Git 2.22
[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. That means that you should not use C99
199 initializers, even if a lot of compilers grok it.
200
201 - Variables have to be declared at the beginning of the block.
202
203 - NULL pointers shall be written as NULL, not as 0.
204
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205 - When declaring pointers, the star sides with the variable
206 name, i.e. "char *string", not "char* string" or
207 "char * string". This makes it easier to understand code
208 like "char *string, c;".
209
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210 - Use whitespace around operators and keywords, but not inside
211 parentheses and not around functions. So:
212
213 while (condition)
214 func(bar + 1);
215
216 and not:
217
218 while( condition )
219 func (bar+1);
220
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221 - We avoid using braces unnecessarily. I.e.
222
223 if (bla) {
224 x = 1;
225 }
226
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227 is frowned upon. But there are a few exceptions:
228
229 - When the statement extends over a few lines (e.g., a while loop
230 with an embedded conditional, or a comment). E.g.:
231
232 while (foo) {
233 if (x)
234 one();
235 else
236 two();
237 }
238
239 if (foo) {
240 /*
241 * This one requires some explanation,
242 * so we're better off with braces to make
243 * it obvious that the indentation is correct.
244 */
245 doit();
246 }
247
248 - When there are multiple arms to a conditional and some of them
249 require braces, enclose even a single line block in braces for
250 consistency. E.g.:
251
252 if (foo) {
253 doit();
254 } else {
255 one();
256 two();
257 three();
258 }
6d0618a8 259
691d0dd0 260 - We try to avoid assignments in the condition of an "if" statement.
0b0b8cd7 261
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262 - Try to make your code understandable. You may put comments
263 in, but comments invariably tend to stale out when the code
264 they were describing changes. Often splitting a function
265 into two makes the intention of the code much clearer.
266
b75a6ca7 267 - Multi-line comments include their delimiters on separate lines from
268 the text. E.g.
269
270 /*
271 * A very long
272 * multi-line comment.
273 */
274
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275 Note however that a comment that explains a translatable string to
276 translators uses a convention of starting with a magic token
66f5f6dc 277 "TRANSLATORS: ", e.g.
cbcfd4e3 278
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279 /*
280 * TRANSLATORS: here is a comment that explains the string to
281 * be translated, that follows immediately after it.
282 */
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283 _("Here is a translatable string explained by the above.");
284
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285 - Double negation is often harder to understand than no negation
286 at all.
287
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288 - There are two schools of thought when it comes to comparison,
289 especially inside a loop. Some people prefer to have the less stable
290 value on the left hand side and the more stable value on the right hand
291 side, e.g. if you have a loop that counts variable i down to the
292 lower bound,
293
294 while (i > lower_bound) {
295 do something;
296 i--;
297 }
298
299 Other people prefer to have the textual order of values match the
300 actual order of values in their comparison, so that they can
301 mentally draw a number line from left to right and place these
302 values in order, i.e.
303
304 while (lower_bound < i) {
305 do something;
306 i--;
307 }
308
309 Both are valid, and we use both. However, the more "stable" the
310 stable side becomes, the more we tend to prefer the former
311 (comparison with a constant, "i > 0", is an extreme example).
312 Just do not mix styles in the same part of the code and mimic
313 existing styles in the neighbourhood.
314
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315 - There are two schools of thought when it comes to splitting a long
316 logical line into multiple lines. Some people push the second and
317 subsequent lines far enough to the right with tabs and align them:
318
319 if (the_beginning_of_a_very_long_expression_that_has_to ||
320 span_more_than_a_single_line_of ||
321 the_source_text) {
322 ...
323
324 while other people prefer to align the second and the subsequent
325 lines with the column immediately inside the opening parenthesis,
326 with tabs and spaces, following our "tabstop is always a multiple
327 of 8" convention:
328
329 if (the_beginning_of_a_very_long_expression_that_has_to ||
330 span_more_than_a_single_line_of ||
331 the_source_text) {
332 ...
333
334 Both are valid, and we use both. Again, just do not mix styles in
335 the same part of the code and mimic existing styles in the
336 neighbourhood.
337
338 - When splitting a long logical line, some people change line before
339 a binary operator, so that the result looks like a parse tree when
340 you turn your head 90-degrees counterclockwise:
341
342 if (the_beginning_of_a_very_long_expression_that_has_to
343 || span_more_than_a_single_line_of_the_source_text) {
344
345 while other people prefer to leave the operator at the end of the
346 line:
347
348 if (the_beginning_of_a_very_long_expression_that_has_to ||
349 span_more_than_a_single_line_of_the_source_text) {
350
351 Both are valid, but we tend to use the latter more, unless the
352 expression gets fairly complex, in which case the former tends to
353 be easier to read. Again, just do not mix styles in the same part
354 of the code and mimic existing styles in the neighbourhood.
