CodingGuidelines: on comparison
[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,
127 and no space inside the parentheses. The opening "{" should also
128 be on the same line.
129
130 (incorrect)
131 my_function(){
132 ...
133
134 (correct)
135 my_function () {
136 ...
03b05c7d 137
009c98ee 138 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 139 [::], [==], or [..]) for portability.
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140
141 - We do not use \{m,n\};
142
143 - We do not use -E;
144
a58088ab 145 - We do not use ? or + (which are \{0,1\} and \{1,\}
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146 respectively in BRE) but that goes without saying as these
147 are ERE elements not BRE (note that \? and \+ are not even part
148 of BRE -- making them accessible from BRE is a GNU extension).
149
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150 - Use Git's gettext wrappers in git-sh-i18n to make the user
151 interface translatable. See "Marking strings for translation" in
152 po/README.
153
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154For C programs:
155
156 - We use tabs to indent, and interpret tabs as taking up to
157 8 spaces.
158
159 - We try to keep to at most 80 characters per line.
160
2de9b711 161 - We try to support a wide range of C compilers to compile Git with,
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162 including old ones. That means that you should not use C99
163 initializers, even if a lot of compilers grok it.
164
165 - Variables have to be declared at the beginning of the block.
166
167 - NULL pointers shall be written as NULL, not as 0.
168
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169 - When declaring pointers, the star sides with the variable
170 name, i.e. "char *string", not "char* string" or
171 "char * string". This makes it easier to understand code
172 like "char *string, c;".
173
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174 - Use whitespace around operators and keywords, but not inside
175 parentheses and not around functions. So:
176
177 while (condition)
178 func(bar + 1);
179
180 and not:
181
182 while( condition )
183 func (bar+1);
184
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185 - We avoid using braces unnecessarily. I.e.
186
187 if (bla) {
188 x = 1;
189 }
190
191 is frowned upon. A gray area is when the statement extends
192 over a few lines, and/or you have a lengthy comment atop of
193 it. Also, like in the Linux kernel, if there is a long list
194 of "else if" statements, it can make sense to add braces to
195 single line blocks.
196
691d0dd0 197 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 198
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199 - Try to make your code understandable. You may put comments
200 in, but comments invariably tend to stale out when the code
201 they were describing changes. Often splitting a function
202 into two makes the intention of the code much clearer.
203
b75a6ca7 204 - Multi-line comments include their delimiters on separate lines from
205 the text. E.g.
206
207 /*
208 * A very long
209 * multi-line comment.
210 */
211
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212 Note however that a comment that explains a translatable string to
213 translators uses a convention of starting with a magic token
214 "TRANSLATORS: " immediately after the opening delimiter, even when
215 it spans multiple lines. We do not add an asterisk at the beginning
216 of each line, either. E.g.
217
218 /* TRANSLATORS: here is a comment that explains the string
219 to be translated, that follows immediately after it */
220 _("Here is a translatable string explained by the above.");
221
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222 - Double negation is often harder to understand than no negation
223 at all.
224
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225 - There are two schools of thought when it comes to comparison,
226 especially inside a loop. Some people prefer to have the less stable
227 value on the left hand side and the more stable value on the right hand
228 side, e.g. if you have a loop that counts variable i down to the
229 lower bound,
230
231 while (i > lower_bound) {
232 do something;
233 i--;
234 }
235
236 Other people prefer to have the textual order of values match the
237 actual order of values in their comparison, so that they can
238 mentally draw a number line from left to right and place these
239 values in order, i.e.
240
241 while (lower_bound < i) {
242 do something;
243 i--;
244 }
245
246 Both are valid, and we use both. However, the more "stable" the
247 stable side becomes, the more we tend to prefer the former
248 (comparison with a constant, "i > 0", is an extreme example).
249 Just do not mix styles in the same part of the code and mimic
250 existing styles in the neighbourhood.
251
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252 - Some clever tricks, like using the !! operator with arithmetic
253 constructs, can be extremely confusing to others. Avoid them,
254 unless there is a compelling reason to use them.
255
256 - Use the API. No, really. We have a strbuf (variable length
257 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 258 string_list for sorted string lists, a hash map (mapping struct
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259 objects) named "struct decorate", amongst other things.
260
261 - When you come up with an API, document it.
262
263 - The first #include in C files, except in platform specific
264 compat/ implementations, should be git-compat-util.h or another
265 header file that includes it, such as cache.h or builtin.h.
266
267 - If you are planning a new command, consider writing it in shell
268 or perl first, so that changes in semantics can be easily
2de9b711 269 changed and discussed. Many Git commands started out like
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270 that, and a few are still scripts.
271
2de9b711 272 - Avoid introducing a new dependency into Git. This means you
6d0618a8 273 usually should stay away from scripting languages not already
2de9b711 274 used in the Git core command set (unless your command is clearly
6d0618a8 275 separate from it, such as an importer to convert random-scm-X
2de9b711 276 repositories to Git).
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277
278 - When we pass <string, length> pair to functions, we should try to
279 pass them in that order.
c455bd89 280
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281 - Use Git's gettext wrappers to make the user interface
282 translatable. See "Marking strings for translation" in po/README.
