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[git/git.git] / Documentation / CodingGuidelines
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6d0618a8 1Like other projects, we also have some guidelines to keep to the
6c3b2afe 2code. For Git in general, a few 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."
2e477d8d 27 Cf. http://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html
dd30800b 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 - If a command sequence joined with && or || or | spans multiple
122 lines, put each command on a separate line and put && and || and |
123 operators at the end of each line, rather than the start. This
124 means you don't need to use \ to join lines, since the above
125 operators imply the sequence isn't finished.
126
127 (incorrect)
128 grep blob verify_pack_result \
129 | awk -f print_1.awk \
130 | sort >actual &&
131 ...
132
133 (correct)
134 grep blob verify_pack_result |
135 awk -f print_1.awk |
136 sort >actual &&
137 ...
138
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139 - We prefer "test" over "[ ... ]".
140
141 - We do not write the noiseword "function" in front of shell
142 functions.
143
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144 - We prefer a space between the function name and the parentheses,
145 and no space inside the parentheses. The opening "{" should also
146 be on the same line.
147
148 (incorrect)
149 my_function(){
150 ...
151
152 (correct)
153 my_function () {
154 ...
03b05c7d 155
009c98ee 156 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 157 [::], [==], or [..]) for portability.
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158
159 - We do not use \{m,n\};
160
161 - We do not use -E;
162
a58088ab 163 - We do not use ? or + (which are \{0,1\} and \{1,\}
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164 respectively in BRE) but that goes without saying as these
165 are ERE elements not BRE (note that \? and \+ are not even part
166 of BRE -- making them accessible from BRE is a GNU extension).
167
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168 - Use Git's gettext wrappers in git-sh-i18n to make the user
169 interface translatable. See "Marking strings for translation" in
170 po/README.
171
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172 - We do not write our "test" command with "-a" and "-o" and use "&&"
173 or "||" to concatenate multiple "test" commands instead, because
174 the use of "-a/-o" is often error-prone. E.g.
175
176 test -n "$x" -a "$a" = "$b"
177
178 is buggy and breaks when $x is "=", but
179
180 test -n "$x" && test "$a" = "$b"
181
182 does not have such a problem.
183
184
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185For C programs:
186
187 - We use tabs to indent, and interpret tabs as taking up to
188 8 spaces.
189
190 - We try to keep to at most 80 characters per line.
191
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192 - As a Git developer we assume you have a reasonably modern compiler
193 and we recommend you to enable the DEVELOPER makefile knob to
194 ensure your patch is clear of all compiler warnings we care about,
195 by e.g. "echo DEVELOPER=1 >>config.mak".
196
2de9b711 197 - We try to support a wide range of C compilers to compile Git with,
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198 including old ones. You should not use features from newer C
199 standard, even if your compiler groks them.
a26fd033 200
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201 There are a few exceptions to this guideline:
202
203 . since early 2012 with e1327023ea, we have been using an enum
204 definition whose last element is followed by a comma. This, like
205 an array initializer that ends with a trailing comma, can be used
206 to reduce the patch noise when adding a new identifer at the end.
207
208 . since mid 2017 with cbc0f81d, we have been using designated
209 initializers for struct (e.g. "struct t v = { .val = 'a' };").
210
211 . since mid 2017 with 512f41cf, we have been using designated
212 initializers for array (e.g. "int array[10] = { [5] = 2 }").
213
214 These used to be forbidden, but we have not heard any breakage
215 report, and they are assumed to be safe.
216
217 - Variables have to be declared at the beginning of the block, before
218 the first statement (i.e. -Wdeclaration-after-statement).
219
220 - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
221 is still not allowed in this codebase.
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222
223 - NULL pointers shall be written as NULL, not as 0.
224
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225 - When declaring pointers, the star sides with the variable
226 name, i.e. "char *string", not "char* string" or
227 "char * string". This makes it easier to understand code
228 like "char *string, c;".
229
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230 - Use whitespace around operators and keywords, but not inside
231 parentheses and not around functions. So:
232
233 while (condition)
234 func(bar + 1);
235
236 and not:
237
238 while( condition )
239 func (bar+1);
240
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241 - We avoid using braces unnecessarily. I.e.
