CodingGuidelines: avoid "test <cond> -a/-o <cond>"
[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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154 - We do not write our "test" command with "-a" and "-o" and use "&&"
155 or "||" to concatenate multiple "test" commands instead, because
156 the use of "-a/-o" is often error-prone. E.g.
157
158 test -n "$x" -a "$a" = "$b"
159
160 is buggy and breaks when $x is "=", but
161
162 test -n "$x" && test "$a" = "$b"
163
164 does not have such a problem.
165
166
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167For C programs:
168
169 - We use tabs to indent, and interpret tabs as taking up to
170 8 spaces.
171
172 - We try to keep to at most 80 characters per line.
173
2de9b711 174 - We try to support a wide range of C compilers to compile Git with,
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175 including old ones. That means that you should not use C99
176 initializers, even if a lot of compilers grok it.
177
178 - Variables have to be declared at the beginning of the block.
179
180 - NULL pointers shall be written as NULL, not as 0.
181
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182 - When declaring pointers, the star sides with the variable
183 name, i.e. "char *string", not "char* string" or
184 "char * string". This makes it easier to understand code
185 like "char *string, c;".
186
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187 - Use whitespace around operators and keywords, but not inside
188 parentheses and not around functions. So:
189
190 while (condition)
191 func(bar + 1);
192
193 and not:
194
195 while( condition )
196 func (bar+1);
197
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198 - We avoid using braces unnecessarily. I.e.
199
200 if (bla) {
201 x = 1;
202 }
203
204 is frowned upon. A gray area is when the statement extends
205 over a few lines, and/or you have a lengthy comment atop of
206 it. Also, like in the Linux kernel, if there is a long list
207 of "else if" statements, it can make sense to add braces to
208 single line blocks.
209
691d0dd0 210 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 211
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212 - Try to make your code understandable. You may put comments
213 in, but comments invariably tend to stale out when the code
214 they were describing changes. Often splitting a function
215 into two makes the intention of the code much clearer.
216
b75a6ca7 217 - Multi-line comments include their delimiters on separate lines from
218 the text. E.g.
219
220 /*
221 * A very long
222 * multi-line comment.
223 */
224
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225 Note however that a comment that explains a translatable string to
226 translators uses a convention of starting with a magic token
227 "TRANSLATORS: " immediately after the opening delimiter, even when
228 it spans multiple lines. We do not add an asterisk at the beginning
229 of each line, either. E.g.
230
231 /* TRANSLATORS: here is a comment that explains the string
232 to be translated, that follows immediately after it */
233 _("Here is a translatable string explained by the above.");
234
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235 - Double negation is often harder to understand than no negation
236 at all.
237
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238 - There are two schools of thought when it comes to comparison,
239 especially inside a loop. Some people prefer to have the less stable
240 value on the left hand side and the more stable value on the right hand
241 side, e.g. if you have a loop that counts variable i down to the
242 lower bound,
243
244 while (i > lower_bound) {
245 do something;
246 i--;
247 }
248
249 Other people prefer to have the textual order of values match the
250 actual order of values in their comparison, so that they can
251 mentally draw a number line from left to right and place these
252 values in order, i.e.
253
254 while (lower_bound < i) {
255 do something;
256 i--;
257 }
258
259 Both are valid, and we use both. However, the more "stable" the
260 stable side becomes, the more we tend to prefer the former
261 (comparison with a constant, "i > 0", is an extreme example).
262 Just do not mix styles in the same part of the code and mimic
263 existing styles in the neighbourhood.
264
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265 - There are two schools of thought when it comes to splitting a long
266 logical line into multiple lines. Some people push the second and
267 subsequent lines far enough to the right with tabs and align them:
268
269 if (the_beginning_of_a_very_long_expression_that_has_to ||
270 span_more_than_a_single_line_of ||
271 the_source_text) {
272 ...
