Master Bash: 40 Essential Shell Script Techniques for Linux

1. Hello World

Programmers often start with a Hello World program to learn a new language. Create a hello-world.sh file with the following content and make it executable.

#!/bin/bash
echo "Hello World"

Make the script executable with chmod a+x hello-world.sh and run it using bash hello-world.sh or ./hello-world.sh. It prints the string passed to echo.

2. Using echo

The echo command prints information in Bash. It supports options such as -n (no newline) and -e (interpret escape sequences). Example script:

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

3. Comments

Comments improve documentation. 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 ( #!/bin/bash) is a shebang, not a comment.

4. Multi‑line comments

Multi‑line comments can be simulated with a : command and a quoted block:

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

5. While loop

The while construct repeats commands while a condition is true.

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

6. For loop

The for loop iterates over a range or list.

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

7. Reading user input

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

8. If statement

if CONDITION
then
  STATEMENTS
fi

The statements execute only when the condition is true.

9. If‑else

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

10. AND operator

Use && to require multiple conditions.

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

11. OR operator

Use || to succeed when any condition is true.

#!/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. Elif

#!/bin/bash
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 statement

#!/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"

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. Substring extraction

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

18. Using cut for extraction

#!/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. 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 value

#!/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 a directory

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

24. Create directory only if it does not exist

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

25. Reading a file

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

26. Deleting a file

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

27. Appending to a 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 if a file exists

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

29. Send email from a 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 extension

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

35. Count files and 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. 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 for 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 a 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"