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