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Bash scripting
CLI
Featured
2020-07-05
Variables
Functions
Interpolation
Brace expansions
Loops
Conditional execution
Command substitution

Getting started

{: .-three-column}

Introduction

{: .-intro}

This is a quick reference to getting started with Bash scripting.

Example

#!/usr/bin/env bash

name="John"
echo "Hello $name!"

Variables

name="John"
echo $name  # see below
echo "$name"
echo "${name}!"

Generally quote your variables unless they contain wildcards to expand or command fragments.

wildcard="*.txt"
options="iv"
cp -$options $wildcard /tmp

String quotes

name="John"
echo "Hi $name"  #=> Hi John
echo 'Hi $name'  #=> Hi $name

Shell execution

echo "I'm in $(pwd)"
echo "I'm in `pwd`"  # obsolescent
# Same

See Command substitution

Conditional execution

git commit && git push
git commit || echo "Commit failed"

Functions

{: id='functions-example'}

get_name() {
  echo "John"
}

echo "You are $(get_name)"

See: Functions

Conditionals

{: id='conditionals-example'}

if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
fi

See: Conditionals

Strict mode

set -euo pipefail
IFS=$'\n\t'

See: Unofficial bash strict mode

Brace expansion

echo {A,B}.js
Expression Description
{A,B} Same as A B
{A,B}.js Same as A.js B.js
{1..5} Same as 1 2 3 4 5
{{1..3},{7..9}} Same as 1 2 3 7 8 9

See: Brace expansion

Parameter expansions

{: .-three-column}

Basics

name="John"
echo "${name}"
echo "${name/J/j}"    #=> "john" (substitution)
echo "${name:0:2}"    #=> "Jo" (slicing)
echo "${name::2}"     #=> "Jo" (slicing)
echo "${name::-1}"    #=> "Joh" (slicing)
echo "${name:(-1)}"   #=> "n" (slicing from right)
echo "${name:(-2):1}" #=> "h" (slicing from right)
echo "${food:-Cake}"  #=> $food or "Cake"
length=2
echo "${name:0:length}"  #=> "Jo"

See: Parameter expansion

str="/path/to/foo.cpp"
echo "${str%.cpp}"    # /path/to/foo
echo "${str%.cpp}.o"  # /path/to/foo.o
echo "${str%/*}"      # /path/to

echo "${str##*.}"     # cpp (extension)
echo "${str##*/}"     # foo.cpp (basepath)

echo "${str#*/}"      # path/to/foo.cpp
echo "${str##*/}"     # foo.cpp

echo "${str/foo/bar}" # /path/to/bar.cpp
str="Hello world"
echo "${str:6:5}"    # "world"
echo "${str: -5:5}"  # "world"
src="/path/to/foo.cpp"
base=${src##*/}   #=> "foo.cpp" (basepath)
dir=${src%$base}  #=> "/path/to/" (dirpath)

Prefix name expansion

prefix_a=one
prefix_b=two
echo ${!prefix_*}  # all variables names starting with `prefix_`
prefix_a prefix_b

Indirection

name=joe
pointer=name
echo ${!pointer}
joe

Substitution

Code Description
${foo%suffix} Remove suffix
${foo#prefix} Remove prefix
--- ---
${foo%%suffix} Remove long suffix
${foo/%suffix} Remove long suffix
${foo##prefix} Remove long prefix
${foo/#prefix} Remove long prefix
--- ---
${foo/from/to} Replace first match
${foo//from/to} Replace all
--- ---
${foo/%from/to} Replace suffix
${foo/#from/to} Replace prefix

Comments

# Single line comment
: '
This is a
multi line
comment
'

Substrings

Expression Description
${foo:0:3} Substring (position, length)
${foo:(-3):3} Substring from the right

Length

Expression Description
${#foo} Length of $foo

Manipulation

str="HELLO WORLD!"
echo "${str,}"   #=> "hELLO WORLD!" (lowercase 1st letter)
echo "${str,,}"  #=> "hello world!" (all lowercase)

str="hello world!"
echo "${str^}"   #=> "Hello world!" (uppercase 1st letter)
echo "${str^^}"  #=> "HELLO WORLD!" (all uppercase)

Default values

Expression Description
${foo:-val} $foo, or val if unset (or null)
${foo:=val} Set $foo to val if unset (or null)
${foo:+val} val if $foo is set (and not null)
${foo:?message} Show error message and exit if $foo is unset (or null)

Omitting the : removes the (non)nullity checks, e.g. ${foo-val} expands to val if unset otherwise $foo.

