quiet]
178 [--utf8 | --no-utf8]
179
180 Parentheses are used for grouping:
181 [(<rev>|<range>)...]
182 (Any number of either <rev> or <range>. Parens are needed to make
183 it clear that "..." pertains to both <rev> and <range>.)
184
185 [(-p <parent>)...]
186 (Any number of option -p, each with one <parent> argument.)
187
188 git remote set-head <name> (-a | -d | <branch>)
189 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
190 brackets) be provided.)
191
192 And a somewhat more contrived example:
193 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
194 Here "=" is outside the brackets, because "--diff-filter=" is a
195 valid usage. "*" has its own pair of brackets, because it can
196 (optionally) be specified only when one or more of the letters is
197 also provided.