Master Bash: 40 Essential Shell Scripts Every Linux User Should Know

Historically, the shell has been the native command-line interpreter for Unix-like systems and remains a core feature of Linux, offering powerful shells such as Bash, Zsh, Tcsh, and Ksh, with programmability as a standout trait.

1.Hello World

Programmers often start learning a new language with a Hello World program that prints "Hello World" to standard output. Create a hello-world.sh file with the following content:

#!/bin/bash
 echo "Hello World"

Make the file executable: $ chmod a+x hello-world.sh Run it with either of these commands:

$ bash hello-world.sh
$ ./hello-world.sh

The script prints the echoed string.

2.Using echo to print

The echo command prints information in Bash, similar to C's printf. Create an echo.sh file:

#!/bin/bash
 echo "Printing text"
 echo -n "Printing text without newline"
 echo -e "
Removing\t special\t characters
"

Run the script to see the effects of the -e option.

3.Using comments

Comments improve documentation and are required for high-quality codebases. Use # to comment a line. Example:

#!/bin/bash
# Adding two values
((sum=25+35))
# Print the result
 echo $sum

The script outputs 60; the first line is a shebang.

4.Multi-line comments

Multi-line comments can be added using a colon and a quoted block:

#!/bin/bash
: '
This script calculates
the square of 5.
'
((area=5*5))
 echo $area

The script prints the substring.

5.While loop

The while construct repeats commands. Example while.sh:

#!/bin/bash
i=0
while [ $i -le 2 ]
do
 echo Number: $i
 ((i++))
done

Loop syntax:

while [ condition ]
do
 commands
... 
done

6.For loop

The for loop iterates efficiently:

#!/bin/bash
for (( counter=1; counter<=10; counter++ ))
do
 echo -n "$counter "
done
 printf "
"

7.Receiving user input

#!/bin/bash
 echo -n "Enter Something:"
 read something
 echo "You Entered: $something"

8.If statement

if CONDITION
then
 STATEMENTS
fi

Example:

#!/bin/bash
 echo -n "Enter a number:"
 read num
 if [[ $num -gt 10 ]]
 then
 echo "Number is greater than 10."
 fi

9.If-else control

#!/bin/bash
 read n
 if [ $n -lt 10 ];
 then
 echo "It is a one digit number"
 else
 echo "It is a two digit number"
 fi

10.Using AND operator

#!/bin/bash
 echo -n "Enter Number:"
 read num
 if [[ ( $num -lt 10 ) && ( $num%2 -eq 0 ) ]]; then
 echo "EvenNumber"
 else
 echo "Odd Number"
 fi

11.Using OR operator

#!/bin/bash
 echo -n "Enter any number:"
 read n
 if [[ ( $n -eq 15 || $n -eq 45 ) ]]; then
 echo "You won"
 else
 echo "You lost!"
 fi

12.Using elif

#!/bin/bash
 echo -n "Enter a number: "
 read num
 if [[ $num -gt 10 ]];
 then
 echo "Number is greater than 10."
 elif [[ $num -eq 10 ]];
 then
 echo "Number is equal to 10."
 else
 echo "Number is less than 10."
 fi

13.Case condition

#!/bin/bash
 echo -n "Enter a number: "
 read num
 case $num in
 100)
 echo "Hundred!!" ;;
 200)
 echo "Double Hundred!!" ;;
 *)
 echo "Neither 100 nor 200" ;;
 esac

14.Command-line arguments

#!/bin/bash
 echo "Total arguments : $#"
 echo "First Argument = $1"
 echo "Second Argument = $2"

Run with $ ./test.sh Hey Howdy.

15.Named parameters

#!/bin/bash
 for arg in "$@"
 do
 index=$(echo $arg | cut -f1 -d=)
 val=$(echo $arg | cut -f2 -d=)
 case $index in
 X) x=$val;;
 Y) y=$val;;
 esac
done
 ((result=x+y))
 echo "X+Y=$result"

16.String concatenation

#!/bin/bash
string1="Ubuntu"
string2="Pit"
string=$string1$string2
 echo "$string is a great resource for Linux beginners."

