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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
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29 - Log messages to explain your changes are as important as the
30 changes themselves. Clearly written code and in-code comments
31 explain how the code works and what is assumed from the surrounding
32 context. The log messages explain what the changes wanted to
33 achieve and why the changes were necessary (more on this in the
34 accompanying SubmittingPatches document).
35
c5e366b1 36Make your code readable and sensible, and don't try to be clever.
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37
38As for more concrete guidelines, just imitate the existing code
39(this is a good guideline, no matter which project you are
dfb047b9 40contributing to). It is always preferable to match the _local_
2de9b711 41convention. New code added to Git suite is expected to match
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42the overall style of existing code. Modifications to existing
43code is expected to match the style the surrounding code already
44uses (even if it doesn't match the overall style of existing code).
45
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46But if you must have a list of rules, here are some language
47specific ones. Note that Documentation/ToolsForGit.txt document
48has a collection of tips to help you use some external tools
49to conform to these guidelines.
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50
51For shell scripts specifically (not exhaustive):
52
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53 - We use tabs for indentation.
54
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55 - Case arms are indented at the same depth as case and esac lines,
56 like this:
57
58 case "$variable" in
59 pattern1)
60 do this
61 ;;
62 pattern2)
63 do that
64 ;;
65 esac
f36a4fa8 66
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67 - Redirection operators should be written with space before, but no
68 space after them. In other words, write 'echo test >"$file"'
69 instead of 'echo test> $file' or 'echo test > $file'. Note that
70 even though it is not required by POSIX to double-quote the
71 redirection target in a variable (as shown above), our code does so
72 because some versions of bash issue a warning without the quotes.
73
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74 (incorrect)
75 cat hello > world < universe
76 echo hello >$world
77
78 (correct)
79 cat hello >world <universe
80 echo hello >"$world"
81
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82 - We prefer $( ... ) for command substitution; unlike ``, it
83 properly nests. It should have been the way Bourne spelled
84 it from day one, but unfortunately isn't.
85
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86 - If you want to find out if a command is available on the user's
87 $PATH, you should use 'type <command>', instead of 'which <command>'.
031fd4b9 88 The output of 'which' is not machine parsable and its exit code
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89 is not reliable across platforms.
90
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91 - We use POSIX compliant parameter substitutions and avoid bashisms;
92 namely:
6d0618a8 93
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94 - We use ${parameter-word} and its [-=?+] siblings, and their
95 colon'ed "unset or null" form.
6d0618a8 96
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97 - We use ${parameter#word} and its [#%] siblings, and their
98 doubled "longest matching" form.
6d0618a8 99
bc979945 100 - No "Substring Expansion" ${parameter:offset:length}.
055467dd 101
bc979945 102 - No shell arrays.
6d0618a8 103
bc979945 104 - No pattern replacement ${parameter/pattern/string}.
6d0618a8 105
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106 - We use Arithmetic Expansion $(( ... )).
107
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108 - We do not use Process Substitution <(list) or >(list).
109
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110 - Do not write control structures on a single line with semicolon.
111 "then" should be on the next line for if statements, and "do"
112 should be on the next line for "while" and "for".
113
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114 (incorrect)
115 if test -f hello; then
116 do this
117 fi
118
119 (correct)
120 if test -f hello
121 then
122 do this
123 fi
124
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125 - If a command sequence joined with && or || or | spans multiple
126 lines, put each command on a separate line and put && and || and |
127 operators at the end of each line, rather than the start. This
128 means you don't need to use \ to join lines, since the above
129 operators imply the sequence isn't finished.
130
131 (incorrect)
132 grep blob verify_pack_result \
133 | awk -f print_1.awk \
134 | sort >actual &&
135 ...
136
137 (correct)
138 grep blob verify_pack_result |
139 awk -f print_1.awk |
140 sort >actual &&
141 ...
142
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143 - We prefer "test" over "[ ... ]".
144
145 - We do not write the noiseword "function" in front of shell
146 functions.
