Documentation / CodingGuidelineson commit Merge branch 'wk/doc-git-has-grown' (0784466)
   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
  21Make your code readable and sensible, and don't try to be clever.
  22
  23As for more concrete guidelines, just imitate the existing code
  24(this is a good guideline, no matter which project you are
  25contributing to). It is always preferable to match the _local_
  26convention. New code added to Git suite is expected to match
  27the overall style of existing code. Modifications to existing
  28code is expected to match the style the surrounding code already
  29uses (even if it doesn't match the overall style of existing code).
  30
  31But if you must have a list of rules, here they are.
  32
  33For shell scripts specifically (not exhaustive):
  34
  35 - We use tabs for indentation.
  36
  37 - Case arms are indented at the same depth as case and esac lines.
  38
  39 - Redirection operators should be written with space before, but no
  40   space after them.  In other words, write 'echo test >"$file"'
  41   instead of 'echo test> $file' or 'echo test > $file'.  Note that
  42   even though it is not required by POSIX to double-quote the
  43   redirection target in a variable (as shown above), our code does so
  44   because some versions of bash issue a warning without the quotes.
  45
  46 - We prefer $( ... ) for command substitution; unlike ``, it
  47   properly nests.  It should have been the way Bourne spelled
  48   it from day one, but unfortunately isn't.
  49
  50 - If you want to find out if a command is available on the user's
  51   $PATH, you should use 'type <command>', instead of 'which <command>'.
  52   The output of 'which' is not machine parseable and its exit code
  53   is not reliable across platforms.
  54
  55 - We use POSIX compliant parameter substitutions and avoid bashisms;
  56   namely:
  57
  58   - We use ${parameter-word} and its [-=?+] siblings, and their
  59     colon'ed "unset or null" form.
  60
  61   - We use ${parameter#word} and its [#%] siblings, and their
  62     doubled "longest matching" form.
  63
  64   - No "Substring Expansion" ${parameter:offset:length}.
  65
  66   - No shell arrays.
  67
  68   - No strlen ${#parameter}.
  69
  70   - No pattern replacement ${parameter/pattern/string}.
  71
  72 - We use Arithmetic Expansion $(( ... )).
  73
  74 - Inside Arithmetic Expansion, spell shell variables with $ in front
  75   of them, as some shells do not grok $((x)) while accepting $(($x))
  76   just fine (e.g. dash older than 0.5.4).
  77
  78 - We do not use Process Substitution <(list) or >(list).
  79
  80 - Do not write control structures on a single line with semicolon.
  81   "then" should be on the next line for if statements, and "do"
  82   should be on the next line for "while" and "for".
  83
  84 - We prefer "test" over "[ ... ]".
  85
  86 - We do not write the noiseword "function" in front of shell
  87   functions.
  88
  89 - We prefer a space between the function name and the parentheses. The
  90   opening "{" should also be on the same line.
  91   E.g.: my_function () {
  92
  93 - As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
  94   [::], [==], nor [..]) for portability.
  95
  96   - We do not use \{m,n\};
  97
  98   - We do not use -E;
  99
 100   - We do not use ? nor + (which are \{0,1\} and \{1,\}
 101     respectively in BRE) but that goes without saying as these
 102     are ERE elements not BRE (note that \? and \+ are not even part
 103     of BRE -- making them accessible from BRE is a GNU extension).
 104
 105 - Use Git's gettext wrappers in git-sh-i18n to make the user
 106   interface translatable. See "Marking strings for translation" in
 107   po/README.
 108
 109For C programs:
 110
 111 - We use tabs to indent, and interpret tabs as taking up to
 112   8 spaces.
 113
 114 - We try to keep to at most 80 characters per line.
 115
 116 - We try to support a wide range of C compilers to compile Git with,
 117   including old ones. That means that you should not use C99
 118   initializers, even if a lot of compilers grok it.
 119
 120 - Variables have to be declared at the beginning of the block.
 121
 122 - NULL pointers shall be written as NULL, not as 0.
 123
 124 - When declaring pointers, the star sides with the variable
 125   name, i.e. "char *string", not "char* string" or
 126   "char * string".  This makes it easier to understand code
 127   like "char *string, c;".
 128
 129 - We avoid using braces unnecessarily.  I.e.
 130
 131        if (bla) {
 132                x = 1;
 133        }
 134
 135   is frowned upon.  A gray area is when the statement extends
 136   over a few lines, and/or you have a lengthy comment atop of
 137   it.  Also, like in the Linux kernel, if there is a long list
 138   of "else if" statements, it can make sense to add braces to
 139   single line blocks.
 140
 141 - We try to avoid assignments inside if().
 142
 143 - Try to make your code understandable.  You may put comments
 144   in, but comments invariably tend to stale out when the code
 145   they were describing changes.  Often splitting a function
 146   into two makes the intention of the code much clearer.
 147
 148 - Double negation is often harder to understand than no negation
 149   at all.
 150
 151 - Some clever tricks, like using the !! operator with arithmetic
 152   constructs, can be extremely confusing to others.  Avoid them,
 153   unless there is a compelling reason to use them.
