Google Shell Style Notes (Revision 1.26)

可执行文件应该没有后缀（强烈推荐）或以.sh后缀。库必须有.sh后缀而且 不能被执行。

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和SGID被禁止。

Use sudo to provide elevated access if you need it.

所有错误信息输出到STDERR

This makes it easier to separate normal status from actual issues.

A function to print out error messages along with other status information is recommended.

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

if ! do_something; then
  err "Unable to do_something"
  exit "${E_DID_NOTHING}"
fi

每个文件以一段它内容的描述开始。

Every file must have a top-level comment including a brief overview of its contents. A copyright notice and author information are optional.

Example:

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

长而不清晰的函数都必须注释。库中的函数都要注释，

All function comments should contain:

• Description of the function
• Global variables used and modified
• Arguments taken
• Returned values other than the default exit status of the last command run

Example:

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

export PATH='/usr/xpg4/bin:/usr/bin:/opt/csw/bin:/opt/goog/bin'

#######################################
# Cleanup files from the backup dir
# Globals:
#   BACKUP_DIR
#   ORACLE_SID
# Arguments:
#   None
# Returns:
#   None
#######################################
cleanup() {
  ...
}

在代码中，对巧妙的，不清晰的，有趣或重要的部分注释。

对那些临时的, 短期的解决方案, 或已经够好但仍不完美的代码使用 TODO 注释.

TODOs should include the string TODO in all caps, followed by your username in parentheses. A colon is optional. It's preferable to put a bug/ticket number next to the TODO item as well.

Examples:

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

缩进2个空格。不要用tab。

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.

最大行长度是80字符。

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."

多个管道如果不能在一行，那么就一行一个。

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

使用while,for.或if时，把 ;do和;then放在一行。

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:

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 [[ "$?" -ne 0 ]]; then
      error_message
    fi
  else
    mkdir -p "${dir}/${ORACLE_SID}"
    if [[ "$?" -ne 0 ]]; then
      error_message
    fi
  fi
done

选项缩进2个空格
在一行的选项中，用空格分割分别在右括号后和;;之前

长或多命令选项需要分成这样形式的多行:匹配项，操作，和;;

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

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 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

优先顺序：保持之前风格;把变量放引号里，用"${var}"代替"$var"，但看如下细节。

They are listed in order of precedence.

1. Stay consistent with what you find for existing code.
2. Quote variables, see Quoting section below.
3. Don't brace-quote single character shell specials / positional parameters, unless strictly necessary or avoiding deep confusion. Prefer brace-quoting 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=${some_var}"
while read f; do
  echo "file=${f}"
done < <(ls -l /tmp)

# Section of discouraged cases

# Unquoted vars, unbraced vars, brace-quoted 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"

• Always quote strings containing variables, command substitutions, spaces or shell meta characters, unless careful unquoted expansion is required.
• 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 [[.
• Use "$@" unless you have a specific reason to use $*.

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

# Simple examples
# "quote command substitutions"
flag="$(some_command and its args "$@" 'quoted separately')"

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

# "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 everytime, and
# $* is wrong almost everytime:
#
# * $* 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 "$#, $@"

使用 $(command) 代替重音符(``)

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)")"

# This is not:
var="`command \`command1\``"

用[[ ... ]], 不要用[, test和/usr/bin/[.

[[ ... ]] reduces errors as no pathname expansion or word splitting takes place between [[ and ]] and [[ … ]] allows for regular expression matching where [ … ] 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.
# For the gory details, see
# E14 at http://tiswww.case.edu/php/chet/bash/FAQ
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

# 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

尽量使用引号，而不是填充字符。

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

# Not this:
if [[ "${my_var}X" = "some_stringX" ]]; then
  do_something
fi
To avoid confusion about what you're testing for, explicitly use -z or -n.