147
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148 - We prefer a space between the function name and the parentheses,
149 and no space inside the parentheses. The opening "{" should also
150 be on the same line.
151
152 (incorrect)
153 my_function(){
154 ...
155
156 (correct)
157 my_function () {
158 ...
03b05c7d 159
009c98ee 160 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
a58088ab 161 [::], [==], or [..]) for portability.
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162
163 - We do not use \{m,n\};
164
a58088ab 165 - We do not use ? or + (which are \{0,1\} and \{1,\}
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166 respectively in BRE) but that goes without saying as these
167 are ERE elements not BRE (note that \? and \+ are not even part
168 of BRE -- making them accessible from BRE is a GNU extension).
169
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170 - Use Git's gettext wrappers in git-sh-i18n to make the user
171 interface translatable. See "Marking strings for translation" in
172 po/README.
173
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174 - We do not write our "test" command with "-a" and "-o" and use "&&"
175 or "||" to concatenate multiple "test" commands instead, because
176 the use of "-a/-o" is often error-prone. E.g.
177
178 test -n "$x" -a "$a" = "$b"
179
180 is buggy and breaks when $x is "=", but
181
182 test -n "$x" && test "$a" = "$b"
183
184 does not have such a problem.
185
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186 - Even though "local" is not part of POSIX, we make heavy use of it
187 in our test suite. We do not use it in scripted Porcelains, and
188 hopefully nobody starts using "local" before they are reimplemented
189 in C ;-)
190
897f964c 191
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192For C programs:
193
194 - We use tabs to indent, and interpret tabs as taking up to
195 8 spaces.
196
197 - We try to keep to at most 80 characters per line.
198
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199 - As a Git developer we assume you have a reasonably modern compiler
200 and we recommend you to enable the DEVELOPER makefile knob to
201 ensure your patch is clear of all compiler warnings we care about,
202 by e.g. "echo DEVELOPER=1 >>config.mak".
203
2de9b711 204 - We try to support a wide range of C compilers to compile Git with,
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205 including old ones. You should not use features from newer C
206 standard, even if your compiler groks them.
a26fd033 207
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208 There are a few exceptions to this guideline:
209
210 . since early 2012 with e1327023ea, we have been using an enum
211 definition whose last element is followed by a comma. This, like
212 an array initializer that ends with a trailing comma, can be used
031fd4b9 213 to reduce the patch noise when adding a new identifier at the end.
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214
215 . since mid 2017 with cbc0f81d, we have been using designated
216 initializers for struct (e.g. "struct t v = { .val = 'a' };").
217
218 . since mid 2017 with 512f41cf, we have been using designated
219 initializers for array (e.g. "int array[10] = { [5] = 2 }").
220
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221 . since early 2021 with 765dc168882, we have been using variadic
222 macros, mostly for printf-like trace and debug macros.
223
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224 These used to be forbidden, but we have not heard any breakage
225 report, and they are assumed to be safe.
226
227 - Variables have to be declared at the beginning of the block, before
228 the first statement (i.e. -Wdeclaration-after-statement).
229
230 - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
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231 is still not allowed in this codebase. We are in the process of
232 allowing it by waiting to see that 44ba10d6 (revision: use C99
233 declaration of variable in for() loop, 2021-11-14) does not get
234 complaints. Let's revisit this around November 2022.
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235
236 - NULL pointers shall be written as NULL, not as 0.
237
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238 - When declaring pointers, the star sides with the variable
239 name, i.e. "char *string", not "char* string" or
240 "char * string". This makes it easier to understand code
241 like "char *string, c;".
242
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243 - Use whitespace around operators and keywords, but not inside
244 parentheses and not around functions. So:
245
246 while (condition)
247 func(bar + 1);
248
249 and not:
250
251 while( condition )
252 func (bar+1);
253
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254 - Do not explicitly compare an integral value with constant 0 or '\0',
255 or a pointer value with constant NULL. For instance, to validate that
256 counted array <ptr, cnt> is initialized but has no elements, write:
257
258 if (!ptr || cnt)
259 BUG("empty array expected");
260
261 and not:
262
263 if (ptr == NULL || cnt != 0);
264 BUG("empty array expected");
265
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266 - We avoid using braces unnecessarily. I.e.
