CodingGuidelines: give an example for control statements
[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
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21 - Fixing style violations while working on a real change as a
22 preparatory clean-up step is good, but otherwise avoid useless code
23 churn for the sake of conforming to the style.
24
25 "Once it _is_ in the tree, it's not really worth the patch noise to
26 go and fix it up."
27 Cf. http://article.gmane.org/gmane.linux.kernel/943020
28
c5e366b1 29Make your code readable and sensible, and don't try to be clever.
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30
31As for more concrete guidelines, just imitate the existing code
32(this is a good guideline, no matter which project you are
dfb047b9 33contributing to). It is always preferable to match the _local_
2de9b711 34convention. New code added to Git suite is expected to match
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35the overall style of existing code. Modifications to existing
36code is expected to match the style the surrounding code already
37uses (even if it doesn't match the overall style of existing code).
38
39But if you must have a list of rules, here they are.
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40
41For shell scripts specifically (not exhaustive):
42
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43 - We use tabs for indentation.
44
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45 - Case arms are indented at the same depth as case and esac lines,
46 like this:
47
48 case "$variable" in
49 pattern1)
50 do this
51 ;;
52 pattern2)
53 do that
54 ;;
55 esac
f36a4fa8 56
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57 - Redirection operators should be written with space before, but no
58 space after them. In other words, write 'echo test >"$file"'
59 instead of 'echo test> $file' or 'echo test > $file'. Note that
60 even though it is not required by POSIX to double-quote the
61 redirection target in a variable (as shown above), our code does so
62 because some versions of bash issue a warning without the quotes.
63
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64 (incorrect)
65 cat hello > world < universe
66 echo hello >$world
67
68 (correct)
69 cat hello >world <universe
70 echo hello >"$world"
71
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72 - We prefer $( ... ) for command substitution; unlike ``, it
73 properly nests. It should have been the way Bourne spelled
74 it from day one, but unfortunately isn't.
75
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76 - If you want to find out if a command is available on the user's
77 $PATH, you should use 'type <command>', instead of 'which <command>'.
78 The output of 'which' is not machine parseable and its exit code
79 is not reliable across platforms.
80
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81 - We use POSIX compliant parameter substitutions and avoid bashisms;
82 namely:
6d0618a8 83
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84 - We use ${parameter-word} and its [-=?+] siblings, and their
85 colon'ed "unset or null" form.
6d0618a8 86
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87 - We use ${parameter#word} and its [#%] siblings, and their
88 doubled "longest matching" form.
6d0618a8 89
bc979945 90 - No "Substring Expansion" ${parameter:offset:length}.
055467dd 91
bc979945 92 - No shell arrays.
6d0618a8 93
bc979945 94 - No strlen ${#parameter}.
6d0618a8 95
bc979945 96 - No pattern replacement ${parameter/pattern/string}.
6d0618a8 97
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98 - We use Arithmetic Expansion $(( ... )).
99
100 - Inside Arithmetic Expansion, spell shell variables with $ in front
101 of them, as some shells do not grok $((x)) while accepting $(($x))
102 just fine (e.g. dash older than 0.5.4).
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103
104 - We do not use Process Substitution <(list) or >(list).
105
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106 - Do not write control structures on a single line with semicolon.
107 "then" should be on the next line for if statements, and "do"
108 should be on the next line for "while" and "for".
109
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110 (incorrect)
111 if test -f hello; then
112 do this
113 fi
114
115 (correct)
116 if test -f hello
117 then
118 do this
119 fi
120
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121 - We prefer "test" over "[ ... ]".
122
123 - We do not write the noiseword "function" in front of shell
124 functions.
125
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126 - We prefer a space between the function name and the parentheses. The
127 opening "{" should also be on the same line.
128 E.g.: my_function () {
129
009c98ee 130 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 131 [::], [==], or [..]) for portability.
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132
133 - We do not use \{m,n\};
134
135 - We do not use -E;
136
a58088ab 137 - We do not use ? or + (which are \{0,1\} and \{1,\}
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138 respectively in BRE) but that goes without saying as these
139 are ERE elements not BRE (note that \? and \+ are not even part
140 of BRE -- making them accessible from BRE is a GNU extension).
141
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142 - Use Git's gettext wrappers in git-sh-i18n to make the user
143 interface translatable. See "Marking strings for translation" in
144 po/README.
