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  - Redirection operators should be written with space before, but no
  39    space after them.  In other words, write 'echo test >"$file"'
  40    instead of 'echo test> $file' or 'echo test > $file'.  Note that
  41    even though it is not required by POSIX to double-quote the
  42    redirection target in a variable (as shown above), our code does so
  43    because some versions of bash issue a warning without the quotes.
  44
  45  - We prefer $( ... ) for command substitution; unlike ``, it
  46    properly nests.  It should have been the way Bourne spelled
  47    it from day one, but unfortunately isn't.
  48
  49  - If you want to find out if a command is available on the user's
  50    $PATH, you should use 'type <command>', instead of 'which <command>'.
  51    The output of 'which' is not machine parseable and its exit code
  52    is not reliable across platforms.
  53
  54  - We use POSIX compliant parameter substitutions and avoid bashisms;
  55    namely:
  56
  57    - We use ${parameter-word} and its [-=?+] siblings, and their
  58      colon'ed "unset or null" form.
  59
  60    - We use ${parameter#word} and its [#%] siblings, and their
  61      doubled "longest matching" form.
  62
  63    - No "Substring Expansion" ${parameter:offset:length}.
  64
  65    - No shell arrays.
  66
  67    - No strlen ${#parameter}.
  68
  69    - No pattern replacement ${parameter/pattern/string}.
  70
  71  - We use Arithmetic Expansion $(( ... )).
  72
  73  - Inside Arithmetic Expansion, spell shell variables with $ in front
  74    of them, as some shells do not grok $((x)) while accepting $(($x))
  75    just fine (e.g. dash older than 0.5.4).
  76
  77  - We do not use Process Substitution <(list) or >(list).
  78
  79  - We prefer "test" over "[ ... ]".
  80
  81  - We do not write the noiseword "function" in front of shell
  82    functions.
  83
  84  - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
  85    [::], [==], nor [..]) for portability.
  86
  87    - We do not use \{m,n\};
  88
  89    - We do not use -E;
  90
  91    - We do not use ? nor + (which are \{0,1\} and \{1,\}
  92      respectively in BRE) but that goes without saying as these
  93      are ERE elements not BRE (note that \? and \+ are not even part
  94      of BRE -- making them accessible from BRE is a GNU extension).
  95
  96  - Use Git's gettext wrappers in git-sh-i18n to make the user
  97    interface translatable. See "Marking strings for translation" in
  98    po/README.
  99
 100 For C programs:
 101
 102  - We use tabs to indent, and interpret tabs as taking up to
 103    8 spaces.
 104
 105  - We try to keep to at most 80 characters per line.
 106
 107  - When declaring pointers, the star sides with the variable
 108    name, i.e. "char *string", not "char* string" or
 109    "char * string".  This makes it easier to understand code
 110    like "char *string, c;".
 111
 112  - We avoid using braces unnecessarily.  I.e.
 113
 114         if (bla) {
 115                 x = 1;
 116         }
 117
 118    is frowned upon.  A gray area is when the statement extends
 119    over a few lines, and/or you have a lengthy comment atop of
 120    it.  Also, like in the Linux kernel, if there is a long list
 121    of "else if" statements, it can make sense to add braces to
 122    single line blocks.
 123
 124  - We try to avoid assignments inside if().
 125
 126  - Try to make your code understandable.  You may put comments
 127    in, but comments invariably tend to stale out when the code
 128    they were describing changes.  Often splitting a function
 129    into two makes the intention of the code much clearer.
 130
 131  - Double negation is often harder to understand than no negation
 132    at all.
 133
 134  - Some clever tricks, like using the !! operator with arithmetic
 135    constructs, can be extremely confusing to others.  Avoid them,
 136    unless there is a compelling reason to use them.
 137
 138  - Use the API.  No, really.  We have a strbuf (variable length
 139    string), several arrays with the ALLOC_GROW() macro, a
 140    string_list for sorted string lists, a hash map (mapping struct
 141    objects) named "struct decorate", amongst other things.
 142
 143  - When you come up with an API, document it.
 144
 145  - The first #include in C files, except in platform specific
 146    compat/ implementations, should be git-compat-util.h or another
 147    header file that includes it, such as cache.h or builtin.h.
 148
 149  - If you are planning a new command, consider writing it in shell
 150    or perl first, so that changes in semantics can be easily
 151    changed and discussed.  Many git commands started out like
 152    that, and a few are still scripts.
 153
 154  - Avoid introducing a new dependency into git. This means you
 155    usually should stay away from scripting languages not already
 156    used in the git core command set (unless your command is clearly
 157    separate from it, such as an importer to convert random-scm-X
 158    repositories to git).
 159
 160  - When we pass <string, length> pair to functions, we should try to
 161    pass them in that order.
 162
 163  - Use Git's gettext wrappers to make the user interface
 164    translatable. See "Marking strings for translation" in po/README.
 165
 166 Writing Documentation:
 167
 168  Every user-visible change should be reflected in the documentation.
 169  The same general rule as for code applies -- imitate the existing
 170  conventions.  A few commented examples follow to provide reference
 171  when writing or modifying command usage strings and synopsis sections
 172  in the manual pages:
 173
 174  Placeholders are spelled in lowercase and enclosed in angle brackets:
 175    <file>
 176    --sort=<key>
 177    --abbrev[=<n>]
 178
 179  Possibility of multiple occurrences is indicated by three dots:
 180    <file>...
 181    (One or more of <file>.)
 182
 183  Optional parts are enclosed in square brackets:
 184    [<extra>]
 185    (Zero or one <extra>.)
 186
 187    --exec-path[=<path>]
 188    (Option with an optional argument.  Note that the "=" is inside the
 189    brackets.)
 190
 191    [<patch>...]
 192    (Zero or more of <patch>.  Note that the dots are inside, not
 193    outside the brackets.)
 194
 195  Multiple alternatives are indicated with vertical bar:
 196    [-q | --quiet]
 197    [--utf8 | --no-utf8]
 198
 199  Parentheses are used for grouping:
 200    [(<rev>|<range>)...]
 201    (Any number of either <rev> or <range>.  Parens are needed to make
 202    it clear that "..." pertains to both <rev> and <range>.)
 203
 204    [(-p <parent>)...]
 205    (Any number of option -p, each with one <parent> argument.)
 206
 207    git remote set-head <name> (-a | -d | <branch>)
 208    (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 209    brackets) be provided.)
 210
 211  And a somewhat more contrived example:
 212    --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 213    Here "=" is outside the brackets, because "--diff-filter=" is a
 214    valid usage.  "*" has its own pair of brackets, because it can
 215    (optionally) be specified only when one or more of the letters is
 216    also provided.