The Bash shell is one of the most powerful components of a Linux system, as well as one of the most compelling reasons to use Linux. Users can interact with Bash through the command line, and write scripts to automate tasks. Although this may sound intimidating to beginning users, it is not hard to get started with Bash scripting.
What is Bash Script?
A Bash script is a text file containing a series of commands written for the Bash shell, used to automate tasks, execute multiple commands, or create simple programs on Unix-based systems like Linux and macOS. That means that if you regularly use the Linux command line, then Bash scripts are pretty simple. Anything that exists in a Bash script can also be executed in the command line. In this tutorial, we will take you through various examples of Bash scripts to show you what it is capable of, and how to utilize different aspects of Bash.
To go from beginner to expert in Bash scripting, you can start by copying some of our examples on to your own system. Use the scripts in this Bash scripting tutorial to modify them, execute them, and keep practicing. We will help you through all the essential aspects of Bash. Let’s begin!
Privileged access to your Linux system as root or via the sudo command.
Conventions
# – requires given linux commands to be executed with root privileges either directly as a root user or by use of sudo command $ – requires given linux commands to be executed as a regular non-privileged user
Hello World Bash Shell Script – Bash Scripting Tutorial
First you need to find out where is your Bash interpreter located. Enter the following into your command line:
$ which bash
/bin/bash
This command reveals that the Bash shell is stored in /bin/bash. This will come into play momentarily.
The next thing you need to do is open our favorite text editor and create a file called hello_world.sh. We will use nano for this step.
$ nano hello_world.sh
Copy and paste the following lines into the new file and save it:
#!/bin/bash
# declare STRING variable
STRING="Hello World"
# print variable on a screen
echo $STRING
WHAT IS SHEBANG
Every bash shell script in this tutorial starts with a 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.
Navigate to the directory where your hello_world.sh bash script is located and make the file executable:
$ chmod +x hello_world.sh
Now you are ready to execute your first bash script:
When writing a Bash script, you are basically putting into it the same commands that you could execute directly on the command line. A perfect example of this is the following script:
#!/bin/bash
tar -czf myhome_directory.tar.gz /home/linuxconfig
This will create a compressed tar file of the home directory for user linuxconfig. The tar command we use in the script could easily just be executed directly on the command line.
So, what’s the advantage of the script? Well, it allows us to quickly call this command without having to remember it or type it every time. We could also easily expand the script later on to be more complex.
Variables in Bash scripts
In this example we declare simple bash variable $STRING and print it on the screen (stdout) with echo command.
#!/bin/bash
STRING="HELLO WORLD!!!"
echo $STRING
The result when we execute the script:
$ ./hello_world.sh
HELLO WORLD!!!
Circling back to our backup script example, let’s use a variable to name our backup file and put a time stamp in the file name by using the date command.
#!/bin/bash
OF=myhome_directory_$(date +%Y%m%d).tar.gz
tar -czf $OF /home/linuxconfig
The result of executing the script:
$ ./backup.sh
$ ls
myhome_directory_$(date +20220209).tar.gz
Now, when we see the file, we can quickly determine that the backup was performed on February 9, 2022.
Global vs. Local variables
In Bash scripting, a global variable is a variable that can be used anywhere inside the script. A local variable will only be used within the function that it is declared in. Check out the example below where we declare both a global variable and local variable. We’ve made some comments in the script to make it a little easier to digest.
#!/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
The result of executing this script:
$ ./variables.sh
global variable
local variable
global variable
Passing arguments to the bash script
When executing a Bash script, it is possible to pass arguments to it in your command. As you can see in the example below, there are multiple ways that a Bash script can interact with the arguments we provide.
#!/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: $#'
Let’s try executing this script and providing three arguments.
$ ./arguments.sh Bash Scripting Tutorial
The results when we execute this script:
Bash Scripting Tutorial -> echo $1 $2 $3
Bash Scripting Tutorial -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}
Bash Scripting Tutorial -> echo $@
Number of arguments passed: 3 -> echo Number of arguments passed: $#
Executing shell commands with bash
The best way to execute a separate shell command inside of a Bash script is by creating a new subshell through the $( ) syntax. Check the example below where we echo the result of running the uname -o command.
