Bash scripting Tutorial

Hello World Bash Shell Script

Attention:

For more verbose and beginner style Bash scripting tutorial visit our Bash Scripting Tutorial for Beginners

First you need to find out where is your bash interpreter located. Enter the following into your command line:

$ which bash

Open up you favorite text editor and create file called hello_world.sh. Insert the following lines to a file:

NOTE:Every bash shell script in this tutorial starts with shebang:"#!" which is not read as a comment. First line is also a place where you put your interpreter which is in this case: /bin/bash.

Here is our first bash shell script example:

#!/bin/bash

# declare STRING variable

STRING="Hello World"

#print variable on a screen

echo $STRING



Navigate to a directory where your hello_world.sh is located and make the file executable:

$ chmod +x hello_world.sh

Now you are ready to execute your first bash script:

./hello_world.sh

Simple Backup bash shell script

#!/bin/bash

tar -czf myhome_directory.tar.gz /home/linuxconfig















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Variables

In this example we declare simple bash variable and print it on the screen ( stdout ) with echo command.

#!/bin/bash

STRING="HELLO WORLD!!!"

echo $STRING

Your backup script and variables:

#!/bin/bash

OF=myhome_directory_$(date +%Y%m%d).tar.gz

tar -czf $OF /home/linuxconfig

Global vs. Local variables

#!/bin/bash

#Define bash global variable

#This variable is global and can be used anywhere in this bash script

VAR="global variable"

function bash {

#Define bash local variable

#This variable is local to bash function only

local VAR="local variable"

echo $VAR

}

echo $VAR

bash

# Note the bash global variable did not change

# "local" is bash reserved word

echo $VAR



Passing arguments to the bash script

#!/bin/bash

# use predefined variables to access passed arguments

#echo arguments to the shell

echo $1 $2 $3 ' -> echo $1 $2 $3'



# We can also store arguments from bash command line in special array

args=("$@")

#echo arguments to the shell

echo ${args[0]} ${args[1]} ${args[2]} ' -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}'



#use $@ to print out all arguments at once

echo $@ ' -> echo $@'



# use $# variable to print out

# number of arguments passed to the bash script

echo Number of arguments passed: $# ' -> echo Number of arguments passed: $#'

/arguments.sh Bash Scripting Tutorial

Executing shell commands with bash

#!/bin/bash

# use backticks " ` ` " to execute shell command

echo `uname -o`

# executing bash command without backticks

echo uname -o

Reading User Input

#!/bin/bash



echo -e "Hi, please type the word: \c "

read word

echo "The word you entered is: $word"

echo -e "Can you please enter two words? "

read word1 word2

echo "Here is your input: \"$word1\" \"$word2\""

echo -e "How do you feel about bash scripting? "

# read command now stores a reply into the default build-in variable $REPLY

read

echo "You said $REPLY, I'm glad to hear that! "

echo -e "What are your favorite colours ? "

# -a makes read command to read into an array

read -a colours

echo "My favorite colours are also ${colours[0]}, ${colours[1]} and ${colours[2]}:-)"

Bash Trap Command

#!/bin/bash

# bash trap command

trap bashtrap INT

# bash clear screen command

clear;

# bash trap function is executed when CTRL-C is pressed:

# bash prints message => Executing bash trap subrutine !

bashtrap()

{

echo "CTRL+C Detected !...executing bash trap !"

}

# for loop from 1/10 to 10/10

for a in `seq 1 10`; do

echo "$a/10 to Exit."

sleep 1;

done

echo "Exit Bash Trap Example!!!"



Arrays

Declare simple bash array

#!/bin/bash

#Declare array with 4 elements

ARRAY=( 'Debian Linux' 'Redhat Linux' Ubuntu Linux )

# get number of elements in the array

ELEMENTS=${#ARRAY[@]}



# echo each element in array

# for loop

for (( i=0;i<$ELEMENTS;i++)); do

echo ${ARRAY[${i}]}

done

Read file into bash array

#!/bin/bash

# Declare array

declare -a ARRAY

# Link filedescriptor 10 with stdin

exec 10<&0

# stdin replaced with a file supplied as a first argument

exec < $1

let count=0



while read LINE; do



ARRAY[$count]=$LINE

((count++))

done



echo Number of elements: ${#ARRAY[@]}

# echo array's content

echo ${ARRAY[@]}

# restore stdin from filedescriptor 10

# and close filedescriptor 10

exec 0<&10 10<&-





Bash script execution with an output:

linuxconfig.org $ cat bash.txt

Bash

Scripting

Tutorial

Guide

linuxconfig.org $ ./bash-script.sh bash.txt

Number of elements: 4

Bash Scripting Tutorial Guide

linuxconfig.org $



Bash if / else / fi statements

Simple Bash if/else statement

Please note the spacing inside the [ and ] brackets! Without the spaces, it won't work!

