【Shell 入门】Google 的 Shell 编码规范_google shell编程规范

shell 编码规范

修订版本 2.02

本规范由多位 Google 员工起草、修订和维护。

目录

背景

使用哪个 Shell

使用 shell 脚本语言编写可执行文件时，Bash 是唯一允许的 Shell 类型。

使用 shell 脚本语言编写的可执行文件必要以 #!bin/bash 和尽可能少的标识符（flag）为开头。Use set to set shell options so that calling your script as bash script_name does not break its functionality.

通过限制所有可执行的 shell 脚本仅可使用 Bash 作为解释器，这可以让我们在所有机器上安装统一的 shell 语言。

何时使用 shell

shell 只能用于编写小型实用工具或简单的脚本化命令。

尽管 shell 脚本并非一门开发语言，在 Google 其被用于编写各式各样的实用脚本。该编码风格指南更像是对其使用的认可而非推荐其用于大范围的部署。

一些指引：

• 如果大多数情况下做的只是调用其他的实用工具且涉及操作的数据量较少或基本没有，那么 shell 的确是一个可接受的选择；
• 如果性能要求很高，请使用除 shell 以外的编程语言；
• 如果你正在编写的脚本超过 $100$ 行，或者使用了不是那么直接的流程控制逻辑，你需要立刻使用更加结构化的语言进行重写。你需要时刻牢记脚本是会慢慢地不断变得更长更复杂，所以请尽早重写你的脚本避免后续花费大量时间；
• 当评估代码的复杂度时（例如：评估是否到了应该使用其他语言重写 shell 脚本所实现的功能），请从维护者的角度出发而非脚本作者的角度出发。

shell 文件和解释器调用

文件扩展名

可执行文件（如 Linux 自带的命令）不应该有文件扩展名（强烈推荐）或者使用 .sh 作为文件扩展名。库文件必须使用 .sh 作为文件扩展名而且不能为可执行文件。

由于在执行一个程序时，没有必要知道该程序是使用何种语言编写的，而且 shell 并不强制使用某个文件扩展名，因此我们推荐不为可执行程序指定扩展名。

However, for libraries it’s important to know what language it is and sometimes there’s a need to have similar libraries in different
languages. This allows library files with identical purposes but different languages to be identically named except for the language-specific suffix.

SUID/SGID

SUID and SGID are forbidden on shell scripts.

There are too many security issues with shell that make it nearly impossible to secure sufficiently to allow SUID/SGID. While bash does make it difficult to run SUID, it’s still possible on some platforms which is why we’re being explicit about banning it.

Use sudo to provide elevated access if you need it.

环境

STDOUT vs STDERR

错误信息需要输出至 STDERR 。这样做可以将正常的执行状态和真正的报错区别开来。

这里建议通过定义一个函数来进行处理，推荐在该函数中打印错误信息以及其他的状态信息。

err() {
  echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $*" >&2
}

if ! do_something; then
  err "Unable to do_something"
  exit 1
fi
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注释

文件头

脚本的开头需要有描述脚本内容的信息。

每一个文件都需要在脚本的开头包含一段对其内容的简短概述。至于版权提醒和作者信息是可有可无的。

示例：

#!/bin/bash
#
# Perform hot backups of Oracle databases.
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函数注释

任何长度很长或者内容晦涩的函数都需要注释。库中的函数无论长短或复杂度都需要注释。

最好的注释能实现的效果是，任何人都可以仅通过阅读脚本或库中对于函数的注释来使用你编写的脚本或者库中的函数，而无需阅读具体的代码。

所有的注释都需要使用下列规则描述预期的 API 行为：

• 函数描述；
• 全局变量：使用或修改的全局变量
• 参数：函数接收的参数
• 输出：输出至 STDOUT 或 STDERR
• 返回值：除了最后执行的命令默认的退出状态外，返回应有的结果。

示例：

#######################################
# Cleanup files from the backup directory.
# Globals:
#   BACKUP_DIR
#   ORACLE_SID
# Arguments:
#   None
#######################################
function cleanup() {
  …
}

#######################################
# Get configuration directory.
# Globals:
#   SOMEDIR
# Arguments:
#   None
# Outputs:
#   Writes location to stdout
#######################################
function get_dir() {
  echo "${SOMEDIR}"
}

#######################################
# Delete a file in a sophisticated manner.
# Arguments:
#   File to delete, a path.
# Returns:
#   0 if thing was deleted, non-zero on error.
#######################################
function del_thing() {
  rm "$1"
}
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Implementation Comments

Comment tricky, non-obvious, interesting or important parts of your code.

