Bashscripting Grundlagen

Original Website

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

Learn bash in y minutes (learnxinyminutes.com)
Bash Guide (mywiki.wooledge.org)

Example

#!/usr/bin/env bash

NAME="John"
echo "Hello $NAME!"

#!/usr/bin/env bash

NAME="John"

echo "Hello $NAME!"

Variables

NAME="John"
echo $NAME
echo "$NAME"
echo "${NAME}!"

NAME="John"

echo $NAME

echo "$NAME"

echo "${NAME}!"

String quotes

NAME="John"
echo "Hi $NAME"  #=&gt; Hi John
echo 'Hi $NAME'  #=&gt; Hi $NAME

NAME="John"

echo "Hi $NAME" #=> Hi John

echo 'Hi $NAME' #=> Hi $NAME

Shell execution

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

echo "I'm in $(pwd)"

echo "I'm in `pwd`"

# Same

See Command substitution

Conditional execution

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

git commit && git push

git commit || echo "Commit failed"

Functions

get_name() {
  echo "John"
}

echo "You are $(get_name)"

get_name() {

echo "John"

}

echo "You are $(get_name)"

See: Functions

Conditionals

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

if [[ -z "$string" ]]; then

echo "String is empty"

elif [[ -n "$string" ]]; then

echo "String is not empty"

See: Conditionals

Strict mode

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

set -euo pipefail

IFS=$'\n\t'

See: Unofficial bash strict mode

Brace expansion

echo {A,B}.js

1 2	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`

See: Brace expansion

Parameter expansions

Basics

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

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}  #=&gt; "Jo"

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="/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"

STR="Hello world"

echo ${STR:6:5} # "world"

echo ${STR: -5:5} # "world"

SRC="/path/to/foo.cpp"
BASE=${SRC##*/}   #=&gt; "foo.cpp" (basepath)
DIR=${SRC%$BASE}  #=&gt; "/path/to/" (dirpath)

SRC="/path/to/foo.cpp"

BASE=${SRC##*/} #=> "foo.cpp" (basepath)

DIR=${SRC%$BASE} #=> "/path/to/" (dirpath)

Substitution

Code	Description
`${FOO%suffix}`	Remove suffix
`${FOO#prefix}`	Remove prefix
`${FOO%%suffix}`	Remove long suffix
`${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

1 2	# Single line comment

: '
This is a
multi 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,}   #=&gt; "hELLO WORLD!" (lowercase 1st letter)
echo ${STR,,}  #=&gt; "hello world!" (all lowercase)

STR="hello world!"
echo ${STR^}   #=&gt; "Hello world!" (uppercase 1st letter)
echo ${STR^^}  #=&gt; "HELLO WORLD!" (all uppercase)

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

Basic for loop

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

for i in /etc/rc.*; do

echo $i

done

C-like for loop

for ((i = 0 ; i &lt; 100 ; i++)); do
  echo $i
done

for ((i = 0 ; i < 100 ; i++)); do

echo $i

done

Ranges

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

for i in {1..5}; do

echo "Welcome $i"

done

With step size

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

for i in {5..50..5}; do

echo "Welcome $i"

done

Reading lines

cat file.txt | while read line; do
  echo $line
done

cat file.txt | while read line; do

echo $line

done

Forever

while true; do
  ···
done

while true; do

···

done

Functions

Defining functions

myfunc() {
    echo "hello $1"
}

myfunc() {

echo "hello $1"

}

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

# Same as above (alternate syntax)

function myfunc() {

echo "hello $1"

}

myfunc "John"

1 2	myfunc "John"

Returning values

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

myfunc() {

local myresult='some value'

echo $myresult

}

result="$(myfunc)"

1 2	result="$(myfunc)"

Raising errors

myfunc() {
  return 1
}

myfunc() {

return 1

}

if myfunc; then
  echo "success"
else
  echo "failure"
fi

if myfunc; then

echo "success"

else

echo "failure"

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

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 \|\| Y ]]`	Or

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

# String

if [[ -z "$string" ]]; then

echo "String is empty"

elif [[ -n "$string" ]]; then

echo "String is not empty"

else

echo "This never happens"

# Combinations
if [[ X &amp;&amp; Y ]]; then
  ...
fi

# Combinations

if [[ X && Y ]]; then

...