#!/bin/bash

directory="./BashScripting"



# bash check if directory exists

if [ -d $directory ]; then

echo "Directory exists"

else

echo "Directory does not exists"

fi

Nested if/else

#!/bin/bash



# Declare variable choice and assign value 4

choice=4

# Print to stdout

echo "1. Bash"

echo "2. Scripting"

echo "3. Tutorial"

echo -n "Please choose a word [1,2 or 3]? "

# Loop while the variable choice is equal 4

# bash while loop

while [ $choice -eq 4 ]; do



# read user input

read choice

# bash nested if/else

if [ $choice -eq 1 ] ; then



echo "You have chosen word: Bash"



else



if [ $choice -eq 2 ] ; then

echo "You have chosen word: Scripting"

else



if [ $choice -eq 3 ] ; then

echo "You have chosen word: Tutorial"

else

echo "Please make a choice between 1-3 !"

echo "1. Bash"

echo "2. Scripting"

echo "3. Tutorial"

echo -n "Please choose a word [1,2 or 3]? "

choice=4

fi

fi

fi

done

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Bash Comparisons

Arithmetic Comparisons

-lt < -gt > -le <= -ge >= -eq == -ne !=

#!/bin/bash

# declare integers

NUM1=2

NUM2=2

if [ $NUM1 -eq $NUM2 ]; then

echo "Both Values are equal"

else

echo "Values are NOT equal"

fi

#!/bin/bash

# declare integers

NUM1=2

NUM2=1

if [ $NUM1 -eq $NUM2 ]; then

echo "Both Values are equal"

else

echo "Values are NOT equal"

fi

#!/bin/bash

# declare integers

NUM1=2

NUM2=1

if [ $NUM1 -eq $NUM2 ]; then

echo "Both Values are equal"

elif [ $NUM1 -gt $NUM2 ]; then

echo "NUM1 is greater then NUM2"

else

echo "NUM2 is greater then NUM1"

fi

String Comparisons

= equal != not equal < less then > greater then -n s1 string s1 is not empty -z s1 string s1 is empty

#!/bin/bash

#Declare string S1

S1="Bash"

#Declare string S2

S2="Scripting"

if [ $S1 = $S2 ]; then

echo "Both Strings are equal"

else

echo "Strings are NOT equal"

fi

#!/bin/bash

#Declare string S1

S1="Bash"

#Declare string S2

S2="Bash"

if [ $S1 = $S2 ]; then

echo "Both Strings are equal"

else

echo "Strings are NOT equal"

fi

Bash File Testing

-b filename Block special file -c filename Special character file -d directoryname Check for directory existence -e filename Check for file existence -f filename Check for regular file existence not a directory -G filename Check if file exists and is owned by effective group ID. -g filename true if file exists and is set-group-id. -k filename Sticky bit -L filename Symbolic link -O filename True if file exists and is owned by the effective user id. -r filename Check if file is a readable -S filename Check if file is socket -s filename Check if file is nonzero size -u filename Check if file set-ser-id bit is set -w filename Check if file is writable -x filename Check if file is executable

#!/bin/bash

file="./file"

if [ -e $file ]; then

echo "File exists"

else

echo "File does not exists"

fi

Similarly for example we can use while loop to check if file does not exists. This script will sleep until file does exists. Note bash negator "!" which negates the -e option.

#!/bin/bash



while [ ! -e myfile ]; do

# Sleep until file does exists/is created

sleep 1

done

Loops

Bash for loop

#!/bin/bash



# bash for loop

for f in $( ls /var/ ); do

echo $f

done

Running for loop from bash shell command line:

$ for f in $( ls /var/ ); do echo $f; done

Bash while loop

#!/bin/bash

COUNT=6

# bash while loop

while [ $COUNT -gt 0 ]; do

echo Value of count is: $COUNT

let COUNT=COUNT-1

done

Bash until loop

#!/bin/bash

COUNT=0

# bash until loop

until [ $COUNT -gt 5 ]; do

echo Value of count is: $COUNT

let COUNT=COUNT+1

done

Control bash loop with

Here is a example of while loop controlled by standard input. Until the redirection chain from STDOUT to STDIN to the read command exists the while loop continues.

#!/bin/bash

# This bash script will locate and replace spaces

# in the filenames

DIR="."