This follows general Google coding comment practice. Don’t comment everything. If there’s a complex algorithm or you’re doing something out of the ordinary, put a short comment in.

TODO Comments

Use TODO comments for code that is temporary, a short-term solution, or good-enough but not perfect.

This matches the convention in the C++ Guide.

TODOs should include the string TODO in all caps, followed by the name, e-mail address, or other identifier of the person
with the best context about the problem referenced by the TODO. The main purpose is to have a consistent TODO that can be searched to find out how to get more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO , it is almost always your name that is given.

Examples:

# TODO(mrmonkey): Handle the unlikely edge cases (bug ####)
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Formatting

While you should follow the style that’s already there for files that you’re modifying, the following are required for any new code.

Indentation

Indent 2 spaces. No tabs.

Use blank lines between blocks to improve readability. Indentation is two spaces. Whatever you do, don’t use tabs. For existing files, stay faithful to the existing indentation.

Line Length and Long Strings

Maximum line length is 80 characters.

If you have to write strings that are longer than 80 characters, this should be done with a here document or an embedded newline if possible. Literal strings that have to be longer than 80 chars and can’t sensibly be split are ok, but it’s strongly preferred to find a way to make it shorter.

# DO use 'here document's
cat <<END
I am an exceptionally long
string.
END

# Embedded newlines are ok too
long_string="I am an exceptionally
long string."
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Pipelines

Pipelines should be split one per line if they don’t all fit on one line.

If a pipeline all fits on one line, it should be on one line.

If not, it should be split at one pipe segment per line with the pipe on the newline and a 2 space indent for the next section of the pipe.
This applies to a chain of commands combined using | as well as to logical compounds using || and &&.

# All fits on one line
command1 | command2

# Long commands
command1 \
  | command2 \
  | command3 \
  | command4
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Loops

Put ; do and ; then on the same line as the while, for or if.

Loops in shell are a bit different, but we follow the same principles as with braces when declaring functions. That is: ; then
and ; do should be on the same line as the if/for/while. else should be on its own line and closing statements should be on their own line vertically aligned with the opening statement.

Example:

# If inside a function, consider declaring the loop variable as
# a local to avoid it leaking into the global environment:
# local dir
for dir in "${dirs_to_cleanup[@]}"; do
  if [[ -d "${dir}/${ORACLE_SID}" ]]; then
    log_date "Cleaning up old files in ${dir}/${ORACLE_SID}"
    rm "${dir}/${ORACLE_SID}/"*
    if (( $? != 0 )); then
      error_message
    fi
  else
    mkdir -p "${dir}/${ORACLE_SID}"
    if (( $? != 0 )); then
      error_message
    fi
  fi
done
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Case statement

• Indent alternatives by 2 spaces.
• A one-line alternative needs a space after the close parenthesis of the pattern and before the ;;.
• Long or multi-command alternatives should be split over multiple lines with the pattern, actions, and ;; on separate lines.

The matching expressions are indented one level from the case and esac.
Multiline actions are indented another level. In general, there is no need to quote match expressions. Pattern expressions should not be preceded by an open parenthesis. Avoid the ;& and ;;& notations.

case "${expression}" in
  a)
    variable="…"
    some_command "${variable}" "${other_expr}" …
    ;;
  absolute)
    actions="relative"
    another_command "${actions}" "${other_expr}" …
    ;;
  *)
    error "Unexpected expression '${expression}'"
    ;;
esac
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Simple commands may be put on the same line as the pattern and ;; as long as the expression remains readable. This is
often appropriate for single-letter option processing. When the actions don’t fit on a single line, put the pattern on a line on its own, then the actions, then ;; also on a line of its own.
When on the same line as the actions, use a space after the close parenthesis of the pattern and another before the ;;.

