Git 1.8.4.3
[git/git.git] / Documentation / CodingGuidelines
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6d0618a8 1Like other projects, we also have some guidelines to keep to the
2de9b711 2code. For Git in general, three rough rules are:
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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
c5e366b1 21Make your code readable and sensible, and don't try to be clever.
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22
23As for more concrete guidelines, just imitate the existing code
24(this is a good guideline, no matter which project you are
dfb047b9 25contributing to). It is always preferable to match the _local_
2de9b711 26convention. New code added to Git suite is expected to match
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27the overall style of existing code. Modifications to existing
28code is expected to match the style the surrounding code already
29uses (even if it doesn't match the overall style of existing code).
30
31But if you must have a list of rules, here they are.
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32
33For shell scripts specifically (not exhaustive):
34
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35 - We use tabs for indentation.
36
37 - Case arms are indented at the same depth as case and esac lines.
38
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39 - Redirection operators should be written with space before, but no
40 space after them. In other words, write 'echo test >"$file"'
41 instead of 'echo test> $file' or 'echo test > $file'. Note that
42 even though it is not required by POSIX to double-quote the
43 redirection target in a variable (as shown above), our code does so
44 because some versions of bash issue a warning without the quotes.
45
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46 - We prefer $( ... ) for command substitution; unlike ``, it
47 properly nests. It should have been the way Bourne spelled
48 it from day one, but unfortunately isn't.
49
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50 - If you want to find out if a command is available on the user's
51 $PATH, you should use 'type <command>', instead of 'which <command>'.
52 The output of 'which' is not machine parseable and its exit code
53 is not reliable across platforms.
54
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55 - We use POSIX compliant parameter substitutions and avoid bashisms;
56 namely:
6d0618a8 57
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58 - We use ${parameter-word} and its [-=?+] siblings, and their
59 colon'ed "unset or null" form.
6d0618a8 60
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61 - We use ${parameter#word} and its [#%] siblings, and their
62 doubled "longest matching" form.
6d0618a8 63
bc979945 64 - No "Substring Expansion" ${parameter:offset:length}.
055467dd 65
bc979945 66 - No shell arrays.
6d0618a8 67
bc979945 68 - No strlen ${#parameter}.
6d0618a8 69
bc979945 70 - No pattern replacement ${parameter/pattern/string}.
6d0618a8 71
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72 - We use Arithmetic Expansion $(( ... )).
73
74 - Inside Arithmetic Expansion, spell shell variables with $ in front
75 of them, as some shells do not grok $((x)) while accepting $(($x))
76 just fine (e.g. dash older than 0.5.4).
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77
78 - We do not use Process Substitution <(list) or >(list).
79
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80 - Do not write control structures on a single line with semicolon.
81 "then" should be on the next line for if statements, and "do"
82 should be on the next line for "while" and "for".
83
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84 - We prefer "test" over "[ ... ]".
85
86 - We do not write the noiseword "function" in front of shell
87 functions.
88
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89 - We prefer a space between the function name and the parentheses. The
90 opening "{" should also be on the same line.
91 E.g.: my_function () {
92
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93 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
94 [::], [==], nor [..]) for portability.
95
96 - We do not use \{m,n\};
97
98 - We do not use -E;
99
100 - We do not use ? nor + (which are \{0,1\} and \{1,\}
101 respectively in BRE) but that goes without saying as these
102 are ERE elements not BRE (note that \? and \+ are not even part
103 of BRE -- making them accessible from BRE is a GNU extension).
104
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105 - Use Git's gettext wrappers in git-sh-i18n to make the user
106 interface translatable. See "Marking strings for translation" in
107 po/README.
108
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109For C programs:
110
111 - We use tabs to indent, and interpret tabs as taking up to
112 8 spaces.
113
114 - We try to keep to at most 80 characters per line.
115
2de9b711 116 - We try to support a wide range of C compilers to compile Git with,
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117 including old ones. That means that you should not use C99
118 initializers, even if a lot of compilers grok it.
119
120 - Variables have to be declared at the beginning of the block.
121
122 - NULL pointers shall be written as NULL, not as 0.
123
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124 - When declaring pointers, the star sides with the variable
125 name, i.e. "char *string", not "char* string" or
126 "char * string". This makes it easier to understand code
127 like "char *string, c;".
128
129 - We avoid using braces unnecessarily. I.e.
130
131 if (bla) {
132 x = 1;
133 }
134
135 is frowned upon. A gray area is when the statement extends
136 over a few lines, and/or you have a lengthy comment atop of
137 it. Also, like in the Linux kernel, if there is a long list
138 of "else if" statements, it can make sense to add braces to
139 single line blocks.
140
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141 - We try to avoid assignments inside if().
142
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143 - Try to make your code understandable. You may put comments
144 in, but comments invariably tend to stale out when the code
145 they were describing changes. Often splitting a function
146 into two makes the intention of the code much clearer.
147
148 - Double negation is often harder to understand than no negation
149 at all.