17.String slicing

#!/bin/bash
Str="Learn Bash Commands from UbuntuPit"
subStr=${Str:0:20}
 echo $subStr

18.Cut for slicing

#!/bin/bash
Str="Learn Bash Commands from UbuntuPit"
subStr=$(echo $Str| cut -d ' ' -f 1-3)
 echo $subStr

19.Adding two values

#!/bin/bash
 echo -n "Enter first number:"
 read x
 echo -n "Enter second number:"
 read y
 (( sum=x+y ))
 echo "The result of addition=$sum"

20.Adding multiple values

#!/bin/bash
sum=0
for (( counter=1; counter<5; counter++ ))
do
 echo -n "Enter Your Number:"
 read n
 (( sum+=n ))
done
 printf "
"
 echo "Result is: $sum"

21.Bash functions

#!/bin/bash
function Add(){
 echo -n "Enter a Number: "
 read x
 echo -n "Enter another Number: "
 read y
 echo "Addition is: $(( x+y ))"
}
Add

22.Functions with return values

#!/bin/bash
function Greet(){
 str="Hello $name, what brings you to UbuntuPit.com?"
 echo $str
}
 echo "-> what's your name?"
 read name
 val=$(Greet)
 echo -e "-> $val"

23.Creating directories

#!/bin/bash
 echo -n "Enter directory name ->"
 read newdir
 cmd="mkdir $newdir"
 eval $cmd

24.Create directory if not exists

#!/bin/bash
 echo -n "Enter directory name ->"
 read dir
 if [ -d "$dir" ];
 then
 echo "Directory exists"
 else
 `mkdir $dir`
 echo "Directory created"
 fi

25.Read file

#!/bin/bash
file='editors.txt'
while read line; do
 echo $line
done < $file

26.Delete file

#!/bin/bash
 echo -n "Enter filename ->"
 read name
 rm -i $name

27.Append to file

#!/bin/bash
 echo "Before appending the file"
 cat editors.txt
 echo "6. NotePad++" >> editors.txt
 echo "After appending the file"
 cat editors.txt

28.Test file existence

#!/bin/bash
filename=$1
if [ -f "$filename" ]; then
 echo "File exists"
else
 echo "File does not exist"
fi

29.Send email from script

#!/bin/bash
recipient="[email protected]"
subject="Greetings"
message="Welcome to UbuntuPit"
`mail -s $subject $recipient <<< $message`

30.Parse date and time

#!/bin/bash
year=`date +%Y`
month=`date +%m`
day=`date +%d`
hour=`date +%H`
minute=`date +%M`
second=`date +%S`
 echo `date`
 echo "Current Date is: $day-$month-$year"
 echo "Current Time is: $hour:$minute:$second"

31.sleep command

#!/bin/bash
 echo "How long to wait?"
 read time
 sleep $time
 echo "Waited for $time seconds!"

32.wait command

#!/bin/bash
 echo "Testing wait command"
 sleep 5 &
 pid=$!
 kill $pid
 wait $pid
 echo $pid was terminated.

33.Show most recently updated file

ls -lrt | grep ^- | awk 'END{print $NF}'

34.Add batch extensions

#!/bin/bash
dir=$1
for file in `ls $1/*`
do
 mv $file $file.UP
done

35.Count files or directories

#!/bin/bash
if [ -d "$@" ]; then
 echo "Files found: $(find "$@" -type f | wc -l)"
 echo "Folders found: $(find "$@" -type d | wc -l)"
else
 echo "[ERROR] Please retry with another folder."
 exit 1
fi

36.Clean log files

#!/bin/bash
LOG_DIR=/var/log
cd $LOG_DIR
cat /dev/null > messages
cat /dev/null > wtmp
 echo "Logs cleaned up."

37.Bash backup script

#!/bin/bash
BACKUPFILE=backup-$(date +%m-%d-%Y)
archive=${1:-$BACKUPFILE}
find . -mtime -1 -type f -print0 | xargs -0 tar rvf "$archive.tar"
 echo "Directory $PWD backed up in archive file \"$archive.tar.gz\"."
 exit 0

38.Check if root user

#!/bin/bash
ROOT_UID=0
if [ "$UID" -eq "$ROOT_UID" ]
then
 echo "You are root."
else
 echo "You are not root"
fi
 exit 0

39.Remove duplicate lines from file

#! /bin/sh
 echo -n "Enter Filename-> "
 read filename
 if [ -f "$filename" ]; then
 sort $filename | uniq | tee sorted.txt
 else
 echo "No $filename in $pwd...try again"
 fi
 exit 0

40.System maintenance

#!/bin/bash
 echo -e "
$(date "+%d-%m-%Y --- %T") --- Starting work
"
apt-get update
apt-get -y upgrade
apt-get -y autoremove
apt-get autoclean
 echo -e "
$(date "+%T") \t Script Terminated"