verbose='false'
aflag=''
bflag=''
files=''
while getopts 'abf:v' flag; do
  case "${flag}" in
    a) aflag='true' ;;
    b) bflag='true' ;;
    f) files="${OPTARG}" ;;
    v) verbose='true' ;;
    *) error "Unexpected option ${flag}" ;;
  esac
done
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Variable expansion

In order of precedence: Stay consistent with what you find; quote your variables; prefer "${var}" over "$var".

These are strongly recommended guidelines but not mandatory regulation. Nonetheless, the fact that it’s a recommendation and
not mandatory doesn’t mean it should be taken lightly or downplayed.

They are listed in order of precedence.

• Stay consistent with what you find for existing code.

• Quote variables, see Quoting section below.

• Don’t brace-delimit single character shell specials / positional parameters, unless strictly necessary or avoiding deep confusion.

  Prefer brace-delimiting all other variables.

  # Section of *recommended* cases.
  
  # Preferred style for 'special' variables:
  echo "Positional: $1" "$5" "$3"
  echo "Specials: !=$!, -=$-, _=$_. ?=$?, #=$# *=$* @=$@ \$=$$ …"
  
  # Braces necessary:
  echo "many parameters: ${10}"
  
  # Braces avoiding confusion:
  # Output is "a0b0c0"
  set -- a b c
  echo "${1}0${2}0${3}0"
  
  # Preferred style for other variables:
  echo "PATH=${PATH}, PWD=${PWD}, mine=${some_var}"
  while read -r f; do
    echo "file=${f}"
  done < <(find /tmp)
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  # Section of *discouraged* cases
  
  # Unquoted vars, unbraced vars, brace-delimited single letter
  # shell specials.
  echo a=$avar "b=$bvar" "PID=${$}" "${1}"
  
  # Confusing use: this is expanded as "${1}0${2}0${3}0",
  # not "${10}${20}${30}
  set -- a b c
  echo "$10$20$30"
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NOTE: Using braces in ${var} is not a form of quoting. “Double quotes” must be used as well.

Quoting

• Always quote strings containing variables, command substitutions, spaces or shell meta characters, unless careful unquoted expansion is required or it’s a shell-internal integer (see next point).
• Use arrays for safe quoting of lists of elements, especially command-line flags. See Arrays below.
• Optionally quote shell-internal, readonly special variables that are defined to be integers: $?, $#, $$, $! (man bash). Prefer quoting of “named” internal integer variables, e.g. PPID etc for consistency.
• Prefer quoting strings that are “words” (as opposed to command options or path names).
• Never quote literal integers.
• Be aware of the quoting rules for pattern matches in [[ … ]]. See the Test, [ … ], and [[ … ]] section below.
• Use "$@" unless you have a specific reason to use $*, such as simply appending the arguments to a string in a message or log.

# 'Single' quotes indicate that no substitution is desired.
# "Double" quotes indicate that substitution is required/tolerated.

# Simple examples

# "quote command substitutions"
# Note that quotes nested inside "$()" don't need escaping.
flag="$(some_command and its args "$@" 'quoted separately')"

# "quote variables"
echo "${flag}"

# Use arrays with quoted expansion for lists.
declare -a FLAGS
FLAGS=( --foo --bar='baz' )
readonly FLAGS
mybinary "${FLAGS[@]}"

# It's ok to not quote internal integer variables.
if (( $# > 3 )); then
  echo "ppid=${PPID}"
fi

# "never quote literal integers"
value=32
# "quote command substitutions", even when you expect integers
number="$(generate_number)"

# "prefer quoting words", not compulsory
readonly USE_INTEGER='true'

# "quote shell meta characters"
echo 'Hello stranger, and well met. Earn lots of $$$'
echo "Process $$: Done making \$\$\$."