242
243 if (bla) {
244 x = 1;
245 }
246
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247 is frowned upon. But there are a few exceptions:
248
249 - When the statement extends over a few lines (e.g., a while loop
250 with an embedded conditional, or a comment). E.g.:
251
252 while (foo) {
253 if (x)
254 one();
255 else
256 two();
257 }
258
259 if (foo) {
260 /*
261 * This one requires some explanation,
262 * so we're better off with braces to make
263 * it obvious that the indentation is correct.
264 */
265 doit();
266 }
267
268 - When there are multiple arms to a conditional and some of them
269 require braces, enclose even a single line block in braces for
270 consistency. E.g.:
271
272 if (foo) {
273 doit();
274 } else {
275 one();
276 two();
277 three();
278 }
6d0618a8 279
691d0dd0 280 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 281
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282 - Try to make your code understandable. You may put comments
283 in, but comments invariably tend to stale out when the code
284 they were describing changes. Often splitting a function
285 into two makes the intention of the code much clearer.
286
b75a6ca7 287 - Multi-line comments include their delimiters on separate lines from
288 the text. E.g.
289
290 /*
291 * A very long
292 * multi-line comment.
293 */
294
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295 Note however that a comment that explains a translatable string to
296 translators uses a convention of starting with a magic token
66f5f6dc 297 "TRANSLATORS: ", e.g.
cbcfd4e3 298
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299 /*
300 * TRANSLATORS: here is a comment that explains the string to
301 * be translated, that follows immediately after it.
302 */
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303 _("Here is a translatable string explained by the above.");
304
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305 - Double negation is often harder to understand than no negation
306 at all.
307
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308 - There are two schools of thought when it comes to comparison,
309 especially inside a loop. Some people prefer to have the less stable
310 value on the left hand side and the more stable value on the right hand
311 side, e.g. if you have a loop that counts variable i down to the
312 lower bound,
313
314 while (i > lower_bound) {
315 do something;
316 i--;
317 }
318
319 Other people prefer to have the textual order of values match the
320 actual order of values in their comparison, so that they can
321 mentally draw a number line from left to right and place these
322 values in order, i.e.
323
324 while (lower_bound < i) {
325 do something;
326 i--;
327 }
328
329 Both are valid, and we use both. However, the more "stable" the
330 stable side becomes, the more we tend to prefer the former
331 (comparison with a constant, "i > 0", is an extreme example).
332 Just do not mix styles in the same part of the code and mimic
333 existing styles in the neighbourhood.
334
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335 - There are two schools of thought when it comes to splitting a long
336 logical line into multiple lines. Some people push the second and
337 subsequent lines far enough to the right with tabs and align them:
338
339 if (the_beginning_of_a_very_long_expression_that_has_to ||
340 span_more_than_a_single_line_of ||
341 the_source_text) {
342 ...
343
344 while other people prefer to align the second and the subsequent
345 lines with the column immediately inside the opening parenthesis,
346 with tabs and spaces, following our "tabstop is always a multiple
347 of 8" convention:
348
349 if (the_beginning_of_a_very_long_expression_that_has_to ||
350 span_more_than_a_single_line_of ||
351 the_source_text) {
352 ...
353
354 Both are valid, and we use both. Again, just do not mix styles in
355 the same part of the code and mimic existing styles in the
356 neighbourhood.
357
358 - When splitting a long logical line, some people change line before
359 a binary operator, so that the result looks like a parse tree when
360 you turn your head 90-degrees counterclockwise:
361
362 if (the_beginning_of_a_very_long_expression_that_has_to
363 || span_more_than_a_single_line_of_the_source_text) {
364
365 while other people prefer to leave the operator at the end of the
366 line:
367
368 if (the_beginning_of_a_very_long_expression_that_has_to ||
369 span_more_than_a_single_line_of_the_source_text) {
370
371 Both are valid, but we tend to use the latter more, unless the
372 expression gets fairly complex, in which case the former tends to
373 be easier to read. Again, just do not mix styles in the same part
374 of the code and mimic existing styles in the neighbourhood.
375
376 - When splitting a long logical line, with everything else being
377 equal, it is preferable to split after the operator at higher
378 level in the parse tree. That is, this is more preferable:
379
380 if (a_very_long_variable * that_is_used_in +
381 a_very_long_expression) {
382 ...
383
384 than
385
386 if (a_very_long_variable *
387 that_is_used_in + a_very_long_expression) {
388 ...