# Controlling a loop with bash read command by redirecting STDOUT as

# a STDIN to while loop

# find will not truncate filenames containing spaces

find $DIR -type f | while read file; do

# using POSIX class [:space:] to find space in the filename

if [[ "$file" = *[[:space:]]* ]]; then

# substitute space with "_" character and consequently rename the file

mv "$file" `echo $file | tr ' ' '_'`

fi;

# end of while loop

done

Bash Functions

!/bin/bash

# BASH FUNCTIONS CAN BE DECLARED IN ANY ORDER

function function_B {

echo Function B.

}

function function_A {

echo $1

}

function function_D {

echo Function D.

}

function function_C {

echo $1

}

# FUNCTION CALLS

# Pass parameter to function A

function_A "Function A."

function_B

# Pass parameter to function C

function_C "Function C."

function_D

Bash Select

#!/bin/bash



PS3='Choose one word: '



# bash select

select word in "linux" "bash" "scripting" "tutorial"

do

echo "The word you have selected is: $word"

# Break, otherwise endless loop

break

done



exit 0

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Case statement conditional

#!/bin/bash

echo "What is your preferred programming / scripting language"

echo "1) bash"

echo "2) perl"

echo "3) phyton"

echo "4) c++"

echo "5) I do not know !"

read case;

#simple case bash structure

# note in this case $case is variable and does not have to

# be named case this is just an example

case $case in

1) echo "You selected bash";;

2) echo "You selected perl";;

3) echo "You selected phyton";;

4) echo "You selected c++";;

5) exit

esac

Bash quotes and quotations

Quotations and quotes are important part of bash and bash scripting. Here are some bash quotes and quotations basics.

Escaping Meta characters

Before we start with quotes and quotations we should know something about escaping meta characters. Escaping will suppress a special meaning of meta characters and therefore meta characters will be read by bash literally. To do this we need to use backslash "\" character. Example:

#!/bin/bash



#Declare bash string variable

BASH_VAR="Bash Script"



# echo variable BASH_VAR

echo $BASH_VAR



#when meta character such us "$" is escaped with "\" it will be read literally

echo $BASH_VAR



# backslash has also special meaning and it can be suppressed with yet another "\"

echo "\"

Single quotes

Single quotes in bash will suppress special meaning of every meta characters. Therefore meta characters will be read literally. It is not possible to use another single quote within two single quotes not even if the single quote is escaped by backslash.

#!/bin/bash



#Declare bash string variable

BASH_VAR="Bash Script"



# echo variable BASH_VAR

echo $BASH_VAR



# meta characters special meaning in bash is suppressed when using single quotes

echo '$BASH_VAR "$BASH_VAR"'

Double Quotes

Double quotes in bash will suppress special meaning of every meta characters except "$", "\" and "`". Any other meta characters will be read literally. It is also possible to use single quote within double quotes. If we need to use double quotes within double quotes bash can read them literally when escaping them with "\". Example:

#!/bin/bash



#Declare bash string variable

BASH_VAR="Bash Script"



# echo variable BASH_VAR

echo $BASH_VAR



# meta characters and its special meaning in bash is

# suppressed when using double quotes except "$", "\" and "`"



echo "It's $BASH_VAR and \"$BASH_VAR\" using backticks: `date`"

Bash quoting with ANSI-C style

There is also another type of quoting and that is ANSI-C. In this type of quoting characters escaped with "\" will gain special meaning according to the ANSI-C standard.

\a alert (bell) \b backspace \e an escape character \f form feed

newline \r carriage return \t horizontal tab \v vertical tab \\ backslash \` single quote

nn octal value of characters ( see [http://www.asciitable.com/ ASCII table] ) \xnn hexadecimal value of characters ( see [http://www.asciitable.com/ ASCII table] )

The syntax fo ansi-c bash quoting is: $'' . Here is an example:

#!/bin/bash



# as a example we have used

as a new line, \x40 is hex value for @

# and is octal value for .

echo $'web: www.linuxconfig.org

email: web\x40linuxconfigorg'

Arithmetic Operations

Bash Addition Calculator Example

#!/bin/bash



let RESULT1=$1+$2

echo $1+$2=$RESULT1 ' -> # let RESULT1=$1+$2'

declare -i RESULT2

RESULT2=$1+$2

echo $1+$2=$RESULT2 ' -> # declare -i RESULT2; RESULT2=$1+$2'

echo $1+$2=$(($1 + $2)) ' -> # $(($1 + $2))'

Bash Arithmetics

#!/bin/bash



echo '### let ###'

