mirror of
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513 lines
17 KiB
Plaintext
513 lines
17 KiB
Plaintext
Like other projects, we also have some guidelines to keep to the
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code. For Git in general, three rough rules are:
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- Most importantly, we never say "It's in POSIX; we'll happily
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ignore your needs should your system not conform to it."
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We live in the real world.
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- However, we often say "Let's stay away from that construct,
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it's not even in POSIX".
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- In spite of the above two rules, we sometimes say "Although
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this is not in POSIX, it (is so convenient | makes the code
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much more readable | has other good characteristics) and
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practically all the platforms we care about support it, so
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let's use it".
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Again, we live in the real world, and it is sometimes a
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judgement call, the decision based more on real world
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constraints people face than what the paper standard says.
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- Fixing style violations while working on a real change as a
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preparatory clean-up step is good, but otherwise avoid useless code
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churn for the sake of conforming to the style.
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"Once it _is_ in the tree, it's not really worth the patch noise to
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go and fix it up."
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Cf. http://article.gmane.org/gmane.linux.kernel/943020
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Make your code readable and sensible, and don't try to be clever.
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As for more concrete guidelines, just imitate the existing code
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(this is a good guideline, no matter which project you are
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contributing to). It is always preferable to match the _local_
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convention. New code added to Git suite is expected to match
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the overall style of existing code. Modifications to existing
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code is expected to match the style the surrounding code already
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uses (even if it doesn't match the overall style of existing code).
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But if you must have a list of rules, here they are.
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For shell scripts specifically (not exhaustive):
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- We use tabs for indentation.
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- Case arms are indented at the same depth as case and esac lines,
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like this:
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case "$variable" in
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pattern1)
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do this
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;;
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pattern2)
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do that
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;;
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esac
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- Redirection operators should be written with space before, but no
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space after them. In other words, write 'echo test >"$file"'
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instead of 'echo test> $file' or 'echo test > $file'. Note that
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even though it is not required by POSIX to double-quote the
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redirection target in a variable (as shown above), our code does so
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because some versions of bash issue a warning without the quotes.
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(incorrect)
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cat hello > world < universe
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echo hello >$world
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(correct)
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cat hello >world <universe
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echo hello >"$world"
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- We prefer $( ... ) for command substitution; unlike ``, it
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properly nests. It should have been the way Bourne spelled
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it from day one, but unfortunately isn't.
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- If you want to find out if a command is available on the user's
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$PATH, you should use 'type <command>', instead of 'which <command>'.
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The output of 'which' is not machine parseable and its exit code
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is not reliable across platforms.
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- We use POSIX compliant parameter substitutions and avoid bashisms;
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namely:
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- We use ${parameter-word} and its [-=?+] siblings, and their
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colon'ed "unset or null" form.
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- We use ${parameter#word} and its [#%] siblings, and their
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doubled "longest matching" form.
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- No "Substring Expansion" ${parameter:offset:length}.
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- No shell arrays.
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- No strlen ${#parameter}.
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- No pattern replacement ${parameter/pattern/string}.
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- We use Arithmetic Expansion $(( ... )).
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- Inside Arithmetic Expansion, spell shell variables with $ in front
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of them, as some shells do not grok $((x)) while accepting $(($x))
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just fine (e.g. dash older than 0.5.4).
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- We do not use Process Substitution <(list) or >(list).
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- Do not write control structures on a single line with semicolon.
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"then" should be on the next line for if statements, and "do"
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should be on the next line for "while" and "for".
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(incorrect)
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if test -f hello; then
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do this
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fi
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(correct)
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if test -f hello
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then
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do this
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fi
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- We prefer "test" over "[ ... ]".
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- We do not write the noiseword "function" in front of shell
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functions.
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- We prefer a space between the function name and the parentheses,
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and no space inside the parentheses. The opening "{" should also
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be on the same line.
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(incorrect)
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my_function(){
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...
