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MUHAMMAD ARBAB ANJUM
MUHAMMAD ARBAB ANJUM

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Shell Scripting - Zero to Hero

#devops #linux #aws #bash

Shell scripting consists of set of commands to perform a task.

Different types of shells:

  1. bash
  2. sh
  3. ksh
  4. tsh
  5. fish
  6. zsh (Extended version of bash with more advanced features)

What is my shell type?

echo $0
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How to check supported shells?

cat /etc/shells
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What is shebang?

It specifies which interpreter should be used to execute the script.

#!/bin/bash
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Basic shell script

#!/bin/bash echo "hello world!"
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How to execute script?

Make sure script have execute permission rwx. 1- ./file.sh 2- path/file.sh 3- bash file.sh
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How to add comments?

1- single line comment #comment 2- Multiline comment <<comment ---- Your Comment Here ---- comment
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Variables:

How to use variables?

#!/bin/bash person="Arbab Anjum" hostName=$(hostname) readonly role="superadmin" echo $person is using $hostName and role is $role ------ > Output: Arbab Anjum is using ubuntu and role is superadmin
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Array:

How to define and get value of an array?

Set Value: myArray = (1 2 3 Hello "World"); Get Value: echo "${myArray[0]}" echo "${myArray[1]}"
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How to find length of an array?

echo "${#myArray[*]}"
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How to get specific values?

echo "${myArray[*]:1}" echo "${myArray[*]:1:2}" // from index 1, get 2 values
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How to update an array?

myArray+=(New 7 8)
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How to store the key value pairs?

declare -A myKeyValArray myKeyValArray=([name]="Arbab Anjum",[city]="London") echo "Name is ${myKeyValArray[name]}"
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String Operations: 

myVar="My name is Muhammad Arbab Anjum" echo "Length: ${#myVar}" echo "Uppercase: ${myVar^^}" echo "Length: ${myVar,,}" echo "Replace (name) with (Name): ${myVar/name/Name}" echo "Slice: ${myVar:6:13}"
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User Interaction: 

echo "What is your name? " read name echo "Name is $name"
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read in same line:

read -p "What is your name: " name echo "My name is $name"
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Arithmetic Operations 

x=10 y=2 let mul=$x*$y # use 'let' to tell it's a mathematical operation echo "Multiplication: $mul" echo "Addition: $(($x+$y))"
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Conditional Statement

If Else

Example: 1

if [[ $marks -gt 40 ]] then echo "Pass!" else echo "Fail!" fi
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Example: 2

if [[ $marks -gt 40 ]] then echo "Above 40!" elif [[ $marks -eq 30 ]] echo "Fail!" then echo "Equal 30!" else echo "Less than 30!" fi
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Example: 3 || Logical operation

if [[ $age -gt 18 ]] && [[ $country -eq "London" ]] then echo "Pass!" else echo "Fail!" fi
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Equal > -eq/== Greater than or equal to > -ge Less than or equal to > -le Not equal to > -ne/!= Greater than > -gt Less than > -lt
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Switch Case

echo "Choose an option below:" echo "a = To see current data" echo "b = list all files in current directory" read choice case $choice in a) date;; b) ls;; *) echo "Not a valid input" esac
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For Loop

Example: 1

for i in 1 2 3 4 5 do echo "Number: $i" done
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Example: 2

for i in {1..10} do echo "Number: $i" done
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Example: 3 - Getting value from file name.txt

FILE='/home/arbab/name.txt' for name in $(cat $FILE) do echo "Name is: $name" done
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Example: 4 - Getting value from file name.txt

myArray=(1 2 3 4 5 6 7) for (( i=0; i<${#myArray[*]}; i++ )) do echo "Value is: ${myArray[$i]}" done
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while Loop

Code keeps executing till the statement is true

count=0 num=10 while [ $count -le $num ] do echo "Number: $count" let count++ done
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Until loop

Code keeps executing till the statement is false

a=10 until[ $a -eq 1 ] do echo "Number: $a" let a-- done
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Read content from CSV file

while IFS="," read f1 f2 f3 do echo "$f1" echo "$f2" echo "$f3" done < file_name.csv
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Function

Example 1:

function myFun{ echo "--------" echo "Welcome" echo "--------" } # Calling Function myFunc
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Example 2:

myFun(){ echo "--------" echo "Welcome" echo "--------" } # Calling Function myFun
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Example 2: With arguments

#!/bin/bash myFun(){ local num1=$1 # $1-> First argument: 5 local num2=$2 # $2-> Second argument: 2 let result=$num1+$num2 echo "--------" echo "Sum: $result" echo "--------" } # Calling Function myFun 5 2
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Example 2: Send arguments while executing file