145
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146For C programs:
147
148 - We use tabs to indent, and interpret tabs as taking up to
149 8 spaces.
150
151 - We try to keep to at most 80 characters per line.
152
2de9b711 153 - We try to support a wide range of C compilers to compile Git with,
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154 including old ones. That means that you should not use C99
155 initializers, even if a lot of compilers grok it.
156
157 - Variables have to be declared at the beginning of the block.
158
159 - NULL pointers shall be written as NULL, not as 0.
160
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161 - When declaring pointers, the star sides with the variable
162 name, i.e. "char *string", not "char* string" or
163 "char * string". This makes it easier to understand code
164 like "char *string, c;".
165
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166 - Use whitespace around operators and keywords, but not inside
167 parentheses and not around functions. So:
168
169 while (condition)
170 func(bar + 1);
171
172 and not:
173
174 while( condition )
175 func (bar+1);
176
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177 - We avoid using braces unnecessarily. I.e.
178
179 if (bla) {
180 x = 1;
181 }
182
183 is frowned upon. A gray area is when the statement extends
184 over a few lines, and/or you have a lengthy comment atop of
185 it. Also, like in the Linux kernel, if there is a long list
186 of "else if" statements, it can make sense to add braces to
187 single line blocks.
188
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189 - We try to avoid assignments inside if().
190
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191 - Try to make your code understandable. You may put comments
192 in, but comments invariably tend to stale out when the code
193 they were describing changes. Often splitting a function
194 into two makes the intention of the code much clearer.
195
b75a6ca7 196 - Multi-line comments include their delimiters on separate lines from
197 the text. E.g.
198
199 /*
200 * A very long
201 * multi-line comment.
202 */
203
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204 Note however that a comment that explains a translatable string to
205 translators uses a convention of starting with a magic token
206 "TRANSLATORS: " immediately after the opening delimiter, even when
207 it spans multiple lines. We do not add an asterisk at the beginning
208 of each line, either. E.g.
209
210 /* TRANSLATORS: here is a comment that explains the string
211 to be translated, that follows immediately after it */
212 _("Here is a translatable string explained by the above.");
213
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214 - Double negation is often harder to understand than no negation
215 at all.
216
217 - Some clever tricks, like using the !! operator with arithmetic
218 constructs, can be extremely confusing to others. Avoid them,
219 unless there is a compelling reason to use them.
220
221 - Use the API. No, really. We have a strbuf (variable length
222 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 223 string_list for sorted string lists, a hash map (mapping struct
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224 objects) named "struct decorate", amongst other things.
225
226 - When you come up with an API, document it.
227
228 - The first #include in C files, except in platform specific
229 compat/ implementations, should be git-compat-util.h or another
230 header file that includes it, such as cache.h or builtin.h.
231
232 - If you are planning a new command, consider writing it in shell
233 or perl first, so that changes in semantics can be easily
2de9b711 234 changed and discussed. Many Git commands started out like
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235 that, and a few are still scripts.
236
2de9b711 237 - Avoid introducing a new dependency into Git. This means you
6d0618a8 238 usually should stay away from scripting languages not already
2de9b711 239 used in the Git core command set (unless your command is clearly
6d0618a8 240 separate from it, such as an importer to convert random-scm-X
2de9b711 241 repositories to Git).
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242
243 - When we pass <string, length> pair to functions, we should try to
244 pass them in that order.
c455bd89 245
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246 - Use Git's gettext wrappers to make the user interface
247 translatable. See "Marking strings for translation" in po/README.
248
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249For Perl programs:
250
251 - Most of the C guidelines above apply.
252
253 - We try to support Perl 5.8 and later ("use Perl 5.008").
254
255 - use strict and use warnings are strongly preferred.
256
257 - Don't overuse statement modifiers unless using them makes the
258 result easier to follow.
259
260 ... do something ...
261 do_this() unless (condition);
262 ... do something else ...
263
264 is more readable than:
265
266 ... do something ...
267 unless (condition) {
268 do_this();
269 }
270 ... do something else ...
271
272 *only* when the condition is so rare that do_this() will be almost
273 always called.
274
275 - We try to avoid assignments inside "if ()" conditions.
276
277 - Learn and use Git.pm if you need that functionality.
