completion: fix zsh check under bash with 'set -u'
[git/git.git] / Documentation / CodingGuidelines
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1Like other projects, we also have some guidelines to keep to the
2code. 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
22As for more concrete guidelines, just imitate the existing code
23(this is a good guideline, no matter which project you are
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24contributing to). It is always preferable to match the _local_
25convention. New code added to git suite is expected to match
26the overall style of existing code. Modifications to existing
27code is expected to match the style the surrounding code already
28uses (even if it doesn't match the overall style of existing code).
29
30But if you must have a list of rules, here they are.
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31
32For shell scripts specifically (not exhaustive):
33
34 - We prefer $( ... ) for command substitution; unlike ``, it
35 properly nests. It should have been the way Bourne spelled
36 it from day one, but unfortunately isn't.
37
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38 - We use POSIX compliant parameter substitutions and avoid bashisms;
39 namely:
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41 - We use ${parameter-word} and its [-=?+] siblings, and their
42 colon'ed "unset or null" form.
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44 - We use ${parameter#word} and its [#%] siblings, and their
45 doubled "longest matching" form.
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bc979945 47 - No "Substring Expansion" ${parameter:offset:length}.
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bc979945 49 - No shell arrays.
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bc979945 51 - No strlen ${#parameter}.
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bc979945 53 - No pattern replacement ${parameter/pattern/string}.
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55 - We use Arithmetic Expansion $(( ... )).
56
57 - Inside Arithmetic Expansion, spell shell variables with $ in front
58 of them, as some shells do not grok $((x)) while accepting $(($x))
59 just fine (e.g. dash older than 0.5.4).
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60
61 - We do not use Process Substitution <(list) or >(list).
62
63 - We prefer "test" over "[ ... ]".
64
65 - We do not write the noiseword "function" in front of shell
66 functions.
67
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68 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
69 [::], [==], nor [..]) for portability.
70
71 - We do not use \{m,n\};
72
73 - We do not use -E;
74
75 - We do not use ? nor + (which are \{0,1\} and \{1,\}
76 respectively in BRE) but that goes without saying as these
77 are ERE elements not BRE (note that \? and \+ are not even part
78 of BRE -- making them accessible from BRE is a GNU extension).
79
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80For C programs:
81
82 - We use tabs to indent, and interpret tabs as taking up to
83 8 spaces.
84
85 - We try to keep to at most 80 characters per line.
86
87 - When declaring pointers, the star sides with the variable
88 name, i.e. "char *string", not "char* string" or
89 "char * string". This makes it easier to understand code
90 like "char *string, c;".
91
92 - We avoid using braces unnecessarily. I.e.
93
94 if (bla) {
95 x = 1;
96 }
97
98 is frowned upon. A gray area is when the statement extends
99 over a few lines, and/or you have a lengthy comment atop of
100 it. Also, like in the Linux kernel, if there is a long list
101 of "else if" statements, it can make sense to add braces to
102 single line blocks.
103
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104 - We try to avoid assignments inside if().
105
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106 - Try to make your code understandable. You may put comments
107 in, but comments invariably tend to stale out when the code
108 they were describing changes. Often splitting a function
109 into two makes the intention of the code much clearer.
110
111 - Double negation is often harder to understand than no negation
112 at all.
113
114 - Some clever tricks, like using the !! operator with arithmetic
115 constructs, can be extremely confusing to others. Avoid them,
116 unless there is a compelling reason to use them.
117
118 - Use the API. No, really. We have a strbuf (variable length
119 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 120 string_list for sorted string lists, a hash map (mapping struct
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121 objects) named "struct decorate", amongst other things.
122
123 - When you come up with an API, document it.
124
125 - The first #include in C files, except in platform specific
126 compat/ implementations, should be git-compat-util.h or another
127 header file that includes it, such as cache.h or builtin.h.
128
129 - If you are planning a new command, consider writing it in shell
130 or perl first, so that changes in semantics can be easily
131 changed and discussed. Many git commands started out like
132 that, and a few are still scripts.
133
134 - Avoid introducing a new dependency into git. This means you
135 usually should stay away from scripting languages not already
136 used in the git core command set (unless your command is clearly
137 separate from it, such as an importer to convert random-scm-X
138 repositories to git).
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139
140 - When we pass <string, length> pair to functions, we should try to
141 pass them in that order.