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(correct)
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my_function () {
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...
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- As to use of grep, stick to a subset of BRE (namely, no \{m,n\},
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[::], [==], or [..]) for portability.
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- We do not use \{m,n\};
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- We do not use -E;
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- We do not use ? or + (which are \{0,1\} and \{1,\}
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respectively in BRE) but that goes without saying as these
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are ERE elements not BRE (note that \? and \+ are not even part
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of BRE -- making them accessible from BRE is a GNU extension).
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- Use Git's gettext wrappers in git-sh-i18n to make the user
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interface translatable. See "Marking strings for translation" in
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po/README.
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- We do not write our "test" command with "-a" and "-o" and use "&&"
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or "||" to concatenate multiple "test" commands instead, because
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the use of "-a/-o" is often error-prone. E.g.
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test -n "$x" -a "$a" = "$b"
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is buggy and breaks when $x is "=", but
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test -n "$x" && test "$a" = "$b"
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does not have such a problem.
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For C programs:
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- We use tabs to indent, and interpret tabs as taking up to
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8 spaces.
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- We try to keep to at most 80 characters per line.
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- We try to support a wide range of C compilers to compile Git with,
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including old ones. That means that you should not use C99
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initializers, even if a lot of compilers grok it.
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- Variables have to be declared at the beginning of the block.
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- NULL pointers shall be written as NULL, not as 0.
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- When declaring pointers, the star sides with the variable
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name, i.e. "char *string", not "char* string" or
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"char * string". This makes it easier to understand code
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like "char *string, c;".
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- Use whitespace around operators and keywords, but not inside
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parentheses and not around functions. So:
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while (condition)
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func(bar + 1);
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and not:
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while( condition )
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func (bar+1);
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- We avoid using braces unnecessarily. I.e.
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if (bla) {
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x = 1;
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}
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is frowned upon. A gray area is when the statement extends
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over a few lines, and/or you have a lengthy comment atop of
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it. Also, like in the Linux kernel, if there is a long list
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of "else if" statements, it can make sense to add braces to
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single line blocks.
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- We try to avoid assignments in the condition of an "if" statement.
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- Try to make your code understandable. You may put comments
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in, but comments invariably tend to stale out when the code
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they were describing changes. Often splitting a function
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into two makes the intention of the code much clearer.
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- Multi-line comments include their delimiters on separate lines from
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the text. E.g.
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/*
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* A very long
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* multi-line comment.
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*/
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Note however that a comment that explains a translatable string to
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translators uses a convention of starting with a magic token
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"TRANSLATORS: " immediately after the opening delimiter, even when
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it spans multiple lines. We do not add an asterisk at the beginning
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of each line, either. E.g.
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/* TRANSLATORS: here is a comment that explains the string
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to be translated, that follows immediately after it */
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_("Here is a translatable string explained by the above.");
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- Double negation is often harder to understand than no negation
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at all.
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- There are two schools of thought when it comes to comparison,
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especially inside a loop. Some people prefer to have the less stable
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value on the left hand side and the more stable value on the right hand
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side, e.g. if you have a loop that counts variable i down to the
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lower bound,
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while (i > lower_bound) {
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do something;
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i--;
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}
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Other people prefer to have the textual order of values match the
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actual order of values in their comparison, so that they can
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mentally draw a number line from left to right and place these
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values in order, i.e.
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while (lower_bound < i) {
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do something;
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i--;
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}
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Both are valid, and we use both. However, the more "stable" the
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stable side becomes, the more we tend to prefer the former
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(comparison with a constant, "i > 0", is an extreme example).
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Just do not mix styles in the same part of the code and mimic
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existing styles in the neighbourhood.
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- There are two schools of thought when it comes to splitting a long
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logical line into multiple lines. Some people push the second and
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subsequent lines far enough to the right with tabs and align them:
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if (the_beginning_of_a_very_long_expression_that_has_to ||
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span_more_than_a_single_line_of ||
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the_source_text) {
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...