#!/bin/bash
# use a subshell $() to execute shell command
echo $(uname -o)
# executing bash command without subshell
echo uname -o
Notice that in the final line of our script, we do not execute the uname command within a subshell, therefore the text is taken literally and output as such.
We can use the read command to read input from the user. This allows a user to interact with a Bash script and help dictate the way it proceeds. Here’s an example:
#!/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]}:-)"
Our Bash script asks multiple questions and then is able to repeat the information back to us through variables and arrays:
$ ./read.sh
Hi, please type the word: Linuxconfig.org
The word you entered is: Linuxconfig.org
Can you please enter two words?
Debian Linux
Here is your input: "Debian" "Linux"
How do you feel about bash scripting?
good
You said good, I'm glad to hear that!
What are your favorite colours ?
blue green black
My favorite colours are also blue, green and black:-)
Bash Trap Command
The trap command can be used in Bash scripts to catch signals sent to the script and then execute a subroutine when they occur. The script below will detect a Ctrl + C interrupt.
#!/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!!!"
In the output below you can see that we try to Ctrl + C two times but the script continues to execute until it reaches its normal exit bash point.
$ ./trap.sh
1/10 to Exit.
2/10 to Exit.
^CCTRL+C Detected !...executing bash trap !
3/10 to Exit.
4/10 to Exit.
5/10 to Exit.
6/10 to Exit.
7/10 to Exit.
^CCTRL+C Detected !...executing bash trap !
8/10 to Exit.
9/10 to Exit.
10/10 to Exit.
Exit Bash Trap Example!!!
Arrays – Bash script tutorial
Bash is capable of storing values in arrays. Check the sections below for two different examples.
Declare simple bash array
This example declares an array with four elements.
#!/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
Executing the script will output the elements of our array:
$ ./arrays.sh
Debian Linux
Redhat Linux
Ubuntu
Linux
Read file into bash array
Rather than filling out all of the elements of our array in the Bash script itself, we can program our script to read input and put it into an 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<&-
Now let’s execute the script and store four elements in the array by using a file’s contents for input.
$ cat bash.txt
Bash
Scripting
Tutorial
Guide
$ ./bash-script.sh bash.txt
Number of elements: 4
Bash Scripting Tutorial Guide
Bash if / else / fi statements
Here is a simple bash if statement that check to see if a directory exists or not. To bash check if directory exists, we use the following code. Depending on the result, it will do one of two things. 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 exist"
fi
The output:
$ ./bash_if_else.sh
Directory does not exist
$ mkdir BashScripting
$ ./bash_if_else.sh
Directory exists
Nested if/else
It is possible to place an if statement inside yet another if statement. This is called nesting. Using bash nested if statements, scripts can get a bit complex depending on how many if statements deep it is.
#!/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
Output from the script:
$ ./nested_if_else.sh
1. Bash
2. Scripting
3. Tutorial
Please choose a word [1,2 or 3]? 5
Please make a choice between 1-3 !
1. Bash
2. Scripting
3. Tutorial
Please choose a word [1,2 or 3]? 2
You have chosen word: Scripting
This example shows a bash while loop that continues executing until a valid choice is made.
Bash Comparisons
Bash can compare two or more values, either integers or strings, to determine if they are equal to each other, or one is greater than the other, etc.
Arithmetic Comparisons
-lt
<
-gt
>
-le
<=
-ge
>=
-eq
==
-ne
!=
Now let’s use these operators in some examples.
#!/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
The result:
$ ./statement.sh
Both values are equal
Let’s try changing one of the numbers.
#!/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
The result:
$ ./statement.sh
Values are NOT equal
Let’s add a little more complexity by including an elif statement and determing which number is larger.
#!/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 than NUM2"
else
echo "NUM2 is greater than NUM1"
fi
The result:
$ ./statement.sh
NUM1 is greater than NUM2
String Comparisons
=
equal
!=
not equal
<
less then
>
greater then
-n s1
string s1 is not empty
-z s1
string s1 is empty
Let’s try comparing two strings to see if they are equal.
#!/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
The result:
$ ./statement.sh
Strings are NOT equal
And again with both string matching.