389
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390 - Some clever tricks, like using the !! operator with arithmetic
391 constructs, can be extremely confusing to others. Avoid them,
392 unless there is a compelling reason to use them.
393
394 - Use the API. No, really. We have a strbuf (variable length
395 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 396 string_list for sorted string lists, a hash map (mapping struct
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397 objects) named "struct decorate", amongst other things.
398
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399 - When you come up with an API, document its functions and structures
400 in the header file that exposes the API to its callers. Use what is
401 in "strbuf.h" as a model for the appropriate tone and level of
402 detail.
6d0618a8 403
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404 - The first #include in C files, except in platform specific compat/
405 implementations, must be either "git-compat-util.h", "cache.h" or
406 "builtin.h". You do not have to include more than one of these.
407
408 - A C file must directly include the header files that declare the
409 functions and the types it uses, except for the functions and types
410 that are made available to it by including one of the header files
411 it must include by the previous rule.
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412
413 - If you are planning a new command, consider writing it in shell
414 or perl first, so that changes in semantics can be easily
2de9b711 415 changed and discussed. Many Git commands started out like
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416 that, and a few are still scripts.
417
2de9b711 418 - Avoid introducing a new dependency into Git. This means you
6d0618a8 419 usually should stay away from scripting languages not already
2de9b711 420 used in the Git core command set (unless your command is clearly
6d0618a8 421 separate from it, such as an importer to convert random-scm-X
2de9b711 422 repositories to Git).
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423
424 - When we pass <string, length> pair to functions, we should try to
425 pass them in that order.
c455bd89 426
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427 - Use Git's gettext wrappers to make the user interface
428 translatable. See "Marking strings for translation" in po/README.
429
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430 - Variables and functions local to a given source file should be marked
431 with "static". Variables that are visible to other source files
432 must be declared with "extern" in header files. However, function
433 declarations should not use "extern", as that is already the default.
434
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435 - You can launch gdb around your program using the shorthand GIT_DEBUGGER.
436 Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or
437 run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to
438 use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
439 ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
440
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441For Perl programs:
442
443 - Most of the C guidelines above apply.
444
445 - We try to support Perl 5.8 and later ("use Perl 5.008").
446
447 - use strict and use warnings are strongly preferred.
448
449 - Don't overuse statement modifiers unless using them makes the
450 result easier to follow.
451
452 ... do something ...
453 do_this() unless (condition);
454 ... do something else ...
455
456 is more readable than:
457
458 ... do something ...
459 unless (condition) {
460 do_this();
461 }
462 ... do something else ...
463
464 *only* when the condition is so rare that do_this() will be almost
465 always called.
466
467 - We try to avoid assignments inside "if ()" conditions.
468
469 - Learn and use Git.pm if you need that functionality.
470
471 - For Emacs, it's useful to put the following in
472 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
473
474 ;; note the first part is useful for C editing, too
475 ((nil . ((indent-tabs-mode . t)
476 (tab-width . 8)
477 (fill-column . 80)))
478 (cperl-mode . ((cperl-indent-level . 8)
479 (cperl-extra-newline-before-brace . nil)
480 (cperl-merge-trailing-else . t))))
481
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482For Python scripts:
483
484 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
485
486 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
487
488 - Where required libraries do not restrict us to Python 2, we try to
489 also be compatible with Python 3.1 and later.
490
491 - When you must differentiate between Unicode literals and byte string
492 literals, it is OK to use the 'b' prefix. Even though the Python
493 documentation for version 2.6 does not mention this prefix, it has
494 been supported since version 2.6.0.
495
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496Error Messages
497
498 - Do not end error messages with a full stop.
499
500 - Do not capitalize ("unable to open %s", not "Unable to open %s")
501
502 - Say what the error is first ("cannot open %s", not "%s: cannot open")
503
504
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505Externally Visible Names
506
507 - For configuration variable names, follow the existing convention:
508
509 . The section name indicates the affected subsystem.
510
511 . The subsection name, if any, indicates which of an unbounded set
512 of things to set the value for.
513
514 . The variable name describes the effect of tweaking this knob.
515
516 The section and variable names that consist of multiple words are
517 formed by concatenating the words without punctuations (e.g. `-`),
518 and are broken using bumpyCaps in documentation as a hint to the
519 reader.
