Documentation / CodingGuidelineson commit Merge branch 'es/worktree-add' (522e2f4)
   1Like other projects, we also have some guidelines to keep to the
   2code.  For Git in general, a few rough rules are:
   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
  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
  29Make your code readable and sensible, and don't try to be clever.
  30
  31As for more concrete guidelines, just imitate the existing code
  32(this is a good guideline, no matter which project you are
  33contributing to). It is always preferable to match the _local_
  34convention. New code added to Git suite is expected to match
  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.
  40
  41For shell scripts specifically (not exhaustive):
  42
  43 - We use tabs for indentation.
  44
  45 - Case arms are indented at the same depth as case and esac lines,
  46   like this:
  47
  48        case "$variable" in
  49        pattern1)
  50                do this
  51                ;;
  52        pattern2)
  53                do that
  54                ;;
  55        esac
  56
  57 - Redirection operators should be written with space before, but no
  58   space after them.  In other words, write 'echo test >"$file"'
  59   instead of 'echo test> $file' or 'echo test > $file'.  Note that
  60   even though it is not required by POSIX to double-quote the
  61   redirection target in a variable (as shown above), our code does so
  62   because some versions of bash issue a warning without the quotes.
  63
  64        (incorrect)
  65        cat hello > world < universe
  66        echo hello >$world
  67
  68        (correct)
  69        cat hello >world <universe
  70        echo hello >"$world"
  71
  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
  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
  81 - We use POSIX compliant parameter substitutions and avoid bashisms;
  82   namely:
  83
  84   - We use ${parameter-word} and its [-=?+] siblings, and their
  85     colon'ed "unset or null" form.
  86
  87   - We use ${parameter#word} and its [#%] siblings, and their
  88     doubled "longest matching" form.
  89
  90   - No "Substring Expansion" ${parameter:offset:length}.
  91
  92   - No shell arrays.
  93
  94   - No strlen ${#parameter}.
  95
  96   - No pattern replacement ${parameter/pattern/string}.
  97
  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).
 103
 104 - We do not use Process Substitution <(list) or >(list).
 105
 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
 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
 121 - We prefer "test" over "[ ... ]".
 122
 123 - We do not write the noiseword "function" in front of shell
 124   functions.
 125
 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                ...
 137
 138 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
 139   [::], [==], or [..]) for portability.
 140
 141   - We do not use \{m,n\};
 142
 143   - We do not use -E;
 144
 145   - We do not use ? or + (which are \{0,1\} and \{1,\}
 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
 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
 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
 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
 174 - We try to support a wide range of C compilers to compile Git with,
 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
 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
 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
 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
 210 - We try to avoid assignments in the condition of an "if" statement.
 211
 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
 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
 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
 235 - Double negation is often harder to understand than no negation
 236   at all.
 237
 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
 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
 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
 326   string_list for sorted string lists, a hash map (mapping struct
 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 compat/
 332   implementations, must be either "git-compat-util.h", "cache.h" or
 333   "builtin.h".  You do not have to include more than one of these.
 334
 335 - A C file must directly include the header files that declare the
 336   functions and the types it uses, except for the functions and types
 337   that are made available to it by including one of the header files
 338   it must include by the previous rule.
 339
 340 - If you are planning a new command, consider writing it in shell
 341   or perl first, so that changes in semantics can be easily
 342   changed and discussed.  Many Git commands started out like
 343   that, and a few are still scripts.
 344
 345 - Avoid introducing a new dependency into Git. This means you
 346   usually should stay away from scripting languages not already
 347   used in the Git core command set (unless your command is clearly
 348   separate from it, such as an importer to convert random-scm-X
 349   repositories to Git).
 350
 351 - When we pass <string, length> pair to functions, we should try to
 352   pass them in that order.
 353
 354 - Use Git's gettext wrappers to make the user interface
 355   translatable. See "Marking strings for translation" in po/README.
 356
 357For Perl programs:
 358
 359 - Most of the C guidelines above apply.
 360
 361 - We try to support Perl 5.8 and later ("use Perl 5.008").
 362
 363 - use strict and use warnings are strongly preferred.
 364
 365 - Don't overuse statement modifiers unless using them makes the
 366   result easier to follow.
 367
 368        ... do something ...
 369        do_this() unless (condition);
 370        ... do something else ...
 371
 372   is more readable than:
 373
 374        ... do something ...
 375        unless (condition) {
 376                do_this();
 377        }
 378        ... do something else ...
 379
 380   *only* when the condition is so rare that do_this() will be almost
 381   always called.
 382
 383 - We try to avoid assignments inside "if ()" conditions.
 384
 385 - Learn and use Git.pm if you need that functionality.
 386
 387 - For Emacs, it's useful to put the following in
 388   GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
 389
 390    ;; note the first part is useful for C editing, too
 391    ((nil . ((indent-tabs-mode . t)
 392                  (tab-width . 8)
 393                  (fill-column . 80)))
 394     (cperl-mode . ((cperl-indent-level . 8)
 395                    (cperl-extra-newline-before-brace . nil)
 396                    (cperl-merge-trailing-else . t))))
 397
 398For Python scripts:
 399
 400 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
 401
 402 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
 403
 404 - Where required libraries do not restrict us to Python 2, we try to
 405   also be compatible with Python 3.1 and later.