Loops

{: .-three-column}

Basic for loop

for i in /etc/rc.*; do
  echo "$i"
done

C-like for loop

for ((i = 0 ; i < 100 ; i++)); do
  echo "$i"
done

Ranges

for i in {1..5}; do
    echo "Welcome $i"
done

With step size

for i in {5..50..5}; do
    echo "Welcome $i"
done

Reading lines

while read -r line; do
  echo "$line"
done <file.txt

Forever

while true; do
  ···
done

Functions

{: .-three-column}

Defining functions

myfunc() {
    echo "hello $1"
}
# Same as above (alternate syntax)
function myfunc {
    echo "hello $1"
}
myfunc "John"

Returning values

myfunc() {
    local myresult='some value'
    echo "$myresult"
}
result=$(myfunc)

Raising errors

myfunc() {
  return 1
}
if myfunc; then
  echo "success"
else
  echo "failure"
fi

Arguments

Expression Description
$# Number of arguments
$* All positional arguments (as a single word)
$@ All positional arguments (as separate strings)
$1 First argument
$_ Last argument of the previous command

Note: $@ and $* must be quoted in order to perform as described. Otherwise, they do exactly the same thing (arguments as separate strings).

See Special parameters.

Conditionals

{: .-three-column}

Conditions

Note that [[ is actually a command/program that returns either 0 (true) or 1 (false). Any program that obeys the same logic (like all base utils, such as grep(1) or ping(1)) can be used as condition, see examples.

Condition Description
[[ -z STRING ]] Empty string
[[ -n STRING ]] Not empty string
[[ STRING == STRING ]] Equal
[[ STRING != STRING ]] Not Equal
--- ---
[[ NUM -eq NUM ]] Equal
[[ NUM -ne NUM ]] Not equal
[[ NUM -lt NUM ]] Less than
[[ NUM -le NUM ]] Less than or equal
[[ NUM -gt NUM ]] Greater than
[[ NUM -ge NUM ]] Greater than or equal
--- ---
[[ STRING =~ STRING ]] Regexp
--- ---
(( NUM < NUM )) Numeric conditions

More conditions

Condition Description
[[ -o noclobber ]] If OPTIONNAME is enabled
--- ---
[[ ! EXPR ]] Not
[[ X && Y ]] And
`[[ X

File conditions

Condition Description
[[ -e FILE ]] Exists
[[ -r FILE ]] Readable
[[ -h FILE ]] Symlink
[[ -d FILE ]] Directory
[[ -w FILE ]] Writable
[[ -s FILE ]] Size is > 0 bytes
[[ -f FILE ]] File
[[ -x FILE ]] Executable
--- ---
[[ FILE1 -nt FILE2 ]] 1 is more recent than 2
[[ FILE1 -ot FILE2 ]] 2 is more recent than 1
[[ FILE1 -ef FILE2 ]] Same files

Example

# String
if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
else
  echo "This never happens"
fi
# Combinations
if [[ X && Y ]]; then
  ...
fi
# Equal
if [[ "$A" == "$B" ]]
# Regex
if [[ "A" =~ . ]]
if (( $a < $b )); then
   echo "$a is smaller than $b"
fi
if [[ -e "file.txt" ]]; then
  echo "file exists"
fi

Arrays

Defining arrays

Fruits=('Apple' 'Banana' 'Orange')
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

Working with arrays

echo "${Fruits[0]}"           # Element #0
echo "${Fruits[-1]}"          # Last element
echo "${Fruits[@]}"           # All elements, space-separated
echo "${#Fruits[@]}"          # Number of elements
echo "${#Fruits}"             # String length of the 1st element
echo "${#Fruits[3]}"          # String length of the Nth element
echo "${Fruits[@]:3:2}"       # Range (from position 3, length 2)
echo "${!Fruits[@]}"          # Keys of all elements, space-separated

Operations

Fruits=("${Fruits[@]}" "Watermelon")    # Push
Fruits+=('Watermelon')                  # Also Push
Fruits=( "${Fruits[@]/Ap*/}" )          # Remove by regex match
unset Fruits[2]                         # Remove one item
Fruits=("${Fruits[@]}")                 # Duplicate
Fruits=("${Fruits[@]}" "${Veggies[@]}") # Concatenate
lines=(`cat "logfile"`)                 # Read from file

Iteration

for i in "${arrayName[@]}"; do
  echo "$i"
done

Dictionaries

{: .-three-column}

Defining

declare -A sounds
sounds[dog]="bark"
sounds[cow]="moo"
sounds[bird]="tweet"
sounds[wolf]="howl"

Declares sound as a Dictionary object (aka associative array).

Working with dictionaries

echo "${sounds[dog]}" # Dog's sound
echo "${sounds[@]}"   # All values
echo "${!sounds[@]}"  # All keys
echo "${#sounds[@]}"  # Number of elements
unset sounds[dog]     # Delete dog

Iteration

Iterate over values

for val in "${sounds[@]}"; do
  echo "$val"
done

Iterate over keys

for key in "${!sounds[@]}"; do
  echo "$key"
done

Options

Options

set -o noclobber  # Avoid overlay files (echo "hi" > foo)
set -o errexit    # Used to exit upon error, avoiding cascading errors
set -o pipefail   # Unveils hidden failures
set -o nounset    # Exposes unset variables

Glob options

shopt -s nullglob    # Non-matching globs are removed  ('*.foo' => '')
shopt -s failglob    # Non-matching globs throw errors
shopt -s nocaseglob  # Case insensitive globs
shopt -s dotglob     # Wildcards match dotfiles ("*.sh" => ".foo.sh")
shopt -s globstar    # Allow ** for recursive matches ('lib/**/*.rb' => 'lib/a/b/c.rb')

Set GLOBIGNORE as a colon-separated list of patterns to be removed from glob matches.