#!/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
The result:
$ ./statement.sh
Both Strings are equal
Bash File Testing
In Bash, we can test to see different characteristics about a file or directory. See the table below for a full list.
-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.
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
The following script will check to see if a file exists or not.
#!/bin/bash
file="./file"
if [ -e $file ]; then
echo "File exists"
else
echo "File does not exist"
fi
The result:
$ ./filetesting.sh
File does not exist
$ touch file
$ ./filetesting.sh
File exists
Similarly for example we can use while loop to check if file does not exist. This script will sleep until file does exist. 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
There are multiple types of loops that can be used in Bash, including for, while, and until. See some of the examples below to learn how to use.
Bash for loop
Using a bash for loop, this script will list every file or directory it finds inside the /var/ directory.
#!/bin/bash
# bash for loop
for f in $( ls /var/ ); do
echo $f
done
A for loop can also be run directly from the command line, no need for a script:
$ for f in $( ls /var/ ); do echo $f; done
The result:
$ ./for_loop.sh
backups
cache
crash
lib
local
lock
log
mail
metrics
opt
run
snap
spool
tmp
Bash while loop
This while loop will continue to loop until our variable reaches a value of 0 or less.
#!/bin/bash
COUNT=6
# bash while loop
while [ $COUNT -gt 0 ]; do
echo Value of count is: $COUNT
let COUNT=COUNT-1
done
The result:
$ ./while_loop.sh
Value of count is: 6
Value of count is: 5
Value of count is: 4
Value of count is: 3
Value of count is: 2
Value of count is: 1
Bash until loop
An until loop works similarly to while.
#!/bin/bash
COUNT=0
# bash until loop
until [ $COUNT -gt 5 ]; do
echo Value of count is: $COUNT
let COUNT=COUNT+1
done
The result:
$ ./until_loop.sh
Value of count is: 0
Value of count is: 1
Value of count is: 2
Value of count is: 3
Value of count is: 4
Value of count is: 5
Control bash loop with input
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
This example shows how to declare a function and call back to it later in the script.
!/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
The result:
$ ./functions.sh
Function A.
Function B.
Function C.
Function D.
Bash Select
The bash select command allows us to prompt the user to make a selection.
#!/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
The result:
$ ./select.sh
1) linux
2) bash
3) scripting
4) tutorial
Choose one word: 2
The word you have selected is: bash
Case statement conditional
The case statement makes it easy to have many different possibilities, whereas an if statement can get lengthy very quickly if you have more than a few possibilities to account for. Here’s a case bash example:
#!/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
The result:
$ ./case.sh
What is your preferred programming / scripting language
1) bash
2) perl
3) phyton
4) c++
5) I do not know !
3
You selected phyton
Bash quotes and quotations
Bash quotes and quotations 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 "\\"
Here’s what it looks like when we execute the script:
$ ./escape_meta.sh
Bash Script
$BASH_VAR
\
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 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`"
The result:
$ ./double_quotes.sh
Bash Script
It's Bash Script and "Bash Script" using backticks: Thu 10 Feb 2022 10:24:15 PM EST
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
\n
newline
\r
carriage return
\t
horizontal tab
\v
vertical tab
\\
backslash
\`
single quote
\nnn
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 for ansi-c bash quoting is: $' ' . Here is an example:
#!/bin/bash
# as a example we have used \n as a new line, \x40 is hex value for @
# and \56 is octal value for .
echo $'web: www.linuxconfig.org\nemail: web\x40linuxconfig\56org'
#!/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
The result:
$ ./round.sh
3.3446
Rounded number with bash: 3
Bash floating point calculations
Using the bash calculatorbc to perform 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
The result:
$ ./simple_bash_calc.sh
Enter input:
10/3.4
Result with 2 digits after decimal point:
2.94
Result with 10 digits after decimal point:
2.9411764705
Result as rounded integer:
2
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
To 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
Closing Thoughts
In this Bash scripting tutorial, we learned how to get started with Bash scripting by learning all the basics of the most common aspects, such as loops, arithmetic, comparisons, etc. This guide has served as an introduction to Bash scripting concepts, and given you some insight into how the Bash shell works. By adapting our examples for your own needs and expanding them as needed, you will be well on your way to mastering Bash scripting.