 406
 407 - When you must differentiate between Unicode literals and byte string
 408   literals, it is OK to use the 'b' prefix.  Even though the Python
 409   documentation for version 2.6 does not mention this prefix, it has
 410   been supported since version 2.6.0.
 411
 412Error Messages
 413
 414 - Do not end error messages with a full stop.
 415
 416 - Do not capitalize ("unable to open %s", not "Unable to open %s")
 417
 418 - Say what the error is first ("cannot open %s", not "%s: cannot open")
 419
 420
 421Externally Visible Names
 422
 423 - For configuration variable names, follow the existing convention:
 424
 425   . The section name indicates the affected subsystem.
 426
 427   . The subsection name, if any, indicates which of an unbounded set
 428     of things to set the value for.
 429
 430   . The variable name describes the effect of tweaking this knob.
 431
 432   The section and variable names that consist of multiple words are
 433   formed by concatenating the words without punctuations (e.g. `-`),
 434   and are broken using bumpyCaps in documentation as a hint to the
 435   reader.
 436
 437   When choosing the variable namespace, do not use variable name for
 438   specifying possibly unbounded set of things, most notably anything
 439   an end user can freely come up with (e.g. branch names).  Instead,
 440   use subsection names or variable values, like the existing variable
 441   branch.<name>.description does.
 442
 443
 444Writing Documentation:
 445
 446 Most (if not all) of the documentation pages are written in the
 447 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
 448 processed into HTML and manpages (e.g. git.html and git.1 in the
 449 same directory).
 450
 451 The documentation liberally mixes US and UK English (en_US/UK)
 452 norms for spelling and grammar, which is somewhat unfortunate.
 453 In an ideal world, it would have been better if it consistently
 454 used only one and not the other, and we would have picked en_US
 455 (if you wish to correct the English of some of the existing
 456 documentation, please see the documentation-related advice in the
 457 Documentation/SubmittingPatches file).
 458
 459 Every user-visible change should be reflected in the documentation.
 460 The same general rule as for code applies -- imitate the existing
 461 conventions.
 462
 463 A few commented examples follow to provide reference when writing or
 464 modifying command usage strings and synopsis sections in the manual
 465 pages:
 466
 467 Placeholders are spelled in lowercase and enclosed in angle brackets:
 468   <file>
 469   --sort=<key>
 470   --abbrev[=<n>]
 471
 472 If a placeholder has multiple words, they are separated by dashes:
 473   <new-branch-name>
 474   --template=<template-directory>
 475
 476 Possibility of multiple occurrences is indicated by three dots:
 477   <file>...
 478   (One or more of <file>.)
 479
 480 Optional parts are enclosed in square brackets:
 481   [<extra>]
 482   (Zero or one <extra>.)
 483
 484   --exec-path[=<path>]
 485   (Option with an optional argument.  Note that the "=" is inside the
 486   brackets.)
 487
 488   [<patch>...]
 489   (Zero or more of <patch>.  Note that the dots are inside, not
 490   outside the brackets.)
 491
 492 Multiple alternatives are indicated with vertical bars:
 493   [-q | --quiet]
 494   [--utf8 | --no-utf8]
 495
 496 Parentheses are used for grouping:
 497   [(<rev> | <range>)...]
 498   (Any number of either <rev> or <range>.  Parens are needed to make
 499   it clear that "..." pertains to both <rev> and <range>.)
 500
 501   [(-p <parent>)...]
 502   (Any number of option -p, each with one <parent> argument.)
 503
 504   git remote set-head <name> (-a | -d | <branch>)
 505   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 506   brackets) be provided.)
 507
 508 And a somewhat more contrived example:
 509   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 510   Here "=" is outside the brackets, because "--diff-filter=" is a
 511   valid usage.  "*" has its own pair of brackets, because it can
 512   (optionally) be specified only when one or more of the letters is
 513   also provided.
 514
 515  A note on notation:
 516   Use 'git' (all lowercase) when talking about commands i.e. something
 517   the user would type into a shell and use 'Git' (uppercase first letter)
 518   when talking about the version control system and its properties.
 519
 520 A few commented examples follow to provide reference when writing or
 521 modifying paragraphs or option/command explanations that contain options
 522 or commands:
 523
 524 Literal examples (e.g. use of command-line options, command names, and
 525 configuration variables) are typeset in monospace, and if you can use
 526 `backticks around word phrases`, do so.
 527   `--pretty=oneline`
 528   `git rev-list`
 529   `remote.pushDefault`
 530
 531 Word phrases enclosed in `backtick characters` are rendered literally
 532 and will not be further expanded. The use of `backticks` to achieve the
 533 previous rule means that literal examples should not use AsciiDoc
 534 escapes.
 535   Correct:
 536      `--pretty=oneline`
 537   Incorrect:
 538      `\--pretty=oneline`
 539
 540 If some place in the documentation needs to typeset a command usage
 541 example with inline substitutions, it is fine to use +monospaced and
 542 inline substituted text+ instead of `monospaced literal text`, and with
 543 the former, the part that should not get substituted must be
 544 quoted/escaped.