355
356 - When splitting a long logical line, with everything else being
357 equal, it is preferable to split after the operator at higher
358 level in the parse tree. That is, this is more preferable:
359
360 if (a_very_long_variable * that_is_used_in +
361 a_very_long_expression) {
362 ...
363
364 than
365
366 if (a_very_long_variable *
367 that_is_used_in + a_very_long_expression) {
368 ...
369
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370 - Some clever tricks, like using the !! operator with arithmetic
371 constructs, can be extremely confusing to others. Avoid them,
372 unless there is a compelling reason to use them.
373
374 - Use the API. No, really. We have a strbuf (variable length
375 string), several arrays with the ALLOC_GROW() macro, a
c455c87c 376 string_list for sorted string lists, a hash map (mapping struct
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377 objects) named "struct decorate", amongst other things.
378
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379 - When you come up with an API, document its functions and structures
380 in the header file that exposes the API to its callers. Use what is
381 in "strbuf.h" as a model for the appropriate tone and level of
382 detail.
6d0618a8 383
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384 - The first #include in C files, except in platform specific compat/
385 implementations, must be either "git-compat-util.h", "cache.h" or
386 "builtin.h". You do not have to include more than one of these.
387
388 - A C file must directly include the header files that declare the
389 functions and the types it uses, except for the functions and types
390 that are made available to it by including one of the header files
391 it must include by the previous rule.
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392
393 - If you are planning a new command, consider writing it in shell
394 or perl first, so that changes in semantics can be easily
2de9b711 395 changed and discussed. Many Git commands started out like
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396 that, and a few are still scripts.
397
2de9b711 398 - Avoid introducing a new dependency into Git. This means you
6d0618a8 399 usually should stay away from scripting languages not already
2de9b711 400 used in the Git core command set (unless your command is clearly
6d0618a8 401 separate from it, such as an importer to convert random-scm-X
2de9b711 402 repositories to Git).
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403
404 - When we pass <string, length> pair to functions, we should try to
405 pass them in that order.
c455bd89 406
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407 - Use Git's gettext wrappers to make the user interface
408 translatable. See "Marking strings for translation" in po/README.
409
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410 - Variables and functions local to a given source file should be marked
411 with "static". Variables that are visible to other source files
412 must be declared with "extern" in header files. However, function
413 declarations should not use "extern", as that is already the default.
414
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415For Perl programs:
416
417 - Most of the C guidelines above apply.
418
419 - We try to support Perl 5.8 and later ("use Perl 5.008").
420
421 - use strict and use warnings are strongly preferred.
422
423 - Don't overuse statement modifiers unless using them makes the
424 result easier to follow.
425
426 ... do something ...
427 do_this() unless (condition);
428 ... do something else ...
429
430 is more readable than:
431
432 ... do something ...
433 unless (condition) {
434 do_this();
435 }
436 ... do something else ...
437
438 *only* when the condition is so rare that do_this() will be almost
439 always called.
440
441 - We try to avoid assignments inside "if ()" conditions.
442
443 - Learn and use Git.pm if you need that functionality.
444
445 - For Emacs, it's useful to put the following in
446 GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
447
448 ;; note the first part is useful for C editing, too
449 ((nil . ((indent-tabs-mode . t)
450 (tab-width . 8)
451 (fill-column . 80)))
452 (cperl-mode . ((cperl-indent-level . 8)
453 (cperl-extra-newline-before-brace . nil)
454 (cperl-merge-trailing-else . t))))
455
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456For Python scripts:
457
458 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
459
460 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
461
462 - Where required libraries do not restrict us to Python 2, we try to
463 also be compatible with Python 3.1 and later.
464
465 - When you must differentiate between Unicode literals and byte string
466 literals, it is OK to use the 'b' prefix. Even though the Python
467 documentation for version 2.6 does not mention this prefix, it has
468 been supported since version 2.6.0.
469
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470Error Messages
471
472 - Do not end error messages with a full stop.
473
474 - Do not capitalize ("unable to open %s", not "Unable to open %s")
475
476 - Say what the error is first ("cannot open %s", not "%s: cannot open")
477
478
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479Externally Visible Names
480
481 - For configuration variable names, follow the existing convention:
482
483 . The section name indicates the affected subsystem.
484
485 . The subsection name, if any, indicates which of an unbounded set
486 of things to set the value for.
487
488 . The variable name describes the effect of tweaking this knob.