# Use this
if [[ -n "${my_var}" ]]; then
  do_something
fi

# Instead of this as errors can occur if ${my_var} expands to a test
# flag
if [[ "${my_var}" ]]; then
  do_something
fi

当做文件名通配符扩展时，使用明确的路径。

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

# This deletes almost everything in the directory by force
psa@bilby$ rm -v *
removed directory: `somedir'
removed `somefile'

# As opposed to:
psa@bilby$ rm -v ./*
removed `./-f'
removed `./-r'
rm: cannot remove `./somedir': Is a directory
removed `./somefile'

eval应该被避免使用。

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)"

使用进程替换或for循环来替代管道到while的形式。在while循环中改变的变量不会传递到父进程，因为 循环的命令运行在子shell。

The implicit subshell in a pipe to while can make it difficult to track down bugs.

last_line='NULL'
your_command | while read line; do
  last_line="${line}"
done

# This will output 'NULL'
echo "${last_line}"

Use a for loop if you are confident that the input will not contain spaces or special characters (usually, this means not user input).

total=0
# Only do this if there are no spaces in return values.
for value in $(command); do
  total+="${value}"
done

Using process substitution allows redirecting output but puts the commands in an explicit subshell rather than the implicit subshell that bash creates for the while loop.

total=0
last_file=
while read count filename; do
  total+="${count}"
  last_file="${filename}"
done < <(your_command | uniq -c)

# This will output the second field of the last line of output from
# the command.
echo "Total = ${total}"
echo "Last one = ${last_file}"

Use while loops where it is not necessary to pass complex results to the parent shell - this is typically where some more complex "parsing" is required. Beware that simple examples are probably more easily done with a tool such as awk. This may also be useful where you specifically don't want to change the parent scope variables.

# Trivial implementation of awk expression:
#   awk '$3 == "nfs" { print $2 " maps to " $1 }' /proc/mounts
cat /proc/mounts | while read src dest type opts rest; do
  if [[ ${type} == "nfs" ]]; then
    echo "NFS ${dest} maps to ${src}"
  fi
done

小写，下划线分割词语。分离库用::. 需要括号在函数名后。关键字function是可选的， 但是必须在一个工程内保持一致。

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 and no space between the function name and the parenthesis.

# Single function
my_func() {
  ...
}

# Part of a package
mypackage::my_func() {
  ...
}

The function keyword is extraneous when "()" is present after the function name, but enhances quick identification of functions.

格式和函数名一样。

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

for zone in ${zones}; do
  something_with "${zone}"
done

所有大写，以下划线分格，在文件顶端定义。

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'

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. Note that declare doesn't operate on global variables within functions, so readonly or export is recommended instead.

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

小写，如果需要，用下划线风格词语。

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

使用readonly和declare -r来确认它们是只读的。

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

用local声明函数内的变量。声明和赋值应当在不同行。

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)" || return

  # DO NOT do this: $? contains the exit code of 'local', not my_func
  local my_var="$(my_func)"
  [[ $? -eq 0 ]] || return

  ...
}

在文件里把所有函数一起放在定量下面。不要在函数间隐藏可执行代码。

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.

Don't hide executable code between functions. Doing so makes the code difficult to follow and results in nasty surprises when debugging.

只要在脚本长到至少含有另外一个函数时，main函数才需要。

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 "$@"

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

一直检查返回值，返回有意义的值。

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 ${dest_dir}" >&2
  exit "${E_BAD_MOVE}"
fi

# Or
mv "${file_list}" "${dest_dir}/"
if [[ "$?" -ne 0 ]]; then
  echo "Unable to move ${file_list} to ${dest_dir}" >&2
  exit "${E_BAD_MOVE}"
fi

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]}" -ne 0 || "${PIPESTATUS[1]}" -ne 0 ]]; then
  echo "Unable to tar files to ${dir}" >&2
fi

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 [[ "${return_codes[0]}" -ne 0 ]]; then
  do_something
fi
if [[ "${return_codes[1]}" -ne 0 ]]; then
  do_something_else
fi

可以选用shell内嵌的功能或一个分开的处理，选择内嵌的功能。

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).

Example:

# Prefer this:
addition=$((${X} + ${Y}))
substitution="${string/#foo/bar}"

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