Documentation / CodingGuidelineson commit Merge branch 'jc/maint-clean-nested-worktree-in-subdir' into maint (5d65c2e)
   1Like other projects, we also have some guidelines to keep to the
   2code.  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
  22As for more concrete guidelines, just imitate the existing code
  23(this is a good guideline, no matter which project you are
  24contributing to). It is always preferable to match the _local_
  25convention. New code added to git suite is expected to match
  26the overall style of existing code. Modifications to existing
  27code is expected to match the style the surrounding code already
  28uses (even if it doesn't match the overall style of existing code).
  29
  30But if you must have a list of rules, here they are.
  31
  32For 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
 100For 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
 166Writing 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.