267
268 if (bla) {
269 x = 1;
270 }
271
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272 is frowned upon. But there are a few exceptions:
273
274 - When the statement extends over a few lines (e.g., a while loop
275 with an embedded conditional, or a comment). E.g.:
276
277 while (foo) {
278 if (x)
279 one();
280 else
281 two();
282 }
283
284 if (foo) {
285 /*
286 * This one requires some explanation,
287 * so we're better off with braces to make
288 * it obvious that the indentation is correct.
289 */
290 doit();
291 }
292
293 - When there are multiple arms to a conditional and some of them
294 require braces, enclose even a single line block in braces for
295 consistency. E.g.:
296
297 if (foo) {
298 doit();
299 } else {
300 one();
301 two();
302 three();
303 }
6d0618a8 304
691d0dd0 305 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 306
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307 - Try to make your code understandable. You may put comments
308 in, but comments invariably tend to stale out when the code
309 they were describing changes. Often splitting a function
310 into two makes the intention of the code much clearer.
311
b75a6ca7 312 - Multi-line comments include their delimiters on separate lines from
313 the text. E.g.
314
315 /*
316 * A very long
317 * multi-line comment.
318 */
319
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320 Note however that a comment that explains a translatable string to
321 translators uses a convention of starting with a magic token
66f5f6dc 322 "TRANSLATORS: ", e.g.
cbcfd4e3 323
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324 /*
325 * TRANSLATORS: here is a comment that explains the string to
326 * be translated, that follows immediately after it.
327 */
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328 _("Here is a translatable string explained by the above.");
329
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330 - Double negation is often harder to understand than no negation
331 at all.
332
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333 - There are two schools of thought when it comes to comparison,
334 especially inside a loop. Some people prefer to have the less stable
335 value on the left hand side and the more stable value on the right hand
336 side, e.g. if you have a loop that counts variable i down to the
337 lower bound,
338
339 while (i > lower_bound) {
340 do something;
341 i--;
342 }
343
344 Other people prefer to have the textual order of values match the
345 actual order of values in their comparison, so that they can
346 mentally draw a number line from left to right and place these
347 values in order, i.e.
348
349 while (lower_bound < i) {
350 do something;
351 i--;
352 }
353
354 Both are valid, and we use both. However, the more "stable" the
355 stable side becomes, the more we tend to prefer the former
356 (comparison with a constant, "i > 0", is an extreme example).
357 Just do not mix styles in the same part of the code and mimic
358 existing styles in the neighbourhood.
359
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360 - There are two schools of thought when it comes to splitting a long
361 logical line into multiple lines. Some people push the second and
362 subsequent lines far enough to the right with tabs and align them:
363
364 if (the_beginning_of_a_very_long_expression_that_has_to ||
365 span_more_than_a_single_line_of ||
366 the_source_text) {
367 ...
368
369 while other people prefer to align the second and the subsequent
370 lines with the column immediately inside the opening parenthesis,
371 with tabs and spaces, following our "tabstop is always a multiple
372 of 8" convention:
373
374 if (the_beginning_of_a_very_long_expression_that_has_to ||
375 span_more_than_a_single_line_of ||
376 the_source_text) {
377 ...
378
379 Both are valid, and we use both. Again, just do not mix styles in
380 the same part of the code and mimic existing styles in the
381 neighbourhood.
382
383 - When splitting a long logical line, some people change line before
384 a binary operator, so that the result looks like a parse tree when
385 you turn your head 90-degrees counterclockwise:
386
387 if (the_beginning_of_a_very_long_expression_that_has_to
388 || span_more_than_a_single_line_of_the_source_text) {
389
390 while other people prefer to leave the operator at the end of the
391 line:
392
393 if (the_beginning_of_a_very_long_expression_that_has_to ||
394 span_more_than_a_single_line_of_the_source_text) {
395
396 Both are valid, but we tend to use the latter more, unless the
397 expression gets fairly complex, in which case the former tends to
398 be easier to read. Again, just do not mix styles in the same part
399 of the code and mimic existing styles in the neighbourhood.