# "command options or path names"
# ($1 is assumed to contain a value here)
grep -li Hugo /dev/null "$1"

# Less simple examples
# "quote variables, unless proven false": ccs might be empty
git send-email --to "${reviewers}" ${ccs:+"--cc" "${ccs}"}

# Positional parameter precautions: $1 might be unset
# Single quotes leave regex as-is.
grep -cP '([Ss]pecial|\|?characters*)$' ${1:+"$1"}

# For passing on arguments,
# "$@" is right almost every time, and
# $* is wrong almost every time:
#
# * $* and $@ will split on spaces, clobbering up arguments
#   that contain spaces and dropping empty strings;
# * "$@" will retain arguments as-is, so no args
#   provided will result in no args being passed on;
#   This is in most cases what you want to use for passing
#   on arguments.
# * "$*" expands to one argument, with all args joined
#   by (usually) spaces,
#   so no args provided will result in one empty string
#   being passed on.
# (Consult `man bash` for the nit-grits ;-)

(set -- 1 "2 two" "3 three tres"; echo $#; set -- "$*"; echo "$#, $@")
(set -- 1 "2 two" "3 three tres"; echo $#; set -- "$@"; echo "$#, $@")
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Features and Bugs

ShellCheck

The ShellCheck project identifies common bugs and warnings for your shell scripts. It is recommended for all scripts, large or small.

Command Substitution

Use $(command) instead of backticks.

Nested backticks require escaping the inner ones with \.
The $(command) format doesn’t change when nested and is easier to read.

Example:

# This is preferred:
var="$(command "$(command1)")"
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# This is not:
var="`command \`command1\``"
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Test, [ … ], and [[ … ]]

[[ … ]] is preferred over [ … ], test and /usr/bin/[.

[[ … ]] reduces errors as no pathname expansion or word splitting takes place between [[ and ]]. In addition, [[ … ]] allows for regular expression matching, while [ … ] does not.

# This ensures the string on the left is made up of characters in
# the alnum character class followed by the string name.
# Note that the RHS should not be quoted here.
if [[ "filename" =~ ^[[:alnum:]]+name ]]; then
  echo "Match"
fi

# This matches the exact pattern "f*" (Does not match in this case)
if [[ "filename" == "f*" ]]; then
  echo "Match"
fi
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# This gives a "too many arguments" error as f* is expanded to the
# contents of the current directory
if [ "filename" == f* ]; then
  echo "Match"
fi
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For the gory details, see E14 at http://tiswww.case.edu/php/chet/bash/FAQ

Testing Strings

Use quotes rather than filler characters where possible.

Bash is smart enough to deal with an empty string in a test. So, given that the code is much easier to read, use tests for empty/non-empty strings or empty strings rather than filler characters.

# Do this:
if [[ "${my_var}" == "some_string" ]]; then
  do_something
fi

# -z (string length is zero) and -n (string length is not zero) are
# preferred over testing for an empty string
if [[ -z "${my_var}" ]]; then
  do_something
fi

# This is OK (ensure quotes on the empty side), but not preferred:
if [[ "${my_var}" == "" ]]; then
  do_something
fi
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# Not this:
if [[ "${my_var}X" == "some_stringX" ]]; then
  do_something
fi
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To avoid confusion about what you’re testing for, explicitly use
-z or -n.

# Use this
if [[ -n "${my_var}" ]]; then
  do_something
fi
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# Instead of this
if [[ "${my_var}" ]]; then
  do_something
fi
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For clarity, use == for equality rather than = even though both work. The former encourages the use of [[ and the latter can be confused with an assignment.
However, be careful when using < and > in [[ … ]] which performs a lexicographical comparison.
Use (( … )) or -lt and -gt for numerical comparison.

# Use this
if [[ "${my_var}" == "val" ]]; then
  do_something
fi

if (( my_var > 3 )); then
  do_something
fi

if [[ "${my_var}" -gt 3 ]]; then
  do_something
fi
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# Instead of this
if [[ "${my_var}" = "val" ]]; then
  do_something
fi

# Probably unintended lexicographical comparison.
if [[ "${my_var}" > 3 ]]; then
  # True for 4, false for 22.
  do_something
fi
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Wildcard Expansion of Filenames

Use an explicit path when doing wildcard expansion of filenames.

