Documentation / CodingGuidelineson commit dir.c: mark more strings for translation (a80897c)
   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://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html
  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 - As a Git developer we assume you have a reasonably modern compiler
 175   and we recommend you to enable the DEVELOPER makefile knob to
 176   ensure your patch is clear of all compiler warnings we care about,
 177   by e.g. "echo DEVELOPER=1 >>config.mak".
 178
 179 - We try to support a wide range of C compilers to compile Git with,
 180   including old ones. That means that you should not use C99
 181   initializers, even if a lot of compilers grok it.
 182
 183 - Variables have to be declared at the beginning of the block.
 184
 185 - NULL pointers shall be written as NULL, not as 0.
 186
 187 - When declaring pointers, the star sides with the variable
 188   name, i.e. "char *string", not "char* string" or
 189   "char * string".  This makes it easier to understand code
 190   like "char *string, c;".
 191
 192 - Use whitespace around operators and keywords, but not inside
 193   parentheses and not around functions. So:
 194
 195        while (condition)
 196                func(bar + 1);
 197
 198   and not:
 199
 200        while( condition )
 201                func (bar+1);
 202
 203 - We avoid using braces unnecessarily.  I.e.
 204
 205        if (bla) {
 206                x = 1;
 207        }
 208
 209   is frowned upon. But there are a few exceptions:
 210
 211        - When the statement extends over a few lines (e.g., a while loop
 212          with an embedded conditional, or a comment). E.g.:
 213
 214                while (foo) {
 215                        if (x)
 216                                one();
 217                        else
 218                                two();
 219                }
 220
 221                if (foo) {
 222                        /*
 223                         * This one requires some explanation,
 224                         * so we're better off with braces to make
 225                         * it obvious that the indentation is correct.
 226                         */
 227                        doit();
 228                }
 229
 230        - When there are multiple arms to a conditional and some of them
 231          require braces, enclose even a single line block in braces for
 232          consistency. E.g.:
 233
 234                if (foo) {
 235                        doit();
 236                } else {
 237                        one();
 238                        two();
 239                        three();
 240                }
 241
 242 - We try to avoid assignments in the condition of an "if" statement.
 243
 244 - Try to make your code understandable.  You may put comments
 245   in, but comments invariably tend to stale out when the code
 246   they were describing changes.  Often splitting a function
 247   into two makes the intention of the code much clearer.
 248
 249 - Multi-line comments include their delimiters on separate lines from
 250   the text.  E.g.
 251
 252        /*
 253         * A very long
 254         * multi-line comment.
 255         */
 256
 257   Note however that a comment that explains a translatable string to
 258   translators uses a convention of starting with a magic token
 259   "TRANSLATORS: ", e.g.
 260
 261        /*
 262         * TRANSLATORS: here is a comment that explains the string to
 263         * be translated, that follows immediately after it.
 264         */
 265        _("Here is a translatable string explained by the above.");
 266
 267 - Double negation is often harder to understand than no negation
 268   at all.
 269
 270 - There are two schools of thought when it comes to comparison,
 271   especially inside a loop. Some people prefer to have the less stable
 272   value on the left hand side and the more stable value on the right hand
 273   side, e.g. if you have a loop that counts variable i down to the
 274   lower bound,
 275
 276        while (i > lower_bound) {
 277                do something;
 278                i--;
 279        }
 280
 281   Other people prefer to have the textual order of values match the
 282   actual order of values in their comparison, so that they can
 283   mentally draw a number line from left to right and place these
 284   values in order, i.e.
 285
 286        while (lower_bound < i) {
 287                do something;
 288                i--;
 289        }
 290
 291   Both are valid, and we use both.  However, the more "stable" the
 292   stable side becomes, the more we tend to prefer the former
 293   (comparison with a constant, "i > 0", is an extreme example).
 294   Just do not mix styles in the same part of the code and mimic
 295   existing styles in the neighbourhood.
 296
 297 - There are two schools of thought when it comes to splitting a long
 298   logical line into multiple lines.  Some people push the second and
 299   subsequent lines far enough to the right with tabs and align them:
 300
 301        if (the_beginning_of_a_very_long_expression_that_has_to ||
 302                span_more_than_a_single_line_of ||
 303                the_source_text) {
 304                ...
 305
 306   while other people prefer to align the second and the subsequent
 307   lines with the column immediately inside the opening parenthesis,
 308   with tabs and spaces, following our "tabstop is always a multiple
 309   of 8" convention:
 310
 311        if (the_beginning_of_a_very_long_expression_that_has_to ||
 312            span_more_than_a_single_line_of ||
 313            the_source_text) {
 314                ...
 315
 316   Both are valid, and we use both.  Again, just do not mix styles in
 317   the same part of the code and mimic existing styles in the
 318   neighbourhood.