400
401 - When splitting a long logical line, with everything else being
402 equal, it is preferable to split after the operator at higher
403 level in the parse tree. That is, this is more preferable:
404
405 if (a_very_long_variable * that_is_used_in +
406 a_very_long_expression) {
407 ...
408
409 than
410
411 if (a_very_long_variable *
412 that_is_used_in + a_very_long_expression) {
413 ...
414
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415 - Some clever tricks, like using the !! operator with arithmetic
416 constructs, can be extremely confusing to others. Avoid them,
417 unless there is a compelling reason to use them.
418
419 - Use the API. No, really. We have a strbuf (variable length
420 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 421 string_list for sorted string lists, a hash map (mapping struct
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422 objects) named "struct decorate", amongst other things.
423
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424 - When you come up with an API, document its functions and structures
425 in the header file that exposes the API to its callers. Use what is
426 in "strbuf.h" as a model for the appropriate tone and level of
427 detail.
6d0618a8 428
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429 - The first #include in C files, except in platform specific compat/
430 implementations, must be either "git-compat-util.h", "cache.h" or
431 "builtin.h". You do not have to include more than one of these.
432
433 - A C file must directly include the header files that declare the
434 functions and the types it uses, except for the functions and types
435 that are made available to it by including one of the header files
436 it must include by the previous rule.
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437
438 - If you are planning a new command, consider writing it in shell
439 or perl first, so that changes in semantics can be easily
2de9b711 440 changed and discussed. Many Git commands started out like
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441 that, and a few are still scripts.
442
2de9b711 443 - Avoid introducing a new dependency into Git. This means you
6d0618a8 444 usually should stay away from scripting languages not already
2de9b711 445 used in the Git core command set (unless your command is clearly
6d0618a8 446 separate from it, such as an importer to convert random-scm-X
2de9b711 447 repositories to Git).
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448
449 - When we pass <string, length> pair to functions, we should try to
450 pass them in that order.
c455bd89 451
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452 - Use Git's gettext wrappers to make the user interface
453 translatable. See "Marking strings for translation" in po/README.
454
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455 - Variables and functions local to a given source file should be marked
456 with "static". Variables that are visible to other source files
457 must be declared with "extern" in header files. However, function
458 declarations should not use "extern", as that is already the default.
459
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460 - You can launch gdb around your program using the shorthand GIT_DEBUGGER.
461 Run `GIT_DEBUGGER=1 ./bin-wrappers/git foo` to simply use gdb as is, or
462 run `GIT_DEBUGGER="<debugger> <debugger-args>" ./bin-wrappers/git foo` to
463 use your own debugger and arguments. Example: `GIT_DEBUGGER="ddd --gdb"
464 ./bin-wrappers/git log` (See `wrap-for-bin.sh`.)
465
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466For Perl programs:
467
468 - Most of the C guidelines above apply.
469
470 - We try to support Perl 5.8 and later ("use Perl 5.008").
471
472 - use strict and use warnings are strongly preferred.
473
474 - Don't overuse statement modifiers unless using them makes the
475 result easier to follow.
476
477 ... do something ...
478 do_this() unless (condition);
479 ... do something else ...
480
481 is more readable than:
482
483 ... do something ...
484 unless (condition) {
485 do_this();
486 }
487 ... do something else ...
488
489 *only* when the condition is so rare that do_this() will be almost
490 always called.
491
492 - We try to avoid assignments inside "if ()" conditions.
493
494 - Learn and use Git.pm if you need that functionality.
495
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496For Python scripts:
497
498 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
499
45a87a83 500 - As a minimum, we aim to be compatible with Python 2.7.
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501
502 - Where required libraries do not restrict us to Python 2, we try to
503 also be compatible with Python 3.1 and later.