489
490 The section and variable names that consist of multiple words are
491 formed by concatenating the words without punctuations (e.g. `-`),
492 and are broken using bumpyCaps in documentation as a hint to the
493 reader.
494
495 When choosing the variable namespace, do not use variable name for
496 specifying possibly unbounded set of things, most notably anything
497 an end user can freely come up with (e.g. branch names). Instead,
498 use subsection names or variable values, like the existing variable
499 branch.<name>.description does.
500
501
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502Writing Documentation:
503
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504 Most (if not all) of the documentation pages are written in the
505 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
506 processed into HTML and manpages (e.g. git.html and git.1 in the
507 same directory).
bb9f2aec 508
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509 The documentation liberally mixes US and UK English (en_US/UK)
510 norms for spelling and grammar, which is somewhat unfortunate.
511 In an ideal world, it would have been better if it consistently
512 used only one and not the other, and we would have picked en_US
513 (if you wish to correct the English of some of the existing
514 documentation, please see the documentation-related advice in the
515 Documentation/SubmittingPatches file).
516
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517 Every user-visible change should be reflected in the documentation.
518 The same general rule as for code applies -- imitate the existing
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519 conventions.
520
521 A few commented examples follow to provide reference when writing or
522 modifying command usage strings and synopsis sections in the manual
523 pages:
c455bd89 524
b1afe49d 525 Placeholders are spelled in lowercase and enclosed in angle brackets:
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526 <file>
527 --sort=<key>
528 --abbrev[=<n>]
529
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530 If a placeholder has multiple words, they are separated by dashes:
531 <new-branch-name>
532 --template=<template-directory>
533
469bfc96 534 Possibility of multiple occurrences is indicated by three dots:
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535 <file>...
536 (One or more of <file>.)
537
538 Optional parts are enclosed in square brackets:
539 [<extra>]
540 (Zero or one <extra>.)
541
542 --exec-path[=<path>]
543 (Option with an optional argument. Note that the "=" is inside the
544 brackets.)
545
546 [<patch>...]
547 (Zero or more of <patch>. Note that the dots are inside, not
548 outside the brackets.)
549
9c9b4f2f 550 Multiple alternatives are indicated with vertical bars:
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551 [-q | --quiet]
552 [--utf8 | --no-utf8]
553
554 Parentheses are used for grouping:
9c9b4f2f 555 [(<rev> | <range>)...]
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556 (Any number of either <rev> or <range>. Parens are needed to make
557 it clear that "..." pertains to both <rev> and <range>.)
558
559 [(-p <parent>)...]
560 (Any number of option -p, each with one <parent> argument.)
561
562 git remote set-head <name> (-a | -d | <branch>)
563 (One and only one of "-a", "-d" or "<branch>" _must_ (no square
564 brackets) be provided.)
565
566 And a somewhat more contrived example:
567 --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
568 Here "=" is outside the brackets, because "--diff-filter=" is a
569 valid usage. "*" has its own pair of brackets, because it can
570 (optionally) be specified only when one or more of the letters is
571 also provided.
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572
573 A note on notation:
574 Use 'git' (all lowercase) when talking about commands i.e. something
575 the user would type into a shell and use 'Git' (uppercase first letter)
576 when talking about the version control system and its properties.
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577
578 A few commented examples follow to provide reference when writing or
579 modifying paragraphs or option/command explanations that contain options
580 or commands:
581
41f5b21f 582 Literal examples (e.g. use of command-line options, command names,
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583 branch names, URLs, pathnames (files and directories), configuration and
584 environment variables) must be typeset in monospace (i.e. wrapped with
585 backticks):
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586 `--pretty=oneline`
587 `git rev-list`
da0005b8 588 `remote.pushDefault`
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589 `http://git.example.com`
590 `.git/config`
41f5b21f 591 `GIT_DIR`
57103dbf 592 `HEAD`
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593
594 An environment variable must be prefixed with "$" only when referring to its
595 value and not when referring to the variable itself, in this case there is
596 nothing to add except the backticks:
597 `GIT_DIR` is specified
598 `$GIT_DIR/hooks/pre-receive`
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599
600 Word phrases enclosed in `backtick characters` are rendered literally
601 and will not be further expanded. The use of `backticks` to achieve the
602 previous rule means that literal examples should not use AsciiDoc
603 escapes.
604 Correct:
605 `--pretty=oneline`
606 Incorrect:
607 `\--pretty=oneline`
608
609 If some place in the documentation needs to typeset a command usage
610 example with inline substitutions, it is fine to use +monospaced and
611 inline substituted text+ instead of `monospaced literal text`, and with
612 the former, the part that should not get substituted must be
613 quoted/escaped.