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Bash Guideline

File Header

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

Example:

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

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.

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 and no space between the function name and the parenthesis.

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# 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.

Variable Names

As for function names.

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

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

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.

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# 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

Variable expansion

In order of precedence: Stay consistent with what you find; quote your variables; prefer ${var} over $var, but see details.
These are meant to be guidelines, as the topic seems too controversial for a mandatory regulation.

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.

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# 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"

STDOUT vs STDERR

All error messages should go to 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.

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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

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.

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# 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)"

Quoting

  • 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.
  • Use "$@" unless you have a specific reason to use $*.
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# '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 "$#, $@")

[ and [[

[[ ... ]] reduces errors as no pathname expansion or word splitting takes place between [[ and ]] and [[ ... ]] allows for regular expression matching where [ ... ] does not.

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# 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 https://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

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.

Warning

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.

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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

  ...
}

Function Location

Put all functions together in the file just below constants. Don't hide executable code between functions.

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.

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:

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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:

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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).

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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

Sources

Extract from Shell Guideline by Google