504
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505
506Program Output
507
508 We make a distinction between a Git command's primary output and
509 output which is merely chatty feedback (for instance, status
510 messages, running transcript, or progress display), as well as error
511 messages. Roughly speaking, a Git command's primary output is that
512 which one might want to capture to a file or send down a pipe; its
513 chatty output should not interfere with these use-cases.
514
515 As such, primary output should be sent to the standard output stream
516 (stdout), and chatty output should be sent to the standard error
517 stream (stderr). Examples of commands which produce primary output
518 include `git log`, `git show`, and `git branch --list` which generate
519 output on the stdout stream.
520
521 Not all Git commands have primary output; this is often true of
522 commands whose main function is to perform an action. Some action
523 commands are silent, whereas others are chatty. An example of a
524 chatty action commands is `git clone` with its "Cloning into
525 '<path>'..." and "Checking connectivity..." status messages which it
526 sends to the stderr stream.
527
528 Error messages from Git commands should always be sent to the stderr
529 stream.
530
531
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532Error Messages
533
534 - Do not end error messages with a full stop.
535
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536 - Do not capitalize the first word, only because it is the first word
537 in the message ("unable to open %s", not "Unable to open %s"). But
538 "SHA-3 not supported" is fine, because the reason the first word is
539 capitalized is not because it is at the beginning of the sentence,
540 but because the word would be spelled in capital letters even when
541 it appeared in the middle of the sentence.
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542
543 - Say what the error is first ("cannot open %s", not "%s: cannot open")
544
545
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546Externally Visible Names
547
548 - For configuration variable names, follow the existing convention:
549
550 . The section name indicates the affected subsystem.
551
552 . The subsection name, if any, indicates which of an unbounded set
553 of things to set the value for.
554
555 . The variable name describes the effect of tweaking this knob.
556
557 The section and variable names that consist of multiple words are
558 formed by concatenating the words without punctuations (e.g. `-`),
559 and are broken using bumpyCaps in documentation as a hint to the
560 reader.
561
562 When choosing the variable namespace, do not use variable name for
563 specifying possibly unbounded set of things, most notably anything
564 an end user can freely come up with (e.g. branch names). Instead,
565 use subsection names or variable values, like the existing variable
566 branch.<name>.description does.
567
568
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569Writing Documentation:
570
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571 Most (if not all) of the documentation pages are written in the
572 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
573 processed into HTML and manpages (e.g. git.html and git.1 in the
574 same directory).
bb9f2aec 575
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576 The documentation liberally mixes US and UK English (en_US/UK)
577 norms for spelling and grammar, which is somewhat unfortunate.
578 In an ideal world, it would have been better if it consistently
579 used only one and not the other, and we would have picked en_US
580 (if you wish to correct the English of some of the existing
581 documentation, please see the documentation-related advice in the
582 Documentation/SubmittingPatches file).
583
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584 In order to ensure the documentation is inclusive, avoid assuming
585 that an unspecified example person is male or female, and think
586 twice before using "he", "him", "she", or "her". Here are some
587 tips to avoid use of gendered pronouns:
588
589 - Prefer succinctness and matter-of-factly describing functionality
590 in the abstract. E.g.
591
592 --short:: Emit output in the short-format.
593
594 and avoid something like these overly verbose alternatives:
595
596 --short:: Use this to emit output in the short-format.
597 --short:: You can use this to get output in the short-format.
598 --short:: A user who prefers shorter output could....
599 --short:: Should a person and/or program want shorter output, he
600 she/they/it can...
601
602 This practice often eliminates the need to involve human actors in
603 your description, but it is a good practice regardless of the
604 avoidance of gendered pronouns.
605
606 - When it becomes awkward to stick to this style, prefer "you" when
c9dba103 607 addressing the hypothetical user, and possibly "we" when
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608 discussing how the program might react to the user. E.g.
609
610 You can use this option instead of --xyz, but we might remove
611 support for it in future versions.
612
613 while keeping in mind that you can probably be less verbose, e.g.
