Documentation/CodingGuidelines - jrn/git - Git at Google

 Like other projects, we also have some guidelines to keep to the
 code.  For Git in general, a few 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://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html

 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

  - If a command sequence joined with && or || or | spans multiple
    lines, put each command on a separate line and put && and || and |
    operators at the end of each line, rather than the start. This
    means you don't need to use \ to join lines, since the above
    operators imply the sequence isn't finished.

 	(incorrect)
 	grep blob verify_pack_result \
 	| awk -f print_1.awk \
 	| sort >actual &&
 	...

 	(correct)
 	grep blob verify_pack_result |
 	awk -f print_1.awk |
 	sort >actual &&
 	...

  - 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.

  - As a Git developer we assume you have a reasonably modern compiler
    and we recommend you to enable the DEVELOPER makefile knob to
    ensure your patch is clear of all compiler warnings we care about,
    by e.g. "echo DEVELOPER=1 >>config.mak".

  - 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. But there are a few exceptions:

 	- When the statement extends over a few lines (e.g., a while loop
 	  with an embedded conditional, or a comment). E.g.:

 		while (foo) {
 			if (x)
 				one();
 			else
 				two();
 		}

 		if (foo) {
 			/*
 			 * This one requires some explanation,
 			 * so we're better off with braces to make
 			 * it obvious that the indentation is correct.
 			 */
 			doit();
 		}

 	- When there are multiple arms to a conditional and some of them
 	  require braces, enclose even a single line block in braces for
 	  consistency. E.g.:

 		if (foo) {
 			doit();
 		} else {
 			one();
 			two();
 			three();
 		}

  - 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: ", 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 its functions and structures
    in the header file that exposes the API to its callers. Use what is
    in "strbuf.h" as a model for the appropriate tone and level of
    detail.

  - 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.

  - Variables and functions local to a given source file should be marked
    with "static". Variables that are visible to other source files
    must be declared with "extern" in header files. However, function
    declarations should not use "extern", as that is already the default.

 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,
  branch names, configuration and environment variables) must be
  typeset in monospace (i.e. wrapped with backticks):
    `--pretty=oneline`
    `git rev-list`
    `remote.pushDefault`
    `GIT_DIR`
    `HEAD`

  An environment variable must be prefixed with "$" only when referring to its
  value and not when referring to the variable itself, in this case there is
  nothing to add except the backticks:
    `GIT_DIR` is specified
    `$GIT_DIR/hooks/pre-receive`

  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.
	Like other projects, we also have some guidelines to keep to the
	code. For Git in general, a few 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://lkml.iu.edu/hypermail/linux/kernel/1001.3/01069.html

	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

	- If a command sequence joined with && or \|\| or \| spans multiple
	lines, put each command on a separate line and put && and \|\| and \|
	operators at the end of each line, rather than the start. This
	means you don't need to use \ to join lines, since the above
	operators imply the sequence isn't finished.

	(incorrect)
	grep blob verify_pack_result \
	\| awk -f print_1.awk \
	\| sort >actual &&
	...

	(correct)
	grep blob verify_pack_result \|
	awk -f print_1.awk \|
	sort >actual &&
	...

	- 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.

	- As a Git developer we assume you have a reasonably modern compiler
	and we recommend you to enable the DEVELOPER makefile knob to
	ensure your patch is clear of all compiler warnings we care about,
	by e.g. "echo DEVELOPER=1 >>config.mak".

	- 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. But there are a few exceptions:

	- When the statement extends over a few lines (e.g., a while loop
	with an embedded conditional, or a comment). E.g.:

	while (foo) {
	if (x)
	one();
	else
	two();
	}

	if (foo) {
	/*
	* This one requires some explanation,
	* so we're better off with braces to make
	* it obvious that the indentation is correct.
	*/
	doit();
	}

	- When there are multiple arms to a conditional and some of them
	require braces, enclose even a single line block in braces for
	consistency. E.g.:

	if (foo) {
	doit();
	} else {
	one();
	two();
	three();
	}

	- 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: ", 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 its functions and structures
	in the header file that exposes the API to its callers. Use what is
	in "strbuf.h" as a model for the appropriate tone and level of
	detail.

	- 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.

	- Variables and functions local to a given source file should be marked
	with "static". Variables that are visible to other source files
	must be declared with "extern" in header files. However, function
	declarations should not use "extern", as that is already the default.

	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,
	branch names, configuration and environment variables) must be
	typeset in monospace (i.e. wrapped with backticks):
	`--pretty=oneline`
	`git rev-list`
	`remote.pushDefault`
	`GIT_DIR`
	`HEAD`

	An environment variable must be prefixed with "$" only when referring to its
	value and not when referring to the variable itself, in this case there is
	nothing to add except the backticks:
	`GIT_DIR` is specified
	`$GIT_DIR/hooks/pre-receive`

	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.