# Equal
if [[ "$A" == "$B" ]]

# Equal

if [[ "$A" == "$B" ]]

# Regex
if [[ "A" =~ . ]]

# Regex

if [[ "A" =~ . ]]

if (( $a &lt; $b )); then
   echo "$a is smaller than $b"
fi

if (( $a < $b )); then

echo "$a is smaller than $b"

if [[ -e "file.txt" ]]; then
  echo "file exists"
fi

if [[ -e "file.txt" ]]; then

echo "file exists"

Arrays

Defining arrays

Fruits=('Apple' 'Banana' 'Orange')

1 2	Fruits=('Apple' 'Banana' 'Orange')

Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="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

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

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

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

echo $i

done

Dictionaries

Defining

declare -A sounds

1 2	declare -A sounds

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

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

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

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

echo $val

done

Iterate over keys

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

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

echo $key

done

Options

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

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

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 `n`th most recent command
`!n`	Expand `n`th 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 `n`th 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 `n`th 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

1 2	$((a + 200)) # Add 200 to $a

$(($RANDOM%200))  # Random number 0..199

1 2	$(($RANDOM%200)) # Random number 0..199

Subshells

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

(cd somedir; echo "I'm now in $PWD")

pwd # still in first directory

Redirection

python hello.py &gt; output.txt   # stdout to (file)
python hello.py &gt;&gt; output.txt  # stdout to (file), append
python hello.py 2&gt; error.log   # stderr to (file)
python hello.py 2&gt;&amp;1           # stderr to stdout
python hello.py 2&gt;/dev/null    # stderr to (null)
python hello.py &amp;&gt;/dev/null    # stdout and stderr to (null)

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 &>/dev/null # stdout and stderr to (null)

python hello.py &lt; foo.txt      # feed foo.txt to stdin for python

1 2	python hello.py < foo.txt # feed foo.txt to stdin for python

Inspecting commands

command -V cd
#=&gt; "cd is a function/alias/whatever"

command -V cd

#=> "cd is a function/alias/whatever"

Trap errors

trap 'echo Error at about $LINENO' ERR

1 2	trap 'echo Error at about $LINENO' ERR

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

set -o errtrace
trap traperr ERR

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

case "$1" in

start | up)

vagrant up

;;

echo "Usage: $0 {start|stop|ssh}"

;;

esac

Source relative

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

1 2	source "${0%/*}/../share/foo.sh"

printf

printf "Hello %s, I'm %s" Sven Olga
#=&gt; "Hello Sven, I'm Olga

printf "1 + 1 = %d" 2
#=&gt; "1 + 1 = 2"

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

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"

Directory of script

DIR="${0%/*}"

1 2	DIR="${0%/*}"

Getting options

while [[ "$1" =~ ^- &amp;&amp; ! "$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

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 &lt;&lt;END
hello world
END

cat <<END

hello world

END

Reading input

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

echo -n "Proceed? [y/n]: "

read ans

echo $ans

read -n 1 ans    # Just one character

1 2	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 argrument of the previous command

See Special parameters.

Go to previous directory

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

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

if ping -c 1 google.com; then

echo "It appears you have a working internet connection"

Grep check

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

if grep -q 'foo' ~/.bash_history; then

echo "You appear to have typed 'foo' in the past"

Also see

Bash-hackers wiki (bash-hackers.org)
Shell vars (bash-hackers.org)
Learn bash in y minutes (learnxinyminutes.com)
Bash Guide (mywiki.wooledge.org)
ShellCheck (shellcheck.net)