# bash addition

let ADDITION=3+5

echo "3 + 5 =" $ADDITION



# bash subtraction

let SUBTRACTION=7-8

echo "7 - 8 =" $SUBTRACTION



# bash multiplication

let MULTIPLICATION=5*8

echo "5 * 8 =" $MULTIPLICATION



# bash division

let DIVISION=4/2

echo "4 / 2 =" $DIVISION



# bash modulus

let MODULUS=9%4

echo "9 % 4 =" $MODULUS



# bash power of two

let POWEROFTWO=2**2

echo "2 ^ 2 =" $POWEROFTWO





echo '### Bash Arithmetic Expansion ###'

# There are two formats for arithmetic expansion: $[ expression ]

# and $(( expression #)) its your choice which you use



echo 4 + 5 = $((4 + 5))

echo 7 - 7 = $[ 7 - 7 ]

echo 4 x 6 = $((3 * 2))

echo 6 / 3 = $((6 / 3))

echo 8 % 7 = $((8 % 7))

echo 2 ^ 8 = $[ 2 ** 8 ]





echo '### Declare ###'



echo -e "Please enter two numbers \c"

# read user input

read num1 num2

declare -i result

result=$num1+$num2

echo "Result is:$result "



# bash convert binary number 10001

result=2#10001

echo $result



# bash convert octal number 16

result=8#16

echo $result



# bash convert hex number 0xE6A

result=16#E6A

echo $result

Round floating point number

#!/bin/bash

# get floating point number

floating_point_number=3.3446

echo $floating_point_number

# round floating point number with bash

for bash_rounded_number in $(printf %.0f $floating_point_number); do

echo "Rounded number with bash:" $bash_rounded_number

done

Bash floating point calculations

#!/bin/bash

# Simple linux bash calculator

echo "Enter input:"

read userinput

echo "Result with 2 digits after decimal point:"

echo "scale=2; ${userinput}" | bc

echo "Result with 10 digits after decimal point:"

echo "scale=10; ${userinput}" | bc

echo "Result as rounded integer:"

echo $userinput | bc

Redirections

STDOUT from bash script to STDERR

#!/bin/bash



echo "Redirect this STDOUT to STDERR" 1>&2

To prove that STDOUT is redirected to STDERR we can redirect script's output to file:



STDERR from bash script to STDOUT

#!/bin/bash



cat $1 2>&1

To prove that STDERR is redirected to STDOUT we can redirect script's output to file:



stdout to screen

The simple way to redirect a standard output ( stdout ) is to simply use any command, because by default stdout is automatically redirected to screen. First create a file "file1":

$ touch file1 $ ls file1 file1

As you can see from the example above execution of ls command produces STDOUT which by default is redirected to screen.

stdout to file

The override the default behavior of STDOUT we can use ">" to redirect this output to file:

$ ls file1 > STDOUT $ cat STDOUT file1

stderr to file

By default STDERR is displayed on the screen:

$ ls file1 STDOUT $ ls file2 ls: cannot access file2: No such file or directory

In the following example we will redirect the standard error ( stderr ) to a file and stdout to a screen as default. Please note that STDOUT is displayed on the screen, however STDERR is redirected to a file called STDERR:

$ ls file1 STDOUT $ ls file1 file2 2> STDERR file1 $ cat STDERR ls: cannot access file2: No such file or directory

stdout to stderr

It is also possible to redirect STDOUT and STDERR to the same file. In the next example we will redirect STDOUT to the same descriptor as STDERR. Both STDOUT and STDERR will be redirected to file "STDERR_STDOUT".

$ ls file1 STDERR STDOUT $ ls file1 file2 2> STDERR_STDOUT 1>&2 $ cat STDERR_STDOUT ls: cannot access file2: No such file or directory file1

File STDERR_STDOUT now contains STDOUT and STDERR.

stderr to stdout

The above example can be reversed by redirecting STDERR to the same descriptor as SDTOUT:

$ ls file1 STDERR STDOUT $ ls file1 file2 > STDERR_STDOUT 2>&1 $ cat STDERR_STDOUT ls: cannot access file2: No such file or directory file1

stderr and stdout to file

Previous two examples redirected both STDOUT and STDERR to a file. Another way to achieve the same effect is illustrated below:

$ ls file1 STDERR STDOUT $ ls file1 file2 &> STDERR_STDOUT $ cat STDERR_STDOUT ls: cannot access file2: No such file or directory file1

or

ls file1 file2 >& STDERR_STDOUT $ cat STDERR_STDOUT ls: cannot access file2: No such file or directory file1