278
279 - For Emacs, it's useful to put the following in
280 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
281
282 ;; note the first part is useful for C editing, too
283 ((nil . ((indent-tabs-mode . t)
284 (tab-width . 8)
285 (fill-column . 80)))
286 (cperl-mode . ((cperl-indent-level . 8)
287 (cperl-extra-newline-before-brace . nil)
288 (cperl-merge-trailing-else . t))))
289
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290For Python scripts:
291
292 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
293
294 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
295
296 - Where required libraries do not restrict us to Python 2, we try to
297 also be compatible with Python 3.1 and later.
298
299 - When you must differentiate between Unicode literals and byte string
300 literals, it is OK to use the 'b' prefix. Even though the Python
301 documentation for version 2.6 does not mention this prefix, it has
302 been supported since version 2.6.0.
303
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304Writing Documentation:
305
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306 Most (if not all) of the documentation pages are written in the
307 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
308 processed into HTML and manpages (e.g. git.html and git.1 in the
309 same directory).
bb9f2aec 310
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311 The documentation liberally mixes US and UK English (en_US/UK)
312 norms for spelling and grammar, which is somewhat unfortunate.
313 In an ideal world, it would have been better if it consistently
314 used only one and not the other, and we would have picked en_US
315 (if you wish to correct the English of some of the existing
316 documentation, please see the documentation-related advice in the
317 Documentation/SubmittingPatches file).
318
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319 Every user-visible change should be reflected in the documentation.
320 The same general rule as for code applies -- imitate the existing
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321 conventions.
322
323 A few commented examples follow to provide reference when writing or
324 modifying command usage strings and synopsis sections in the manual
325 pages:
c455bd89 326
b1afe49d 327 Placeholders are spelled in lowercase and enclosed in angle brackets:
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328 <file>
329 --sort=<key>
330 --abbrev[=<n>]
331
469bfc96 332 Possibility of multiple occurrences is indicated by three dots:
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333 <file>...
334 (One or more of <file>.)
335
336 Optional parts are enclosed in square brackets:
337 [<extra>]
338 (Zero or one <extra>.)
339
340 --exec-path[=<path>]
341 (Option with an optional argument. Note that the "=" is inside the
342 brackets.)
343
344 [<patch>...]
345 (Zero or more of <patch>. Note that the dots are inside, not
346 outside the brackets.)
347
348 Multiple alternatives are indicated with vertical bar:
349 [-q | --quiet]
350 [--utf8 | --no-utf8]
351
352 Parentheses are used for grouping:
353 [(<rev>|<range>)...]
354 (Any number of either <rev> or <range>. Parens are needed to make
355 it clear that "..." pertains to both <rev> and <range>.)
356
357 [(-p <parent>)...]
358 (Any number of option -p, each with one <parent> argument.)
359
360 git remote set-head <name> (-a | -d | <branch>)
361 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
362 brackets) be provided.)
363
364 And a somewhat more contrived example:
365 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
366 Here "=" is outside the brackets, because "--diff-filter=" is a
367 valid usage. "*" has its own pair of brackets, because it can
368 (optionally) be specified only when one or more of the letters is
369 also provided.
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370
371 A note on notation:
372 Use 'git' (all lowercase) when talking about commands i.e. something
373 the user would type into a shell and use 'Git' (uppercase first letter)
374 when talking about the version control system and its properties.
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375
376 A few commented examples follow to provide reference when writing or
377 modifying paragraphs or option/command explanations that contain options
378 or commands:
379
380 Literal examples (e.g. use of command-line options, command names, and
381 configuration variables) are typeset in monospace, and if you can use
382 `backticks around word phrases`, do so.
383 `--pretty=oneline`
384 `git rev-list`
385 `remote.pushdefault`
386
387 Word phrases enclosed in `backtick characters` are rendered literally
388 and will not be further expanded. The use of `backticks` to achieve the
389 previous rule means that literal examples should not use AsciiDoc
390 escapes.
391 Correct:
392 `--pretty=oneline`
393 Incorrect:
394 `\--pretty=oneline`
395
396 If some place in the documentation needs to typeset a command usage
397 example with inline substitutions, it is fine to use +monospaced and
398 inline substituted text+ instead of `monospaced literal text`, and with
399 the former, the part that should not get substituted must be
400 quoted/escaped.