614
615 Use this instead of --xyz. This option might be removed in future
616 versions.
617
618 - If you still need to refer to an example person that is
619 third-person singular, you may resort to "singular they" to avoid
620 "he/she/him/her", e.g.
621
622 A contributor asks their upstream to pull from them.
623
624 Note that this sounds ungrammatical and unnatural to those who
625 learned that "they" is only used for third-person plural, e.g.
626 those who learn English as a second language in some parts of the
627 world.
628
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629 Every user-visible change should be reflected in the documentation.
630 The same general rule as for code applies -- imitate the existing
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631 conventions.
632
633 A few commented examples follow to provide reference when writing or
634 modifying command usage strings and synopsis sections in the manual
635 pages:
c455bd89 636
b1afe49d 637 Placeholders are spelled in lowercase and enclosed in angle brackets:
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638 <file>
639 --sort=<key>
640 --abbrev[=<n>]
641
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642 If a placeholder has multiple words, they are separated by dashes:
643 <new-branch-name>
644 --template=<template-directory>
645
469bfc96 646 Possibility of multiple occurrences is indicated by three dots:
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647 <file>...
648 (One or more of <file>.)
649
650 Optional parts are enclosed in square brackets:
651 [<extra>]
652 (Zero or one <extra>.)
653
654 --exec-path[=<path>]
655 (Option with an optional argument. Note that the "=" is inside the
656 brackets.)
657
658 [<patch>...]
659 (Zero or more of <patch>. Note that the dots are inside, not
660 outside the brackets.)
661
9c9b4f2f 662 Multiple alternatives are indicated with vertical bars:
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663 [-q | --quiet]
664 [--utf8 | --no-utf8]
665
666 Parentheses are used for grouping:
9c9b4f2f 667 [(<rev> | <range>)...]
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668 (Any number of either <rev> or <range>. Parens are needed to make
669 it clear that "..." pertains to both <rev> and <range>.)
670
671 [(-p <parent>)...]
672 (Any number of option -p, each with one <parent> argument.)
673
674 git remote set-head <name> (-a | -d | <branch>)
675 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
676 brackets) be provided.)
677
678 And a somewhat more contrived example:
679 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
680 Here "=" is outside the brackets, because "--diff-filter=" is a
681 valid usage. "*" has its own pair of brackets, because it can
682 (optionally) be specified only when one or more of the letters is
683 also provided.
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684
685 A note on notation:
686 Use 'git' (all lowercase) when talking about commands i.e. something
687 the user would type into a shell and use 'Git' (uppercase first letter)
688 when talking about the version control system and its properties.
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689
690 A few commented examples follow to provide reference when writing or
691 modifying paragraphs or option/command explanations that contain options
692 or commands:
693
41f5b21f 694 Literal examples (e.g. use of command-line options, command names,
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695 branch names, URLs, pathnames (files and directories), configuration and
696 environment variables) must be typeset in monospace (i.e. wrapped with
697 backticks):
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698 `--pretty=oneline`
699 `git rev-list`
da0005b8 700 `remote.pushDefault`
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701 `http://git.example.com`
702 `.git/config`
41f5b21f 703 `GIT_DIR`
57103dbf 704 `HEAD`
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705
706 An environment variable must be prefixed with "$" only when referring to its
707 value and not when referring to the variable itself, in this case there is
708 nothing to add except the backticks:
709 `GIT_DIR` is specified
710 `$GIT_DIR/hooks/pre-receive`
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711
712 Word phrases enclosed in `backtick characters` are rendered literally
713 and will not be further expanded. The use of `backticks` to achieve the
714 previous rule means that literal examples should not use AsciiDoc
715 escapes.
716 Correct:
717 `--pretty=oneline`
718 Incorrect:
719 `\--pretty=oneline`
720
721 If some place in the documentation needs to typeset a command usage
722 example with inline substitutions, it is fine to use +monospaced and
723 inline substituted text+ instead of `monospaced literal text`, and with
724 the former, the part that should not get substituted must be
725 quoted/escaped.