520
521 When choosing the variable namespace, do not use variable name for
522 specifying possibly unbounded set of things, most notably anything
523 an end user can freely come up with (e.g. branch names). Instead,
524 use subsection names or variable values, like the existing variable
525 branch.<name>.description does.
526
527
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528Writing Documentation:
529
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530 Most (if not all) of the documentation pages are written in the
531 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
532 processed into HTML and manpages (e.g. git.html and git.1 in the
533 same directory).
bb9f2aec 534
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535 The documentation liberally mixes US and UK English (en_US/UK)
536 norms for spelling and grammar, which is somewhat unfortunate.
537 In an ideal world, it would have been better if it consistently
538 used only one and not the other, and we would have picked en_US
539 (if you wish to correct the English of some of the existing
540 documentation, please see the documentation-related advice in the
541 Documentation/SubmittingPatches file).
542
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543 Every user-visible change should be reflected in the documentation.
544 The same general rule as for code applies -- imitate the existing
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545 conventions.
546
547 A few commented examples follow to provide reference when writing or
548 modifying command usage strings and synopsis sections in the manual
549 pages:
c455bd89 550
b1afe49d 551 Placeholders are spelled in lowercase and enclosed in angle brackets:
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552 <file>
553 --sort=<key>
554 --abbrev[=<n>]
555
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556 If a placeholder has multiple words, they are separated by dashes:
557 <new-branch-name>
558 --template=<template-directory>
559
469bfc96 560 Possibility of multiple occurrences is indicated by three dots:
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561 <file>...
562 (One or more of <file>.)
563
564 Optional parts are enclosed in square brackets:
565 [<extra>]
566 (Zero or one <extra>.)
567
568 --exec-path[=<path>]
569 (Option with an optional argument. Note that the "=" is inside the
570 brackets.)
571
572 [<patch>...]
573 (Zero or more of <patch>. Note that the dots are inside, not
574 outside the brackets.)
575
9c9b4f2f 576 Multiple alternatives are indicated with vertical bars:
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577 [-q | --quiet]
578 [--utf8 | --no-utf8]
579
580 Parentheses are used for grouping:
9c9b4f2f 581 [(<rev> | <range>)...]
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582 (Any number of either <rev> or <range>. Parens are needed to make
583 it clear that "..." pertains to both <rev> and <range>.)
584
585 [(-p <parent>)...]
586 (Any number of option -p, each with one <parent> argument.)
587
588 git remote set-head <name> (-a | -d | <branch>)
589 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
590 brackets) be provided.)
591
592 And a somewhat more contrived example:
593 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
594 Here "=" is outside the brackets, because "--diff-filter=" is a
595 valid usage. "*" has its own pair of brackets, because it can
596 (optionally) be specified only when one or more of the letters is
597 also provided.
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598
599 A note on notation:
600 Use 'git' (all lowercase) when talking about commands i.e. something
601 the user would type into a shell and use 'Git' (uppercase first letter)
602 when talking about the version control system and its properties.
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603
604 A few commented examples follow to provide reference when writing or
605 modifying paragraphs or option/command explanations that contain options
606 or commands:
607
41f5b21f 608 Literal examples (e.g. use of command-line options, command names,
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609 branch names, URLs, pathnames (files and directories), configuration and
610 environment variables) must be typeset in monospace (i.e. wrapped with
611 backticks):
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612 `--pretty=oneline`
613 `git rev-list`
da0005b8 614 `remote.pushDefault`
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615 `http://git.example.com`
616 `.git/config`
41f5b21f 617 `GIT_DIR`
57103dbf 618 `HEAD`
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619
620 An environment variable must be prefixed with "$" only when referring to its
621 value and not when referring to the variable itself, in this case there is
622 nothing to add except the backticks:
623 `GIT_DIR` is specified
624 `$GIT_DIR/hooks/pre-receive`
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625
626 Word phrases enclosed in `backtick characters` are rendered literally
627 and will not be further expanded. The use of `backticks` to achieve the
628 previous rule means that literal examples should not use AsciiDoc
629 escapes.
630 Correct:
631 `--pretty=oneline`
632 Incorrect:
633 `\--pretty=oneline`
634
635 If some place in the documentation needs to typeset a command usage
636 example with inline substitutions, it is fine to use +monospaced and
637 inline substituted text+ instead of `monospaced literal text`, and with
638 the former, the part that should not get substituted must be
639 quoted/escaped.