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while other people prefer to align the second and the subsequent
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lines with the column immediately inside the opening parenthesis,
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with tabs and spaces, following our "tabstop is always a multiple
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of 8" convention:
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if (the_beginning_of_a_very_long_expression_that_has_to ||
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span_more_than_a_single_line_of ||
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the_source_text) {
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...
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Both are valid, and we use both. Again, just do not mix styles in
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the same part of the code and mimic existing styles in the
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neighbourhood.
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- When splitting a long logical line, some people change line before
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a binary operator, so that the result looks like a parse tree when
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you turn your head 90-degrees counterclockwise:
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if (the_beginning_of_a_very_long_expression_that_has_to
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|| span_more_than_a_single_line_of_the_source_text) {
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while other people prefer to leave the operator at the end of the
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line:
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if (the_beginning_of_a_very_long_expression_that_has_to ||
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span_more_than_a_single_line_of_the_source_text) {
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Both are valid, but we tend to use the latter more, unless the
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expression gets fairly complex, in which case the former tends to
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be easier to read. Again, just do not mix styles in the same part
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of the code and mimic existing styles in the neighbourhood.
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- When splitting a long logical line, with everything else being
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equal, it is preferable to split after the operator at higher
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level in the parse tree. That is, this is more preferable:
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if (a_very_long_variable * that_is_used_in +
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a_very_long_expression) {
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...
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than
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if (a_very_long_variable *
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that_is_used_in + a_very_long_expression) {
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...
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- Some clever tricks, like using the !! operator with arithmetic
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constructs, can be extremely confusing to others. Avoid them,
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unless there is a compelling reason to use them.
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- Use the API. No, really. We have a strbuf (variable length
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string), several arrays with the ALLOC_GROW() macro, a
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string_list for sorted string lists, a hash map (mapping struct
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objects) named "struct decorate", amongst other things.
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- When you come up with an API, document it.
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- The first #include in C files, except in platform specific
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compat/ implementations, should be git-compat-util.h or another
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header file that includes it, such as cache.h or builtin.h.
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- If you are planning a new command, consider writing it in shell
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or perl first, so that changes in semantics can be easily
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changed and discussed. Many Git commands started out like
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that, and a few are still scripts.
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- Avoid introducing a new dependency into Git. This means you
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usually should stay away from scripting languages not already
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used in the Git core command set (unless your command is clearly
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separate from it, such as an importer to convert random-scm-X
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repositories to Git).
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- When we pass <string, length> pair to functions, we should try to
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pass them in that order.
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- Use Git's gettext wrappers to make the user interface
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translatable. See "Marking strings for translation" in po/README.
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For Perl programs:
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- Most of the C guidelines above apply.
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- We try to support Perl 5.8 and later ("use Perl 5.008").
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- use strict and use warnings are strongly preferred.
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- Don't overuse statement modifiers unless using them makes the
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result easier to follow.
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... do something ...
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do_this() unless (condition);
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... do something else ...
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is more readable than:
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... do something ...
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unless (condition) {
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do_this();
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}
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... do something else ...
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*only* when the condition is so rare that do_this() will be almost
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always called.
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- We try to avoid assignments inside "if ()" conditions.
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- Learn and use Git.pm if you need that functionality.
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- For Emacs, it's useful to put the following in
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GIT_CHECKOUT/.dir-locals.el, assuming you use cperl-mode:
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;; note the first part is useful for C editing, too
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((nil . ((indent-tabs-mode . t)
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(tab-width . 8)
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(fill-column . 80)))
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(cperl-mode . ((cperl-indent-level . 8)
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(cperl-extra-newline-before-brace . nil)
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(cperl-merge-trailing-else . t))))
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For Python scripts:
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- We follow PEP-8 (http://www.python.org/dev/peps/pep-0008/).
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- As a minimum, we aim to be compatible with Python 2.6 and 2.7.