 319
 320 - When splitting a long logical line, some people change line before
 321   a binary operator, so that the result looks like a parse tree when
 322   you turn your head 90-degrees counterclockwise:
 323
 324        if (the_beginning_of_a_very_long_expression_that_has_to
 325            || span_more_than_a_single_line_of_the_source_text) {
 326
 327   while other people prefer to leave the operator at the end of the
 328   line:
 329
 330        if (the_beginning_of_a_very_long_expression_that_has_to ||
 331            span_more_than_a_single_line_of_the_source_text) {
 332
 333   Both are valid, but we tend to use the latter more, unless the
 334   expression gets fairly complex, in which case the former tends to
 335   be easier to read.  Again, just do not mix styles in the same part
 336   of the code and mimic existing styles in the neighbourhood.
 337
 338 - When splitting a long logical line, with everything else being
 339   equal, it is preferable to split after the operator at higher
 340   level in the parse tree.  That is, this is more preferable:
 341
 342        if (a_very_long_variable * that_is_used_in +
 343            a_very_long_expression) {
 344                ...
 345
 346   than
 347
 348        if (a_very_long_variable *
 349            that_is_used_in + a_very_long_expression) {
 350                ...
 351
 352 - Some clever tricks, like using the !! operator with arithmetic
 353   constructs, can be extremely confusing to others.  Avoid them,
 354   unless there is a compelling reason to use them.
 355
 356 - Use the API.  No, really.  We have a strbuf (variable length
 357   string), several arrays with the ALLOC_GROW() macro, a
 358   string_list for sorted string lists, a hash map (mapping struct
 359   objects) named "struct decorate", amongst other things.
 360
 361 - When you come up with an API, document it.
 362
 363 - The first #include in C files, except in platform specific compat/
 364   implementations, must be either "git-compat-util.h", "cache.h" or
 365   "builtin.h".  You do not have to include more than one of these.
 366
 367 - A C file must directly include the header files that declare the
 368   functions and the types it uses, except for the functions and types
 369   that are made available to it by including one of the header files
 370   it must include by the previous rule.
 371
 372 - If you are planning a new command, consider writing it in shell
 373   or perl first, so that changes in semantics can be easily
 374   changed and discussed.  Many Git commands started out like
 375   that, and a few are still scripts.
 376
 377 - Avoid introducing a new dependency into Git. This means you
 378   usually should stay away from scripting languages not already
 379   used in the Git core command set (unless your command is clearly
 380   separate from it, such as an importer to convert random-scm-X
 381   repositories to Git).
 382
 383 - When we pass <string, length> pair to functions, we should try to
 384   pass them in that order.
 385
 386 - Use Git's gettext wrappers to make the user interface
 387   translatable. See "Marking strings for translation" in po/README.
 388
 389 - Variables and functions local to a given source file should be marked
 390   with "static". Variables that are visible to other source files
 391   must be declared with "extern" in header files. However, function
 392   declarations should not use "extern", as that is already the default.
 393
 394For Perl programs:
 395
 396 - Most of the C guidelines above apply.
 397
 398 - We try to support Perl 5.8 and later ("use Perl 5.008").
 399
 400 - use strict and use warnings are strongly preferred.
 401
 402 - Don't overuse statement modifiers unless using them makes the
 403   result easier to follow.
 404
 405        ... do something ...
 406        do_this() unless (condition);
 407        ... do something else ...
 408
 409   is more readable than:
 410
 411        ... do something ...
 412        unless (condition) {
 413                do_this();
 414        }
 415        ... do something else ...
 416
 417   *only* when the condition is so rare that do_this() will be almost
 418   always called.
 419
 420 - We try to avoid assignments inside "if ()" conditions.
 421
 422 - Learn and use Git.pm if you need that functionality.
 423
 424 - For Emacs, it's useful to put the following in
 425   GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
 426
 427    ;; note the first part is useful for C editing, too
 428    ((nil . ((indent-tabs-mode . t)
 429                  (tab-width . 8)
 430                  (fill-column . 80)))
 431     (cperl-mode . ((cperl-indent-level . 8)
 432                    (cperl-extra-newline-before-brace . nil)
 433                    (cperl-merge-trailing-else . t))))
 434
 435For Python scripts:
 436
 437 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
 438
 439 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
 440
 441 - Where required libraries do not restrict us to Python 2, we try to
 442   also be compatible with Python 3.1 and later.
 443
 444 - When you must differentiate between Unicode literals and byte string
 445   literals, it is OK to use the 'b' prefix.  Even though the Python
 446   documentation for version 2.6 does not mention this prefix, it has
 447   been supported since version 2.6.0.
 448
 449Error Messages
 450
 451 - Do not end error messages with a full stop.
 452
 453 - Do not capitalize ("unable to open %s", not "Unable to open %s")
 454
 455 - Say what the error is first ("cannot open %s", not "%s: cannot open")
 456
 457
 458Externally Visible Names
 459
 460 - For configuration variable names, follow the existing convention:
 461
 462   . The section name indicates the affected subsystem.