 154
 155 - Use the API.  No, really.  We have a strbuf (variable length
 156   string), several arrays with the ALLOC_GROW() macro, a
 157   string_list for sorted string lists, a hash map (mapping struct
 158   objects) named "struct decorate", amongst other things.
 159
 160 - When you come up with an API, document it.
 161
 162 - The first #include in C files, except in platform specific
 163   compat/ implementations, should be git-compat-util.h or another
 164   header file that includes it, such as cache.h or builtin.h.
 165
 166 - If you are planning a new command, consider writing it in shell
 167   or perl first, so that changes in semantics can be easily
 168   changed and discussed.  Many Git commands started out like
 169   that, and a few are still scripts.
 170
 171 - Avoid introducing a new dependency into Git. This means you
 172   usually should stay away from scripting languages not already
 173   used in the Git core command set (unless your command is clearly
 174   separate from it, such as an importer to convert random-scm-X
 175   repositories to Git).
 176
 177 - When we pass <string, length> pair to functions, we should try to
 178   pass them in that order.
 179
 180 - Use Git's gettext wrappers to make the user interface
 181   translatable. See "Marking strings for translation" in po/README.
 182
 183For Perl programs:
 184
 185 - Most of the C guidelines above apply.
 186
 187 - We try to support Perl 5.8 and later ("use Perl 5.008").
 188
 189 - use strict and use warnings are strongly preferred.
 190
 191 - Don't overuse statement modifiers unless using them makes the
 192   result easier to follow.
 193
 194        ... do something ...
 195        do_this() unless (condition);
 196        ... do something else ...
 197
 198   is more readable than:
 199
 200        ... do something ...
 201        unless (condition) {
 202                do_this();
 203        }
 204        ... do something else ...
 205
 206   *only* when the condition is so rare that do_this() will be almost
 207   always called.
 208
 209 - We try to avoid assignments inside "if ()" conditions.
 210
 211 - Learn and use Git.pm if you need that functionality.
 212
 213 - For Emacs, it's useful to put the following in
 214   GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
 215
 216    ;; note the first part is useful for C editing, too
 217    ((nil . ((indent-tabs-mode . t)
 218                  (tab-width . 8)
 219                  (fill-column . 80)))
 220     (cperl-mode . ((cperl-indent-level . 8)
 221                    (cperl-extra-newline-before-brace . nil)
 222                    (cperl-merge-trailing-else . t))))
 223
 224For Python scripts:
 225
 226 - We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
 227
 228 - As a minimum, we aim to be compatible with Python 2.6 and 2.7.
 229
 230 - Where required libraries do not restrict us to Python 2, we try to
 231   also be compatible with Python 3.1 and later.
 232
 233 - When you must differentiate between Unicode literals and byte string
 234   literals, it is OK to use the 'b' prefix.  Even though the Python
 235   documentation for version 2.6 does not mention this prefix, it has
 236   been supported since version 2.6.0.
 237
 238Writing Documentation:
 239
 240 Most (if not all) of the documentation pages are written in the
 241 AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
 242 processed into HTML and manpages (e.g. git.html and git.1 in the
 243 same directory).
 244
 245 Every user-visible change should be reflected in the documentation.
 246 The same general rule as for code applies -- imitate the existing
 247 conventions.  A few commented examples follow to provide reference
 248 when writing or modifying command usage strings and synopsis sections
 249 in the manual pages:
 250
 251 Placeholders are spelled in lowercase and enclosed in angle brackets:
 252   <file>
 253   --sort=<key>
 254   --abbrev[=<n>]
 255
 256 Possibility of multiple occurrences is indicated by three dots:
 257   <file>...
 258   (One or more of <file>.)
 259
 260 Optional parts are enclosed in square brackets:
 261   [<extra>]
 262   (Zero or one <extra>.)
 263
 264   --exec-path[=<path>]
 265   (Option with an optional argument.  Note that the "=" is inside the
 266   brackets.)
 267
 268   [<patch>...]
 269   (Zero or more of <patch>.  Note that the dots are inside, not
 270   outside the brackets.)
 271
 272 Multiple alternatives are indicated with vertical bar:
 273   [-q | --quiet]
 274   [--utf8 | --no-utf8]
 275
 276 Parentheses are used for grouping:
 277   [(<rev>|<range>)...]
 278   (Any number of either <rev> or <range>.  Parens are needed to make
 279   it clear that "..." pertains to both <rev> and <range>.)
 280
 281   [(-p <parent>)...]
 282   (Any number of option -p, each with one <parent> argument.)
 283
 284   git remote set-head <name> (-a | -d | <branch>)
 285   (One and only one of "-a", "-d" or "<branch>" _must_ (no square
 286   brackets) be provided.)
 287
 288 And a somewhat more contrived example:
 289   --diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
 290   Here "=" is outside the brackets, because "--diff-filter=" is a
 291   valid usage.  "*" has its own pair of brackets, because it can
 292   (optionally) be specified only when one or more of the letters is
 293   also provided.
 294
 295  A note on notation:
 296   Use 'git' (all lowercase) when talking about commands i.e. something
 297   the user would type into a shell and use 'Git' (uppercase first letter)
 298   when talking about the version control system and its properties.