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- Where required libraries do not restrict us to Python 2, we try to
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also be compatible with Python 3.1 and later.
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- When you must differentiate between Unicode literals and byte string
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literals, it is OK to use the 'b' prefix. Even though the Python
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documentation for version 2.6 does not mention this prefix, it has
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been supported since version 2.6.0.
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Error Messages
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- Do not end error messages with a full stop.
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- Do not capitalize ("unable to open %s", not "Unable to open %s")
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- Say what the error is first ("cannot open %s", not "%s: cannot open")
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Writing Documentation:
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Most (if not all) of the documentation pages are written in the
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AsciiDoc format in *.txt files (e.g. Documentation/git.txt), and
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processed into HTML and manpages (e.g. git.html and git.1 in the
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same directory).
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The documentation liberally mixes US and UK English (en_US/UK)
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norms for spelling and grammar, which is somewhat unfortunate.
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In an ideal world, it would have been better if it consistently
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used only one and not the other, and we would have picked en_US
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(if you wish to correct the English of some of the existing
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documentation, please see the documentation-related advice in the
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Documentation/SubmittingPatches file).
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Every user-visible change should be reflected in the documentation.
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The same general rule as for code applies -- imitate the existing
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conventions.
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A few commented examples follow to provide reference when writing or
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modifying command usage strings and synopsis sections in the manual
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pages:
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Placeholders are spelled in lowercase and enclosed in angle brackets:
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<file>
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--sort=<key>
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--abbrev[=<n>]
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Possibility of multiple occurrences is indicated by three dots:
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<file>...
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(One or more of <file>.)
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Optional parts are enclosed in square brackets:
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[<extra>]
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(Zero or one <extra>.)
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--exec-path[=<path>]
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(Option with an optional argument. Note that the "=" is inside the
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brackets.)
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[<patch>...]
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(Zero or more of <patch>. Note that the dots are inside, not
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outside the brackets.)
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Multiple alternatives are indicated with vertical bar:
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[-q | --quiet]
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[--utf8 | --no-utf8]
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Parentheses are used for grouping:
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[(<rev>|<range>)...]
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(Any number of either <rev> or <range>. Parens are needed to make
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it clear that "..." pertains to both <rev> and <range>.)
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[(-p <parent>)...]
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(Any number of option -p, each with one <parent> argument.)
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git remote set-head <name> (-a | -d | <branch>)
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(One and only one of "-a", "-d" or "<branch>" _must_ (no square
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brackets) be provided.)
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And a somewhat more contrived example:
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--diff-filter=[(A|C|D|M|R|T|U|X|B)...[*]]
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Here "=" is outside the brackets, because "--diff-filter=" is a
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valid usage. "*" has its own pair of brackets, because it can
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(optionally) be specified only when one or more of the letters is
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also provided.
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A note on notation:
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Use 'git' (all lowercase) when talking about commands i.e. something
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the user would type into a shell and use 'Git' (uppercase first letter)
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when talking about the version control system and its properties.
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A few commented examples follow to provide reference when writing or
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modifying paragraphs or option/command explanations that contain options
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or commands:
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Literal examples (e.g. use of command-line options, command names, and
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configuration variables) are typeset in monospace, and if you can use
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`backticks around word phrases`, do so.
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`--pretty=oneline`
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`git rev-list`
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`remote.pushdefault`
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Word phrases enclosed in `backtick characters` are rendered literally
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and will not be further expanded. The use of `backticks` to achieve the
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previous rule means that literal examples should not use AsciiDoc
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escapes.
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Correct:
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`--pretty=oneline`
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Incorrect:
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`\--pretty=oneline`
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If some place in the documentation needs to typeset a command usage
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example with inline substitutions, it is fine to use +monospaced and
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inline substituted text+ instead of `monospaced literal text`, and with
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the former, the part that should not get substituted must be
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quoted/escaped.
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