150
151 - Some clever tricks, like using the !! operator with arithmetic
152 constructs, can be extremely confusing to others. Avoid them,
153 unless there is a compelling reason to use them.
154
155 - Use the API. No, really. We have a strbuf (variable length
156 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 157 string_list for sorted string lists, a hash map (mapping struct
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158 objects) named "struct decorate", amongst other things.
159
160 - When you come up with an API, document it.
161
162 - The first #include in C files, except in platform specific
163 compat/ implementations, should be git-compat-util.h or another
164 header file that includes it, such as cache.h or builtin.h.
165
166 - If you are planning a new command, consider writing it in shell
167 or perl first, so that changes in semantics can be easily
2de9b711 168 changed and discussed. Many Git commands started out like
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169 that, and a few are still scripts.
170
2de9b711 171 - Avoid introducing a new dependency into Git. This means you
6d0618a8 172 usually should stay away from scripting languages not already
2de9b711 173 used in the Git core command set (unless your command is clearly
6d0618a8 174 separate from it, such as an importer to convert random-scm-X
2de9b711 175 repositories to Git).
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176
177 - When we pass <string, length> pair to functions, we should try to
178 pass them in that order.
c455bd89 179
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180 - Use Git's gettext wrappers to make the user interface
181 translatable. See "Marking strings for translation" in po/README.
182
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183For Perl programs:
184
185 - Most of the C guidelines above apply.
186
187 - We try to support Perl 5.8 and later ("use Perl 5.008").
188
189 - use strict and use warnings are strongly preferred.
190
191 - Don't overuse statement modifiers unless using them makes the
192 result easier to follow.
193
194 ... do something ...
195 do_this() unless (condition);
196 ... do something else ...
197
198 is more readable than:
199
200 ... do something ...
201 unless (condition) {
202 do_this();
203 }
204 ... do something else ...
205
206 *only* when the condition is so rare that do_this() will be almost
207 always called.
208
209 - We try to avoid assignments inside "if ()" conditions.
210
211 - Learn and use Git.pm if you need that functionality.
212
213 - For Emacs, it's useful to put the following in
214 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
215
216 ;; note the first part is useful for C editing, too
217 ((nil . ((indent-tabs-mode . t)
218 (tab-width . 8)
219 (fill-column . 80)))
220 (cperl-mode . ((cperl-indent-level . 8)
221 (cperl-extra-newline-before-brace . nil)
222 (cperl-merge-trailing-else . t))))
223
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224For Python scripts:
225
226 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
227
228 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
229
230 - Where required libraries do not restrict us to Python 2, we try to
231 also be compatible with Python 3.1 and later.
232
233 - When you must differentiate between Unicode literals and byte string
234 literals, it is OK to use the 'b' prefix. Even though the Python
235 documentation for version 2.6 does not mention this prefix, it has
236 been supported since version 2.6.0.
237
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238Writing Documentation:
239
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240 Most (if not all) of the documentation pages are written in the
241 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
242 processed into HTML and manpages (e.g. git.html and git.1 in the
243 same directory).
bb9f2aec 244
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245 Every user-visible change should be reflected in the documentation.
246 The same general rule as for code applies -- imitate the existing
247 conventions. A few commented examples follow to provide reference
248 when writing or modifying command usage strings and synopsis sections
249 in the manual pages:
250
b1afe49d 251 Placeholders are spelled in lowercase and enclosed in angle brackets:
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252 <file>
253 --sort=<key>
254 --abbrev[=<n>]
255
469bfc96 256 Possibility of multiple occurrences is indicated by three dots:
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257 <file>...
258 (One or more of <file>.)
259
260 Optional parts are enclosed in square brackets:
261 [<extra>]
262 (Zero or one <extra>.)
263
264 --exec-path[=<path>]
265 (Option with an optional argument. Note that the "=" is inside the
266 brackets.)
267
268 [<patch>...]
269 (Zero or more of <patch>. Note that the dots are inside, not
270 outside the brackets.)
271
272 Multiple alternatives are indicated with vertical bar:
273 [-q | --quiet]
274 [--utf8 | --no-utf8]
275
276 Parentheses are used for grouping:
277 [(<rev>|<range>)...]
278 (Any number of either <rev> or <range>. Parens are needed to make
279 it clear that "..." pertains to both <rev> and <range>.)
280
281 [(-p <parent>)...]
282 (Any number of option -p, each with one <parent> argument.)
283
284 git remote set-head <name> (-a | -d | <branch>)
285 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
286 brackets) be provided.)
287
288 And a somewhat more contrived example:
289 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
290 Here "=" is outside the brackets, because "--diff-filter=" is a
291 valid usage. "*" has its own pair of brackets, because it can
292 (optionally) be specified only when one or more of the letters is
293 also provided.
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294
295 A note on notation:
296 Use 'git' (all lowercase) when talking about commands i.e. something
297 the user would type into a shell and use 'Git' (uppercase first letter)
298 when talking about the version control system and its properties.