As filenames can begin with a -, it’s a lot safer to expand wildcards with ./* instead of *.

# Here's the contents of the directory:
# -f  -r  somedir  somefile

# Incorrectly deletes almost everything in the directory by force
psa@bilby$ rm -v *
removed directory: `somedir'
removed `somefile'
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# As opposed to:
psa@bilby$ rm -v ./*
removed `./-f'
removed `./-r'
rm: cannot remove `./somedir': Is a directory
removed `./somefile'
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Eval

eval should be avoided.

Eval munges the input when used for assignment to variables and can set variables without making it possible to check what those variables were.

# What does this set?
# Did it succeed? In part or whole?
eval $(set_my_variables)

# What happens if one of the returned values has a space in it?
variable="$(eval some_function)"
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Arrays

Bash arrays should be used to store lists of elements, to avoid quoting complications. This particularly applies to argument lists. Arrays should not be used to facilitate more complex data structures (see When to use Shell above).

Arrays store an ordered collection of strings, and can be safely expanded into individual elements for a command or loop.

Using a single string for multiple command arguments should be avoided, as it inevitably leads to authors using eval or trying to nest quotes inside the string, which does not give reliable or readable results and leads to needless complexity.

# An array is assigned using parentheses, and can be appended to
# with +=( … ).
declare -a flags
flags=(--foo --bar='baz')
flags+=(--greeting="Hello ${name}")
mybinary "${flags[@]}"
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# Don’t use strings for sequences.
flags='--foo --bar=baz'
flags+=' --greeting="Hello world"'  # This won’t work as intended.
mybinary ${flags}
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# Command expansions return single strings, not arrays. Avoid
# unquoted expansion in array assignments because it won’t
# work correctly if the command output contains special
# characters or whitespace.

# This expands the listing output into a string, then does special keyword
# expansion, and then whitespace splitting.  Only then is it turned into a
# list of words.  The ls command may also change behavior based on the user's
# active environment!
declare -a files=($(ls /directory))

# The get_arguments writes everything to STDOUT, but then goes through the
# same expansion process above before turning into a list of arguments.
mybinary $(get_arguments)
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Arrays Pros

• Using Arrays allows lists of things without confusing quoting semantics. Conversely, not using arrays leads to misguided
  attempts to nest quoting inside a string.
• Arrays make it possible to safely store sequences/lists of arbitrary strings, including strings containing whitespace.

Arrays Cons

Using arrays can risk a script’s complexity growing.

Arrays Decision

Arrays should be used to safely create and pass around lists. In particular, when building a set of command arguments, use arrays to avoid confusing quoting issues. Use quoted expansion – "${array[@]}" – to access arrays. However, if more advanced data manipulation is required, shell scripting should be avoided altogether; see above.

Pipes to While

Use process substitution or the readarray builtin (bash4+) in preference to piping to while. Pipes create a subshell, so any variables modified within a pipeline do not propagate to the parent shell.

The implicit subshell in a pipe to while can introduce subtle bugs that are hard to track down.

last_line='NULL'
your_command | while read -r line; do
  if [[ -n "${line}" ]]; then
    last_line="${line}"
  fi
done

# This will always output 'NULL'!
echo "${last_line}"
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Using process substitution also creates a subshell. However, it allows
redirecting from a subshell to a while without putting the while (or any
other command) in a subshell.

last_line='NULL'
while read line; do
  if [[ -n "${line}" ]]; then
    last_line="${line}"
  fi
done < <(your_command)

# This will output the last non-empty line from your_command
echo "${last_line}"
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Alternatively, use the readarray builtin to read the file into an array, then loop over the array’s contents. Notice that (for the same reason as above) you need to use a process substitution with readarray rather than a pipe, but with the advantage that the input generation for the loop is located before it, rather than after.

last_line='NULL'
readarray -t lines < <(your_command)
for line in "${lines[@]}"; do
  if [[ -n "${line}" ]]; then
    last_line="${line}"
  fi
done
echo "${last_line}"
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Note: Be cautious using a for-loop to iterate over output, as in for var in $(...), as the output is split by whitespace, not by line. Sometimes you will
know this is safe because the output can’t contain any unexpected whitespace,
but where this isn’t obvious or doesn’t improve readability (such as a long
command inside $(...)), a while read loop or readarray is often safer
and clearer.