History

Commands

Command Description
history Show history
shopt -s histverify Don't execute expanded result immediately

Expansions

Expression Description
!$ Expand last parameter of most recent command
!* Expand all parameters of most recent command
!-n Expand nth most recent command
!n Expand nth command in history
!<command> Expand most recent invocation of command <command>

Operations

Code Description
!! Execute last command again
!!:s/<FROM>/<TO>/ Replace first occurrence of <FROM> to <TO> in most recent command
!!:gs/<FROM>/<TO>/ Replace all occurrences of <FROM> to <TO> in most recent command
!$:t Expand only basename from last parameter of most recent command
!$:h Expand only directory from last parameter of most recent command

!! and !$ can be replaced with any valid expansion.

Slices

Code Description
!!:n Expand only nth token from most recent command (command is 0; first argument is 1)
!^ Expand first argument from most recent command
!$ Expand last token from most recent command
!!:n-m Expand range of tokens from most recent command
!!:n-$ Expand nth token to last from most recent command

!! can be replaced with any valid expansion i.e. !cat, !-2, !42, etc.

Miscellaneous

Numeric calculations

$((a + 200))      # Add 200 to $a
$(($RANDOM%200))  # Random number 0..199
declare -i count  # Declare as type integer
count+=1          # Increment

Subshells

(cd somedir; echo "I'm now in $PWD")
pwd # still in first directory

Redirection

python hello.py > output.txt            # stdout to (file)
python hello.py >> output.txt           # stdout to (file), append
python hello.py 2> error.log            # stderr to (file)
python hello.py 2>&1                    # stderr to stdout
python hello.py 2>/dev/null             # stderr to (null)
python hello.py >output.txt 2>&1        # stdout and stderr to (file), equivalent to &>
python hello.py &>/dev/null             # stdout and stderr to (null)
echo "$0: warning: too many users" >&2  # print diagnostic message to stderr
python hello.py < foo.txt      # feed foo.txt to stdin for python
diff <(ls -r) <(ls)            # Compare two stdout without files

Inspecting commands

command -V cd
#=> "cd is a function/alias/whatever"

Trap errors

trap 'echo Error at about $LINENO' ERR

or

traperr() {
  echo "ERROR: ${BASH_SOURCE[1]} at about ${BASH_LINENO[0]}"
}

set -o errtrace
trap traperr ERR

Case/switch

case "$1" in
  start | up)
    vagrant up
    ;;

  *)
    echo "Usage: $0 {start|stop|ssh}"
    ;;
esac

Source relative

source "${0%/*}/../share/foo.sh"

printf

printf "Hello %s, I'm %s" Sven Olga
#=> "Hello Sven, I'm Olga

printf "1 + 1 = %d" 2
#=> "1 + 1 = 2"

printf "This is how you print a float: %f" 2
#=> "This is how you print a float: 2.000000"

printf '%s\n' '#!/bin/bash' 'echo hello' >file
# format string is applied to each group of arguments
printf '%i+%i=%i\n' 1 2 3  4 5 9

Transform strings

Command option Description
-c Operations apply to characters not in the given set
-d Delete characters
-s Replaces repeated characters with single occurrence
-t Truncates
[:upper:] All upper case letters
[:lower:] All lower case letters
[:digit:] All digits
[:space:] All whitespace
[:alpha:] All letters
[:alnum:] All letters and digits

Example

echo "Welcome To Devhints" | tr '[:lower:]' '[:upper:]'
WELCOME TO DEVHINTS

Directory of script

dir=${0%/*}

Getting options

while [[ "$1" =~ ^- && ! "$1" == "--" ]]; do case $1 in
  -V | --version )
    echo "$version"
    exit
    ;;
  -s | --string )
    shift; string=$1
    ;;
  -f | --flag )
    flag=1
    ;;
esac; shift; done
if [[ "$1" == '--' ]]; then shift; fi

Heredoc

cat <<END
hello world
END

Reading input

echo -n "Proceed? [y/n]: "
read -r ans
echo "$ans"

The -r option disables a peculiar legacy behavior with backslashes.

read -n 1 ans    # Just one character

Special variables

Expression Description
$? Exit status of last task
$! PID of last background task
$$ PID of shell
$0 Filename of the shell script
$_ Last argument of the previous command
${PIPESTATUS[n]} return value of piped commands (array)

See Special parameters.

Go to previous directory

pwd # /home/user/foo
cd bar/
pwd # /home/user/foo/bar
cd -
pwd # /home/user/foo

Check for command's result

if ping -c 1 google.com; then
  echo "It appears you have a working internet connection"
fi

Grep check

if grep -q 'foo' ~/.bash_history; then
  echo "You appear to have typed 'foo' in the past"
fi

Also see

{: .-one-column}