Bash

Hello Summary

# variable default value if unset
v=${1:-world}
echo "hello ${v}" # hello world (if no args)

# arrays
l=("hello")
l+=("world")
echo "${#l[@]}" # 2
echo "${l[*]}" # hello world

# conditions
if [ 5 -ge 0 ] && [ 10 -ge 0 ]; then # can also be written [[ ... && ...]]
  echo "both positives" # both positive
fi

# 'function' and 'tricky::' are optional
function tricky::variable() {
  a=5
  local b=10
}

echo "a=${a}, b=${b}" # a=,b=
tricky::variable # call function
echo "a=${a}, b=${b}" # a=5,b=

# function (can be used as a command)
run_ls() {
  local directory="$1"
  ls ${directory}
}

# redirect the standard and error output
run_ls "/tmp" >/dev/null 2>&1 && echo "success" || echo "failed" # success

run_ls "/nonexistingdir/lala" >/dev/null 2>&1 \
&& echo "success" \
|| echo "failed" # failed

# in case you want to stop the script, you can add (set -e) and do something like
# ... || (echo "failed" && exit 1)

# loops
for i in $(seq 1 5); do echo -n "${i}, "; done # 1, 2, 3, 4, 5,
while true; do echo "test"; sleep 1s; break; done # test

# case
case ${option} in
  -l|--list)
    ls
  ;;
  *)
    echo "default" # default
  ;;
esac

Completion

Here's an intersting example with the command ''pass''

# completion file for bash

# Copyright (C) 2012 Jason A. Donenfeld `<Jason@zx2c4.com>` and
# Brian Mattern `<rephorm@rephorm.com>`. All Rights Reserved.
# This file is licensed under the GPLv2+. Please see COPYING for more information.

_pass_complete_entries () {
	prefix="${PASSWORD_STORE_DIR:-$HOME/.password-store/}"
	suffix=".gpg"
	autoexpand=${1:-0}

	local IFS=$'\n'
	local items=($(compgen -f $prefix$cur))
	for item in ${items[@]}; do
		if [[ $item == $prefix.* ]]; then
			continue
		fi

		# if there is a unique match, and it is a directory with one entry
		# autocomplete the subentry as well (recursively)
		if [[ ${#items[@]} -eq 1 && $autoexpand -eq 1 ]]; then
			while [[ -d $item ]]; do
				local subitems=($(compgen -f "$item/"))
				if [[ ${#subitems[@]} -eq 1 ]]; then
					item="${subitems[0]}"
				else
					break
				fi
			done
		fi

		# append / to directories
		[[ -d $item ]] && item="$item/"

		item="${item%$suffix}"
		COMPREPLY+=("${item#$prefix}")
	done
}

_pass_complete_keys () {
	local IFS=$'\n'
	local GPG=gpg2
	which $GPG >/dev/null || GPG=gpg
	# Extract names and email addresses from gpg --list-keys
	local keys="$($GPG --list-secret-keys --with-colons | cut -d : -f 10 | sort -u | sed '/^$/d')"
	COMPREPLY+=($(compgen -W "${keys}" -- ${cur}))
}

_pass()
{
	COMPREPLY=()
	local cur="${COMP_WORDS[COMP_CWORD]}"
	local commands="init ls show insert generate edit rm git help version"
	if [[ $COMP_CWORD -gt 1 ]]; then
		case "${COMP_WORDS[1]}" in
			init)
				COMPREPLY+=($(compgen -W "-e --reencrypt" -- ${cur}))
				_pass_complete_keys
				;;
			ls|list|edit)
				_pass_complete_entries
				;;
			show|-*)
				COMPREPLY+=($(compgen -W "-c --clip" -- ${cur}))
				_pass_complete_entries 1
				;;
			insert)
				COMPREPLY+=($(compgen -W "-e --echo -m --multiline -f --force" -- ${cur}))
				_pass_complete_entries
				;;
			generate)
				COMPREPLY+=($(compgen -W "-n --no-symbols -c --clip -f --force" -- ${cur}))
				_pass_complete_entries
				;;
			rm|remove|delete)
				COMPREPLY+=($(compgen -W "-r --recursive -f --force" -- ${cur}))
				_pass_complete_entries
				;;
			git)
				COMPREPLY+=($(compgen -W "init push pull config log reflog" -- ${cur}))
				;;
		esac
	else
		COMPREPLY+=($(compgen -W "${commands}" -- ${cur}))
		_pass_complete_entries 1
	fi
}

complete -o filenames -o nospace -F _pass pass

Environment

To retrieve environment variable: env{bash}

Note: Could also use set but it return the register functions too.

Constant

readonly var=value

Arithmetic

n1=1
n2=1

# $(( expression ))

a=$(( n1+n2 )) # 2
b=$(( n1/n2 )) # 1
c=$(( n1-n2 )) # 0

See more on official doc

Shell Parameter Expansion

Official source

Example :

bash --version # GNU bash, version 4.2.25(1)-release (x86_64-pc-linux-gnu)

test="n=5=6=7"
echo ${test} # n=5=6=7

echo ${test#*=} # 5=6=7
echo ${test#*=*=} # 6=7
echo ${test##*=} # 7

echo ${test%=*} # n=5=6
echo ${test%=*=*} # n=5
echo ${test%%=*} # n

Bash Regex

Example with host variable which start by "user." and continue by whatever or start by host1.

if [[ "$HOST" =~ ^user.*|^host1 ]]; then
  echo "yes"
fi

FAQ / Tips

Debug bash script

bash -x script

Note: You could also add ''-v'' option

Or directly in script :

#!/bin/bash

### Turn on debug mode ###
set -x

# Run shell commands

echo "Hello $(LOGNAME)"
echo "Today is $(date)"
echo "Users currently on the machine, and their processes:"
w

### Turn OFF debug mode ###
set +x

Note: set permit also to change option of shell (see more).

Breakpoint

bash -c "$SHELL"

source

Slugify

echo $str | iconv -t ascii//TRANSLIT | sed -r s/[~\^]+//g | sed -r s/[^a-zA-Z0-9]+/-/g | sed -r s/^-+\|-+$//g | tr A-Z a-z

source

Use bash completion from another command

function _mycmd(){
  local current choice
  current=${COMP_WORDS[COMP_CWORD]}

  case "$COMP_CWORD" in
  1)
    choice="opt1 opt2"
    COMPREPLY=( $( compgen -W '$choice' -- $current ) )
  ;;
  2)
    COMPREPLY=( $(compgen -W 'the_other_command' -- $current) )
  ;;
  *)
    # idea based on sudo bash completion
    local PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
    _command_offset 2
    return
  ;;
  esac
}

complete -F _mycmd mycmd

Note/Todo: the command will look like mycmd opt1 the_other_command .... I did not find a way to get rid of the_other_command to make it works

Inspired from:

lock a file (multiple access)

# lock
exec 3>filename # open a file handle; this part will always succeed
flock -x 3      # lock the file handle; this part will block

# release
exec 3>&-       # close the file handle

equivalent code in a block

{
  flock -x 3
  ...other stuff here...
} 3>filename

with new bash version, no need to fd number:

# this requires a very new bash -- 4.2 or so.
exec {lock_fd}>filename  # open filename, store FD number in lock_fd
flock -x "$lock_fd"      # pass that FD number to flock
exec {lock_fd}>&-         # later: release the lock

source