Arithmetic

Always use (( … )) or $(( … )) rather than let or $[ … ] or expr.

Never use the $[ … ] syntax, the expr command, or the let built-in.

< and > don’t perform numerical comparison inside [[ … ]] expressions (they perform lexicographical comparisons instead; see Testing Strings). For preference, don’t use [[ … ]] at all for numeric comparisons, use (( … )) instead.

It is recommended to avoid using (( … )) as a standalone statement, and otherwise be wary of its expression evaluating to zero

• particularly with set -e enabled. For example, set -e; i=0; (( i++ )) will cause the shell to exit.

# Simple calculation used as text - note the use of $(( … )) within
# a string.
echo "$(( 2 + 2 )) is 4"

# When performing arithmetic comparisons for testing
if (( a < b )); then
  …
fi

# Some calculation assigned to a variable.
(( i = 10 * j + 400 ))
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# This form is non-portable and deprecated
i=$[2 * 10]

# Despite appearances, 'let' isn't one of the declarative keywords,
# so unquoted assignments are subject to globbing wordsplitting.
# For the sake of simplicity, avoid 'let' and use (( … ))
let i="2 + 2"

# The expr utility is an external program and not a shell builtin.
i=$( expr 4 + 4 )

# Quoting can be error prone when using expr too.
i=$( expr 4 '*' 4 )
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Stylistic considerations aside, the shell’s built-in arithmetic is many times faster than expr.

When using variables, the ${var} (and $var) forms are not required within $(( … )). The shell knows to look up var for you, and omitting the ${…} leads to cleaner code. This is slightly contrary to the previous rule about always using braces, so this is a recommendation only.

# N.B.: Remember to declare your variables as integers when
# possible, and to prefer local variables over globals.
local -i hundred=$(( 10 * 10 ))
declare -i five=$(( 10 / 2 ))

# Increment the variable "i" by three.
# Note that:
#  - We do not write ${i} or $i.
#  - We put a space after the (( and before the )).
(( i += 3 ))

# To decrement the variable "i" by five:
(( i -= 5 ))

# Do some complicated computations.
# Note that normal arithmetic operator precedence is observed.
hr=2
min=5
sec=30
echo $(( hr * 3600 + min * 60 + sec )) # prints 7530 as expected
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Naming Conventions

Function Names

Lower-case, with underscores to separate words. Separate libraries with ::. Parentheses are required after the function name. The keyword function is optional, but must be used consistently throughout a project.

If you’re writing single functions, use lowercase and separate words with underscore. If you’re writing a package, separate package names with ::. Braces must be on the same line as the function name (as with other languages at Google) and no space between the function name and the parenthesis.

# Single function
my_func() {
  …
}

# Part of a package
mypackage::my_func() {
  …
}
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The function keyword is extraneous when “()” is present after the function name, but enhances quick identification of functions.

Variable Names

As for function names.

Variables names for loops should be similarly named for any variable you’re looping through.

for zone in "${zones[@]}"; do
  something_with "${zone}"
done
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Constants and Environment Variable Names

All caps, separated with underscores, declared at the top of the file.

Constants and anything exported to the environment should be capitalized.

# Constant
readonly PATH_TO_FILES='/some/path'

# Both constant and environment
declare -xr ORACLE_SID='PROD'
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Some things become constant at their first setting (for example, via getopts). Thus, it’s OK to set a constant in getopts or based on a condition, but it should be made readonly immediately afterwards.
For the sake of clarity readonly or export is recommended instead of the equivalent declare commands.

VERBOSE='false'
while getopts 'v' flag; do
  case "${flag}" in
    v) VERBOSE='true' ;;
  esac
done
readonly VERBOSE
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Source Filenames

Lowercase, with underscores to separate words if desired.