 463
 464   . The subsection name, if any, indicates which of an unbounded set
 465     of things to set the value for.
 466
 467   . The variable name describes the effect of tweaking this knob.
 468
 469   The section and variable names that consist of multiple words are
 470   formed by concatenating the words without punctuations (e.g. `-`),
 471   and are broken using bumpyCaps in documentation as a hint to the
 472   reader.
 473
 474   When choosing the variable namespace, do not use variable name for
 475   specifying possibly unbounded set of things, most notably anything
 476   an end user can freely come up with (e.g. branch names).  Instead,
 477   use subsection names or variable values, like the existing variable
 478   branch.<name>.description does.
 479
 480
 481Writing Documentation:
 482
 483 Most (if not all) of the documentation pages are written in the
 484 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
 485 processed into HTML and manpages (e.g. git.html and git.1 in the
 486 same directory).
 487
 488 The documentation liberally mixes US and UK English (en_US/UK)
 489 norms for spelling and grammar, which is somewhat unfortunate.
 490 In an ideal world, it would have been better if it consistently
 491 used only one and not the other, and we would have picked en_US
 492 (if you wish to correct the English of some of the existing
 493 documentation, please see the documentation-related advice in the
 494 Documentation/SubmittingPatches file).
 495
 496 Every user-visible change should be reflected in the documentation.
 497 The same general rule as for code applies -- imitate the existing
 498 conventions.
 499
 500 A few commented examples follow to provide reference when writing or
 501 modifying command usage strings and synopsis sections in the manual
 502 pages:
 503
 504 Placeholders are spelled in lowercase and enclosed in angle brackets:
 505   <file>
 506   --sort=<key>
 507   --abbrev[=<n>]
 508
 509 If a placeholder has multiple words, they are separated by dashes:
 510   <new-branch-name>
 511   --template=<template-directory>
 512
 513 Possibility of multiple occurrences is indicated by three dots:
 514   <file>...
 515   (One or more of <file>.)
 516
 517 Optional parts are enclosed in square brackets:
 518   [<extra>]
 519   (Zero or one <extra>.)
 520
 521   --exec-path[=<path>]
 522   (Option with an optional argument.  Note that the "=" is inside the
 523   brackets.)
 524
 525   [<patch>...]
 526   (Zero or more of <patch>.  Note that the dots are inside, not
 527   outside the brackets.)
 528
 529 Multiple alternatives are indicated with vertical bars:
 530   [-q | --quiet]
 531   [--utf8 | --no-utf8]
 532
 533 Parentheses are used for grouping:
 534   [(<rev> | <range>)...]
 535   (Any number of either <rev> or <range>.  Parens are needed to make
 536   it clear that "..." pertains to both <rev> and <range>.)
 537
 538   [(-p <parent>)...]
 539   (Any number of option -p, each with one <parent> argument.)
 540
 541   git remote set-head <name> (-a | -d | <branch>)
 542   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 543   brackets) be provided.)
 544
 545 And a somewhat more contrived example:
 546   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 547   Here "=" is outside the brackets, because "--diff-filter=" is a
 548   valid usage.  "*" has its own pair of brackets, because it can
 549   (optionally) be specified only when one or more of the letters is
 550   also provided.
 551
 552  A note on notation:
 553   Use 'git' (all lowercase) when talking about commands i.e. something
 554   the user would type into a shell and use 'Git' (uppercase first letter)
 555   when talking about the version control system and its properties.
 556
 557 A few commented examples follow to provide reference when writing or
 558 modifying paragraphs or option/command explanations that contain options
 559 or commands:
 560
 561 Literal examples (e.g. use of command-line options, command names,
 562 branch names, configuration and environment variables) must be
 563 typeset in monospace (i.e. wrapped with backticks):
 564   `--pretty=oneline`
 565   `git rev-list`
 566   `remote.pushDefault`
 567   `GIT_DIR`
 568   `HEAD`
 569
 570 An environment variable must be prefixed with "$" only when referring to its
 571 value and not when referring to the variable itself, in this case there is
 572 nothing to add except the backticks:
 573   `GIT_DIR` is specified
 574   `$GIT_DIR/hooks/pre-receive`
 575
 576 Word phrases enclosed in `backtick characters` are rendered literally
 577 and will not be further expanded. The use of `backticks` to achieve the
 578 previous rule means that literal examples should not use AsciiDoc
 579 escapes.
 580   Correct:
 581      `--pretty=oneline`
 582   Incorrect:
 583      `\--pretty=oneline`
 584
 585 If some place in the documentation needs to typeset a command usage
 586 example with inline substitutions, it is fine to use +monospaced and
 587 inline substituted text+ instead of `monospaced literal text`, and with
 588 the former, the part that should not get substituted must be
 589 quoted/escaped.