Documentation/CodingGuidelines

   1 Like other projects, we also have some guidelines to keep to the
   2 code.  For git in general, three 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
  22 As for more concrete guidelines, just imitate the existing code
  23 (this is a good guideline, no matter which project you are
  24 contributing to). It is always preferable to match the _local_
  25 convention. New code added to git suite is expected to match
  26 the overall style of existing code. Modifications to existing
  27 code is expected to match the style the surrounding code already
  28 uses (even if it doesn't match the overall style of existing code).
  29
  30 But if you must have a list of rules, here they are.
  31
  32 For shell scripts specifically (not exhaustive):
  33
  34  - We use tabs for indentation.
  35
  36  - Case arms are indented at the same depth as case and esac lines.
  37
  38  - We prefer $( ... ) for command substitution; unlike ``, it
  39    properly nests.  It should have been the way Bourne spelled
  40    it from day one, but unfortunately isn't.
  41
  42  - We use POSIX compliant parameter substitutions and avoid bashisms;
  43    namely:
  44
  45    - We use ${parameter-word} and its [-=?+] siblings, and their
  46      colon'ed "unset or null" form.
  47
  48    - We use ${parameter#word} and its [#%] siblings, and their
  49      doubled "longest matching" form.
  50
  51    - No "Substring Expansion" ${parameter:offset:length}.
  52
  53    - No shell arrays.
  54
  55    - No strlen ${#parameter}.
  56
  57    - No pattern replacement ${parameter/pattern/string}.
  58
  59  - We use Arithmetic Expansion $(( ... )).
  60
  61  - Inside Arithmetic Expansion, spell shell variables with $ in front
  62    of them, as some shells do not grok $((x)) while accepting $(($x))
  63    just fine (e.g. dash older than 0.5.4).
  64
  65  - We do not use Process Substitution <(list) or >(list).
  66
  67  - We prefer "test" over "[ ... ]".
  68
  69  - We do not write the noiseword "function" in front of shell
  70    functions.
  71
  72  - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
  73    [::], [==], nor [..]) for portability.
  74
  75    - We do not use \{m,n\};
  76
  77    - We do not use -E;
  78
  79    - We do not use ? nor + (which are \{0,1\} and \{1,\}
  80      respectively in BRE) but that goes without saying as these
  81      are ERE elements not BRE (note that \? and \+ are not even part
  82      of BRE -- making them accessible from BRE is a GNU extension).
  83
  84  - Use Git's gettext wrappers in git-sh-i18n to make the user
  85    interface translatable. See "Marking strings for translation" in
  86    po/README.
  87
  88 For C programs:
  89
  90  - We use tabs to indent, and interpret tabs as taking up to
  91    8 spaces.
  92
  93  - We try to keep to at most 80 characters per line.
  94
  95  - When declaring pointers, the star sides with the variable
  96    name, i.e. "char *string", not "char* string" or
  97    "char * string".  This makes it easier to understand code
  98    like "char *string, c;".
  99
 100  - We avoid using braces unnecessarily.  I.e.
 101
 102         if (bla) {
 103                 x = 1;
 104         }
 105
 106    is frowned upon.  A gray area is when the statement extends
 107    over a few lines, and/or you have a lengthy comment atop of
 108    it.  Also, like in the Linux kernel, if there is a long list
 109    of "else if" statements, it can make sense to add braces to
 110    single line blocks.
 111
 112  - We try to avoid assignments inside if().
 113
 114  - Try to make your code understandable.  You may put comments
 115    in, but comments invariably tend to stale out when the code
 116    they were describing changes.  Often splitting a function
 117    into two makes the intention of the code much clearer.
 118
 119  - Double negation is often harder to understand than no negation
 120    at all.
 121
 122  - Some clever tricks, like using the !! operator with arithmetic
 123    constructs, can be extremely confusing to others.  Avoid them,
 124    unless there is a compelling reason to use them.
 125
 126  - Use the API.  No, really.  We have a strbuf (variable length
 127    string), several arrays with the ALLOC_GROW() macro, a
 128    string_list for sorted string lists, a hash map (mapping struct
 129    objects) named "struct decorate", amongst other things.
 130
 131  - When you come up with an API, document it.
 132
 133  - The first #include in C files, except in platform specific
 134    compat/ implementations, should be git-compat-util.h or another
 135    header file that includes it, such as cache.h or builtin.h.
 136
 137  - If you are planning a new command, consider writing it in shell
 138    or perl first, so that changes in semantics can be easily
 139    changed and discussed.  Many git commands started out like
 140    that, and a few are still scripts.
 141
 142  - Avoid introducing a new dependency into git. This means you
 143    usually should stay away from scripting languages not already
 144    used in the git core command set (unless your command is clearly
 145    separate from it, such as an importer to convert random-scm-X
 146    repositories to git).
 147
 148  - When we pass <string, length> pair to functions, we should try to
 149    pass them in that order.
 150
 151  - Use Git's gettext wrappers to make the user interface
 152    translatable. See "Marking strings for translation" in po/README.
 153
 154 Writing Documentation:
 155
 156  Every user-visible change should be reflected in the documentation.
 157  The same general rule as for code applies -- imitate the existing
 158  conventions.  A few commented examples follow to provide reference
 159  when writing or modifying command usage strings and synopsis sections
 160  in the manual pages:
 161
 162  Placeholders are spelled in lowercase and enclosed in angle brackets:
 163    <file>
 164    --sort=<key>
 165    --abbrev[=<n>]
 166
 167  Possibility of multiple occurrences is indicated by three dots:
 168    <file>...
 169    (One or more of <file>.)
 170
 171  Optional parts are enclosed in square brackets:
 172    [<extra>]
 173    (Zero or one <extra>.)
 174
 175    --exec-path[=<path>]
 176    (Option with an optional argument.  Note that the "=" is inside the
 177    brackets.)
 178
 179    [<patch>...]
 180    (Zero or more of <patch>.  Note that the dots are inside, not
 181    outside the brackets.)
 182
 183  Multiple alternatives are indicated with vertical bar:
 184    [-q | --quiet]
 185    [--utf8 | --no-utf8]
 186
 187  Parentheses are used for grouping:
 188    [(<rev>|<range>)...]
 189    (Any number of either <rev> or <range>.  Parens are needed to make
 190    it clear that "..." pertains to both <rev> and <range>.)
 191
 192    [(-p <parent>)...]
 193    (Any number of option -p, each with one <parent> argument.)
 194
 195    git remote set-head <name> (-a | -d | <branch>)
 196    (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 197    brackets) be provided.)
 198
 199  And a somewhat more contrived example:
 200    --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 201    Here "=" is outside the brackets, because "--diff-filter=" is a
 202    valid usage.  "*" has its own pair of brackets, because it can
 203    (optionally) be specified only when one or more of the letters is
 204    also provided.