This is for consistency with other code styles in Google:
maketemplate or make_template but not make-template.

Read-only Variables

Use readonly or declare -r to ensure they’re read only.

As globals are widely used in shell, it’s important to catch errors when working with them. When you declare a variable that is meant to be read-only, make this explicit.

zip_version="$(dpkg --status zip | grep Version: | cut -d ' ' -f 2)"
if [[ -z "${zip_version}" ]]; then
  error_message
else
  readonly zip_version
fi
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Use Local Variables

Declare function-specific variables with local. Declaration and assignment should be on different lines.

Ensure that local variables are only seen inside a function and its children by using local when declaring them. This avoids
polluting the global name space and inadvertently setting variables that may have significance outside the function.

Declaration and assignment must be separate statements when the assignment value is provided by a command substitution; as the local builtin does not propagate the exit code from the command substitution.

my_func2() {
  local name="$1"

  # Separate lines for declaration and assignment:
  local my_var
  my_var="$(my_func)"
  (( $? == 0 )) || return

  …
}
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my_func2() {
  # DO NOT do this:
  # $? will always be zero, as it contains the exit code of 'local', not my_func
  local my_var="$(my_func)"
  (( $? == 0 )) || return

  …
}
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Function Location

Put all functions together in the file just below constants. Don’t hide executable code between functions. Doing so makes the code difficult to follow and results in nasty surprises when debugging.

If you’ve got functions, put them all together near the top of the file. Only includes, set statements and setting constants
may be done before declaring functions.

main

A function called main is required for scripts long enough to contain at least one other function.

In order to easily find the start of the program, put the main program in a function called main as the bottom most function. This provides consistency with the rest of the code base as well as allowing you to define more variables as local (which can’t be done if the main code is not a function). The last non-comment line in the file should be a call to main:

main "$@"
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Obviously, for short scripts where it’s just a linear flow, main is overkill and so is not required.

Calling Commands

Checking Return Values

Always check return values and give informative return values.

For unpiped commands, use $? or check directly via an if statement to keep it simple.

Example:

if ! mv "${file_list[@]}" "${dest_dir}/"; then
  echo "Unable to move ${file_list[*]} to ${dest_dir}" >&2
  exit 1
fi

# Or
mv "${file_list[@]}" "${dest_dir}/"
if (( $? != 0 )); then
  echo "Unable to move ${file_list[*]} to ${dest_dir}" >&2
  exit 1
fi
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Bash also has the PIPESTATUS variable that allows checking of the return code from all parts of a pipe. If it’s only necessary to check success or failure of the whole pipe, then the following is acceptable:

tar -cf - ./* | ( cd "${dir}" && tar -xf - )
if (( PIPESTATUS[0] != 0 || PIPESTATUS[1] != 0 )); then
  echo "Unable to tar files to ${dir}" >&2
fi
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However, as PIPESTATUS will be overwritten as soon as you do any other command, if you need to act differently on errors based on where it happened in the pipe, you’ll need to assign PIPESTATUS to another variable immediately after running the command (don’t forget that [ is a command and will wipe out PIPESTATUS).

tar -cf - ./* | ( cd "${DIR}" && tar -xf - )
return_codes=( "${PIPESTATUS[@]}" )
if (( return_codes[0] != 0 )); then
  do_something
fi
if (( return_codes[1] != 0 )); then
  do_something_else
fi
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Builtin Commands vs. External Commands

Given the choice between invoking a shell builtin and invoking a separate process, choose the builtin.

We prefer the use of builtins such as the Parameter Expansion functions in bash(1) as it’s more robust and portable (especially when compared to things like sed).

Examples:

# Prefer this:
addition=$(( X + Y ))
substitution="${string/#foo/bar}"
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# Instead of this:
addition="$(expr "${X}" + "${Y}")"
substitution="$(echo "${string}" | sed -e 's/^foo/bar/')"
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Conclusion

Use common sense and BE CONSISTENT.

Please take a few minutes to read the Parting Words section at the bottom of the C++ Guide.

Revision 2.02