283
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284For Perl programs:
285
286 - Most of the C guidelines above apply.
287
288 - We try to support Perl 5.8 and later ("use Perl 5.008").
289
290 - use strict and use warnings are strongly preferred.
291
292 - Don't overuse statement modifiers unless using them makes the
293 result easier to follow.
294
295 ... do something ...
296 do_this() unless (condition);
297 ... do something else ...
298
299 is more readable than:
300
301 ... do something ...
302 unless (condition) {
303 do_this();
304 }
305 ... do something else ...
306
307 *only* when the condition is so rare that do_this() will be almost
308 always called.
309
310 - We try to avoid assignments inside "if ()" conditions.
311
312 - Learn and use Git.pm if you need that functionality.
313
314 - For Emacs, it's useful to put the following in
315 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
316
317 ;; note the first part is useful for C editing, too
318 ((nil . ((indent-tabs-mode . t)
319 (tab-width . 8)
320 (fill-column . 80)))
321 (cperl-mode . ((cperl-indent-level . 8)
322 (cperl-extra-newline-before-brace . nil)
323 (cperl-merge-trailing-else . t))))
324
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325For Python scripts:
326
327 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
328
329 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
330
331 - Where required libraries do not restrict us to Python 2, we try to
332 also be compatible with Python 3.1 and later.
333
334 - When you must differentiate between Unicode literals and byte string
335 literals, it is OK to use the 'b' prefix. Even though the Python
336 documentation for version 2.6 does not mention this prefix, it has
337 been supported since version 2.6.0.
338
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339Writing Documentation:
340
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341 Most (if not all) of the documentation pages are written in the
342 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
343 processed into HTML and manpages (e.g. git.html and git.1 in the
344 same directory).
bb9f2aec 345
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346 The documentation liberally mixes US and UK English (en_US/UK)
347 norms for spelling and grammar, which is somewhat unfortunate.
348 In an ideal world, it would have been better if it consistently
349 used only one and not the other, and we would have picked en_US
350 (if you wish to correct the English of some of the existing
351 documentation, please see the documentation-related advice in the
352 Documentation/SubmittingPatches file).
353
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354 Every user-visible change should be reflected in the documentation.
355 The same general rule as for code applies -- imitate the existing
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356 conventions.
357
358 A few commented examples follow to provide reference when writing or
359 modifying command usage strings and synopsis sections in the manual
360 pages:
c455bd89 361
b1afe49d 362 Placeholders are spelled in lowercase and enclosed in angle brackets:
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363 <file>
364 --sort=<key>
365 --abbrev[=<n>]
366
469bfc96 367 Possibility of multiple occurrences is indicated by three dots:
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368 <file>...
369 (One or more of <file>.)
370
371 Optional parts are enclosed in square brackets:
372 [<extra>]
373 (Zero or one <extra>.)
374
375 --exec-path[=<path>]
376 (Option with an optional argument. Note that the "=" is inside the
377 brackets.)
378
379 [<patch>...]
380 (Zero or more of <patch>. Note that the dots are inside, not
381 outside the brackets.)
382
383 Multiple alternatives are indicated with vertical bar:
384 [-q | --quiet]
385 [--utf8 | --no-utf8]
386
387 Parentheses are used for grouping:
388 [(<rev>|<range>)...]
389 (Any number of either <rev> or <range>. Parens are needed to make
390 it clear that "..." pertains to both <rev> and <range>.)
391
392 [(-p <parent>)...]
393 (Any number of option -p, each with one <parent> argument.)
394
395 git remote set-head <name> (-a | -d | <branch>)
396 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
397 brackets) be provided.)
398
399 And a somewhat more contrived example:
400 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
401 Here "=" is outside the brackets, because "--diff-filter=" is a
402 valid usage. "*" has its own pair of brackets, because it can
403 (optionally) be specified only when one or more of the letters is
404 also provided.
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405
406 A note on notation:
407 Use 'git' (all lowercase) when talking about commands i.e. something
408 the user would type into a shell and use 'Git' (uppercase first letter)
409 when talking about the version control system and its properties.
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410
411 A few commented examples follow to provide reference when writing or
412 modifying paragraphs or option/command explanations that contain options
413 or commands:
414
415 Literal examples (e.g. use of command-line options, command names, and
416 configuration variables) are typeset in monospace, and if you can use
417 `backticks around word phrases`, do so.
418 `--pretty=oneline`
419 `git rev-list`
420 `remote.pushdefault`
421
422 Word phrases enclosed in `backtick characters` are rendered literally
423 and will not be further expanded. The use of `backticks` to achieve the
424 previous rule means that literal examples should not use AsciiDoc
425 escapes.
426 Correct:
427 `--pretty=oneline`
428 Incorrect:
429 `\--pretty=oneline`
430
431 If some place in the documentation needs to typeset a command usage
432 example with inline substitutions, it is fine to use +monospaced and
433 inline substituted text+ instead of `monospaced literal text`, and with
434 the former, the part that should not get substituted must be
435 quoted/escaped.