273
274 while other people prefer to align the second and the subsequent
275 lines with the column immediately inside the opening parenthesis,
276 with tabs and spaces, following our "tabstop is always a multiple
277 of 8" convention:
278
279 if (the_beginning_of_a_very_long_expression_that_has_to ||
280 span_more_than_a_single_line_of ||
281 the_source_text) {
282 ...
283
284 Both are valid, and we use both. Again, just do not mix styles in
285 the same part of the code and mimic existing styles in the
286 neighbourhood.
287
288 - When splitting a long logical line, some people change line before
289 a binary operator, so that the result looks like a parse tree when
290 you turn your head 90-degrees counterclockwise:
291
292 if (the_beginning_of_a_very_long_expression_that_has_to
293 || span_more_than_a_single_line_of_the_source_text) {
294
295 while other people prefer to leave the operator at the end of the
296 line:
297
298 if (the_beginning_of_a_very_long_expression_that_has_to ||
299 span_more_than_a_single_line_of_the_source_text) {
300
301 Both are valid, but we tend to use the latter more, unless the
302 expression gets fairly complex, in which case the former tends to
303 be easier to read. Again, just do not mix styles in the same part
304 of the code and mimic existing styles in the neighbourhood.
305
306 - When splitting a long logical line, with everything else being
307 equal, it is preferable to split after the operator at higher
308 level in the parse tree. That is, this is more preferable:
309
310 if (a_very_long_variable * that_is_used_in +
311 a_very_long_expression) {
312 ...
313
314 than
315
316 if (a_very_long_variable *
317 that_is_used_in + a_very_long_expression) {
318 ...
319
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320 - Some clever tricks, like using the !! operator with arithmetic
321 constructs, can be extremely confusing to others. Avoid them,
322 unless there is a compelling reason to use them.
323
324 - Use the API. No, really. We have a strbuf (variable length
325 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 326 string_list for sorted string lists, a hash map (mapping struct
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327 objects) named "struct decorate", amongst other things.
328
329 - When you come up with an API, document it.
330
331 - The first #include in C files, except in platform specific
332 compat/ implementations, should be git-compat-util.h or another
333 header file that includes it, such as cache.h or builtin.h.
334
335 - If you are planning a new command, consider writing it in shell
336 or perl first, so that changes in semantics can be easily
2de9b711 337 changed and discussed. Many Git commands started out like
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338 that, and a few are still scripts.
339
2de9b711 340 - Avoid introducing a new dependency into Git. This means you
6d0618a8 341 usually should stay away from scripting languages not already
2de9b711 342 used in the Git core command set (unless your command is clearly
6d0618a8 343 separate from it, such as an importer to convert random-scm-X
2de9b711 344 repositories to Git).
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345
346 - When we pass <string, length> pair to functions, we should try to
347 pass them in that order.
c455bd89 348
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349 - Use Git's gettext wrappers to make the user interface
350 translatable. See "Marking strings for translation" in po/README.
351
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352For Perl programs:
353
354 - Most of the C guidelines above apply.
355
356 - We try to support Perl 5.8 and later ("use Perl 5.008").
357
358 - use strict and use warnings are strongly preferred.
359
360 - Don't overuse statement modifiers unless using them makes the
361 result easier to follow.
362
363 ... do something ...
364 do_this() unless (condition);
365 ... do something else ...
366
367 is more readable than:
368
369 ... do something ...
370 unless (condition) {
371 do_this();
372 }
373 ... do something else ...
374
375 *only* when the condition is so rare that do_this() will be almost
376 always called.
377
378 - We try to avoid assignments inside "if ()" conditions.
379
380 - Learn and use Git.pm if you need that functionality.
381
382 - For Emacs, it's useful to put the following in
383 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
384
385 ;; note the first part is useful for C editing, too
386 ((nil . ((indent-tabs-mode . t)
387 (tab-width . 8)
388 (fill-column . 80)))
389 (cperl-mode . ((cperl-indent-level . 8)
390 (cperl-extra-newline-before-brace . nil)
391 (cperl-merge-trailing-else . t))))
392
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393For Python scripts:
394
395 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
396
397 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
398
399 - Where required libraries do not restrict us to Python 2, we try to
400 also be compatible with Python 3.1 and later.
