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  - Do not write control structures on a single line with semicolon.
  80    "then" should be on the next line for if statements, and "do"
  81    should be on the next line for "while" and "for".
  82
  83  - We prefer "test" over "[ ... ]".
  84
  85  - We do not write the noiseword "function" in front of shell
  86    functions.
  87
  88  - We prefer a space between the function name and the parentheses. The
  89    opening "{" should also be on the same line.
  90    E.g.: my_function () {
  91
  92  - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
  93    [::], [==], nor [..]) for portability.
  94
  95    - We do not use \{m,n\};
  96
  97    - We do not use -E;
  98
  99    - We do not use ? nor + (which are \{0,1\} and \{1,\}
 100      respectively in BRE) but that goes without saying as these
 101      are ERE elements not BRE (note that \? and \+ are not even part
 102      of BRE -- making them accessible from BRE is a GNU extension).
 103
 104  - Use Git's gettext wrappers in git-sh-i18n to make the user
 105    interface translatable. See "Marking strings for translation" in
 106    po/README.
 107
 108 For C programs:
 109
 110  - We use tabs to indent, and interpret tabs as taking up to
 111    8 spaces.
 112
 113  - We try to keep to at most 80 characters per line.
 114
 115  - When declaring pointers, the star sides with the variable
 116    name, i.e. "char *string", not "char* string" or
 117    "char * string".  This makes it easier to understand code
 118    like "char *string, c;".
 119
 120  - We avoid using braces unnecessarily.  I.e.
 121
 122         if (bla) {
 123                 x = 1;
 124         }
 125
 126    is frowned upon.  A gray area is when the statement extends
 127    over a few lines, and/or you have a lengthy comment atop of
 128    it.  Also, like in the Linux kernel, if there is a long list
 129    of "else if" statements, it can make sense to add braces to
 130    single line blocks.
 131
 132  - We try to avoid assignments inside if().
 133
 134  - Try to make your code understandable.  You may put comments
 135    in, but comments invariably tend to stale out when the code
 136    they were describing changes.  Often splitting a function
 137    into two makes the intention of the code much clearer.
 138
 139  - Double negation is often harder to understand than no negation
 140    at all.
 141
 142  - Some clever tricks, like using the !! operator with arithmetic
 143    constructs, can be extremely confusing to others.  Avoid them,
 144    unless there is a compelling reason to use them.
 145
 146  - Use the API.  No, really.  We have a strbuf (variable length
 147    string), several arrays with the ALLOC_GROW() macro, a
 148    string_list for sorted string lists, a hash map (mapping struct
 149    objects) named "struct decorate", amongst other things.
 150
 151  - When you come up with an API, document it.
 152
 153  - The first #include in C files, except in platform specific
 154    compat/ implementations, should be git-compat-util.h or another
 155    header file that includes it, such as cache.h or builtin.h.
 156
 157  - If you are planning a new command, consider writing it in shell
 158    or perl first, so that changes in semantics can be easily
 159    changed and discussed.  Many git commands started out like
 160    that, and a few are still scripts.
 161
 162  - Avoid introducing a new dependency into git. This means you
 163    usually should stay away from scripting languages not already
 164    used in the git core command set (unless your command is clearly
 165    separate from it, such as an importer to convert random-scm-X
 166    repositories to git).
 167
 168  - When we pass <string, length> pair to functions, we should try to
 169    pass them in that order.
 170
 171  - Use Git's gettext wrappers to make the user interface
 172    translatable. See "Marking strings for translation" in po/README.
 173
 174 Writing Documentation:
 175
 176  Every user-visible change should be reflected in the documentation.
 177  The same general rule as for code applies -- imitate the existing
 178  conventions.  A few commented examples follow to provide reference
 179  when writing or modifying command usage strings and synopsis sections
 180  in the manual pages:
 181
 182  Placeholders are spelled in lowercase and enclosed in angle brackets:
 183    <file>
 184    --sort=<key>
 185    --abbrev[=<n>]
 186
 187  Possibility of multiple occurrences is indicated by three dots:
 188    <file>...
 189    (One or more of <file>.)
 190
 191  Optional parts are enclosed in square brackets:
 192    [<extra>]
 193    (Zero or one <extra>.)
 194
 195    --exec-path[=<path>]
 196    (Option with an optional argument.  Note that the "=" is inside the
 197    brackets.)
 198
 199    [<patch>...]
 200    (Zero or more of <patch>.  Note that the dots are inside, not
 201    outside the brackets.)
 202
 203  Multiple alternatives are indicated with vertical bar:
 204    [-q | --quiet]
 205    [--utf8 | --no-utf8]
 206
 207  Parentheses are used for grouping:
 208    [(<rev>|<range>)...]
 209    (Any number of either <rev> or <range>.  Parens are needed to make
 210    it clear that "..." pertains to both <rev> and <range>.)
 211
 212    [(-p <parent>)...]
 213    (Any number of option -p, each with one <parent> argument.)
 214
 215    git remote set-head <name> (-a | -d | <branch>)
 216    (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 217    brackets) be provided.)
 218
 219  And a somewhat more contrived example:
 220    --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 221    Here "=" is outside the brackets, because "--diff-filter=" is a
 222    valid usage.  "*" has its own pair of brackets, because it can
 223    (optionally) be specified only when one or more of the letters is
 224    also provided.