./file.sh arbab anjum file.sh: echo $1 # result: arbab echo $@ # result: arbab anjum -> $@ means all arguments echo $# # result: 2 -> number of arguments
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Useful Concept

  • break - To stop the loop
  • continue - To stop current iteration of loop and start from the next iteration
  • sleep - To create delay between two execution e.g; sleep 1m/1s
  • exit - To stop script to a point. e.g; exit 1
  • $? - exit status - gives you status of previous command if that was successful - status 0 means success
  • basename - strip directory info and only give filename
  • dirname - strip the filename and gives dir path
  • realpath - gives you full path for a file
  • RANDOM - generate random number - echo $RANDOM
  • UID - User ID of the logged in user - echo $UID - UID of root user is 0

Debugging: 

Add `set -x` in the script to enable debugging
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Scripts:

Check if file exist or not

#!/bin/bash FILPATH=/home/test/file.csv if[[ -f $FILEPATH ]] then echo "FILE EXIST" else echo "FILE DON't EXIST" fi
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If you want to exit script when a command fail

set -e # use this echo "Hello" cd /root #fail and skip next execution pwd
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Run script in background

nohup ./script.sh
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Run Script at specific time using At command 

at 12:09 AM # Run and hit enter it will ask what to do bash ./script.sh atq -> to check scheduled job atrm <id> -> to remove a scheduled job
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Crontab - Script automation 

crontab -l # view existing cronjobs crontab -e # Create New Format: * * * * * cd /home/arbab/ && ./runfile.sh Stars from Right To Left * -> Day of Week (0-6) Sunday=0 * -> month 1-12 * -> Day of month (1-31) * -> hour 0-23 * -> minute 0-59 https://crontab.guru
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Mini Projects

Monitoring free RAM Space

#!/bin/bash FREE_SPACE=$(free -mt | grep "Total" | awk '{print $4}') TH=500 if [[ $FREE_SPACE -lt $TH ]] then echo "Normal Memory!" else echo "Running out of memory!" fi #--------- Comments --------- #free -mt -> show total free memory #grep "Total" -> pick the line containing word total #awk '{print $4}' print 4th column value
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Monitoring free disk Space and send an alert email

#!/bin/bash FREE_DISK_SPACE=$(df -H | egrep -v "Filesystem|tmpfs" | grep "sda2" | awk '{print $5}' | tr -d %) TH=90 TO=user@gmail.com if [[ $FREE_DISK_SPACE -lt $TH ]] then echo "Disk has enough space!" | mail -s "Disk Space Alert" $TO else echo "Low disk space!" fi #--------- Comments --------- #configure postfix to send emails #df -h -> show all disk usage #egrep -v "Filesystem|tmpfs" -> Remove Filsystem and tmps line from output #grep "sda2" -> search sda2 #awk '{print $5}' print 5th column value #tr -d % -> truncate/delete % from output
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Move files having size more than 20MB to achieve folder

!/bin/bash # Version: 1 # Date: 19 April 2025 #Variables BASE=/home/arbab/myFiles DAYS=10 DEPTH=1 RUN=0 #check if the directory is present or not if [[ ! -d $BASE ]] then echo "Dir doesn't exist: $BASE" exit 1 fi #create 'archive' folder if not present if [[ ! -d $BASE/archive ]] then mkdir archive fi #Find list of files largest than 20MB for i in `find $BASE -maxdepth $DEPTH -type f -size +20M` do if [[ $RUN -eq 0 ]] then echo "[$(date "+%y-%m-%d %H:%M:%S")] archiving $i ==> $BASE/archive" gzip $i || exit 1 mv $i.gz $BASE/archive || exit 1 fi done ## || runs the command on right side when left side result is false
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Automate user account creation

#!/bin/bash # script should execute with sudo/root access. if [[ "${UID}" -ne 0 ]] then echo 'Please run with root or sudo user' exit 1 fi # user should provide atleast one argument as username else guide him if [[ "${#}" -lt 1 ]] then echo 'Usage: ${0} USER_NAME [COMMENT]...' exit 1 fi # ${0} -> Tells the name of script # store first argument as username USER_NAME="{$1}" # All other arguments as comment shift COMMENT="{$@}" # create a password RANDOM_PASS=$(date +%s%N) # seconds and nano seconds # create a user useradd -c "${COMMENT}" -m $USER_NAME # -c -> comment # -m -> Make Dir # Check if user created successfully or not if [[ $? -ne 0 ]] then echo "Failed to create user" exit 1 fi # set password for user echo $PASSWORD | passwd --stdin $USER_NAME # Check if password is successfully set or not if [[ $? -ne 0 ]] then echo "Failed to create user" exit 1 fi # force password change on first login passwd -e $USER_NAME # Display username password and hostname echo "Username: $USER_NAME" echo "Username: $PASSWORD" echo "Hostname: $(hostname)"
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