401
402 - When you must differentiate between Unicode literals and byte string
403 literals, it is OK to use the 'b' prefix. Even though the Python
404 documentation for version 2.6 does not mention this prefix, it has
405 been supported since version 2.6.0.
406
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407Writing Documentation:
408
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409 Most (if not all) of the documentation pages are written in the
410 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
411 processed into HTML and manpages (e.g. git.html and git.1 in the
412 same directory).
bb9f2aec 413
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414 The documentation liberally mixes US and UK English (en_US/UK)
415 norms for spelling and grammar, which is somewhat unfortunate.
416 In an ideal world, it would have been better if it consistently
417 used only one and not the other, and we would have picked en_US
418 (if you wish to correct the English of some of the existing
419 documentation, please see the documentation-related advice in the
420 Documentation/SubmittingPatches file).
421
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422 Every user-visible change should be reflected in the documentation.
423 The same general rule as for code applies -- imitate the existing
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424 conventions.
425
426 A few commented examples follow to provide reference when writing or
427 modifying command usage strings and synopsis sections in the manual
428 pages:
c455bd89 429
b1afe49d 430 Placeholders are spelled in lowercase and enclosed in angle brackets:
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431 <file>
432 --sort=<key>
433 --abbrev[=<n>]
434
469bfc96 435 Possibility of multiple occurrences is indicated by three dots:
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436 <file>...
437 (One or more of <file>.)
438
439 Optional parts are enclosed in square brackets:
440 [<extra>]
441 (Zero or one <extra>.)
442
443 --exec-path[=<path>]
444 (Option with an optional argument. Note that the "=" is inside the
445 brackets.)
446
447 [<patch>...]
448 (Zero or more of <patch>. Note that the dots are inside, not
449 outside the brackets.)
450
451 Multiple alternatives are indicated with vertical bar:
452 [-q | --quiet]
453 [--utf8 | --no-utf8]
454
455 Parentheses are used for grouping:
456 [(<rev>|<range>)...]
457 (Any number of either <rev> or <range>. Parens are needed to make
458 it clear that "..." pertains to both <rev> and <range>.)
459
460 [(-p <parent>)...]
461 (Any number of option -p, each with one <parent> argument.)
462
463 git remote set-head <name> (-a | -d | <branch>)
464 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
465 brackets) be provided.)
466
467 And a somewhat more contrived example:
468 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
469 Here "=" is outside the brackets, because "--diff-filter=" is a
470 valid usage. "*" has its own pair of brackets, because it can
471 (optionally) be specified only when one or more of the letters is
472 also provided.
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473
474 A note on notation:
475 Use 'git' (all lowercase) when talking about commands i.e. something
476 the user would type into a shell and use 'Git' (uppercase first letter)
477 when talking about the version control system and its properties.
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478
479 A few commented examples follow to provide reference when writing or
480 modifying paragraphs or option/command explanations that contain options
481 or commands:
482
483 Literal examples (e.g. use of command-line options, command names, and
484 configuration variables) are typeset in monospace, and if you can use
485 `backticks around word phrases`, do so.
486 `--pretty=oneline`
487 `git rev-list`
488 `remote.pushdefault`
489
490 Word phrases enclosed in `backtick characters` are rendered literally
491 and will not be further expanded. The use of `backticks` to achieve the
492 previous rule means that literal examples should not use AsciiDoc
493 escapes.
494 Correct:
495 `--pretty=oneline`
496 Incorrect:
497 `\--pretty=oneline`
498
499 If some place in the documentation needs to typeset a command usage
500 example with inline substitutions, it is fine to use +monospaced and
501 inline substituted text+ instead of `monospaced literal text`, and with
502 the former, the part that should not get substituted must be
503 quoted/escaped.