Shell Scripting Variables – Complete Beginner Tutorial
Learn how to declare, access, and use variables in Bash with real-world examples.
5.1) Variables
Create a Variable
name="Muni"
echo $name
Environment Variable
export PATH=/usr/local/bin:$PATH
Read Input
echo "Enter your name:"
read username
echo "Welcome $username"
Command Substitution
today=$(date)
echo "Today is: $today"
5.2) Loops (for, while)
For Loop Example
for i in 1 2 3 4 5
do
echo "Number: $i"
done
For Loop – Files in Directory
for file in *.log
do
echo "Processing $file"
done
While Loop
count=1
while [ $count -le 5 ]
do
echo "Count: $count"
count=$((count+1))
done
Infinite While Loop
while true
do
echo "Running..."
sleep 2
done
5.3) If-Else Conditions
Basic If
num=10
if [ $num -gt 5 ]; then
echo "Greater than 5"
fi
If-Else
if [ -f /etc/passwd ]; then
echo "File exists"
else
echo "File does not exist"
fi
Elif
read -p "Enter score: " score
if [ $score -ge 90 ]; then
echo "Grade A"
elif [ $score -ge 60 ]; then
echo "Grade B"
else
echo "Fail"
fi
5.4) Functions
Simple Function
welcome() {
echo "Welcome to Shell Scripting!"
}
welcome
Function with Parameters
add() {
echo "Sum = $(($1 + $2))"
}
add 5 10
Return Values
check_user() {
id $1 > /dev/null 2>&1
return $?
}
check_user muni
echo "Exit status: $?"
5.5) Arrays
Create Array
fruits=("apple" "banana" "orange")
Print Single Element
echo ${fruits[1]}
Print All
echo ${fruits[@]}
Array Loop
for item in ${fruits[@]}
do
echo $item
done
5.6) Automation Scripts
A) User Creation Automation
#!/bin/bash
for user in user1 user2 user3
do
useradd $user
echo "User $user created"
done
B) Backup Automation Script
#!/bin/bash
src="/var/www"
dst="/backup/www-$(date +%F).tar.gz"
tar -czf $dst $src
echo "Backup completed: $dst"
C) Service Monitoring Script
#!/bin/bash
service=httpd
if systemctl is-active --quiet $service; then
echo "$service is running"
else
systemctl start $service
echo "$service restarted"
fi
5.7) Log Monitoring Scripts
A) Real-Time Error Watcher
#!/bin/bash
tail -Fn0 /var/log/messages | \
while read line
do
echo "$line" | grep -i "error" && echo "Error detected!"
done
B) Apache Log Monitor
#!/bin/bash
log="/var/log/httpd/access.log"
grep "$(date +%d/%b/%Y)" $log | wc -l
C) Detect Failed SSH Logins
grep "Failed password" /var/log/secure
5.8) Cron-Based Script Execution
Edit Crontab
crontab -e
Run Script Every Day at 7 PM
0 19 * * * /scripts/backup.sh
Run Script Every 5 Minutes
*/5 * * * * /scripts/monitor.sh
Log Cron Output
0 1 * * * /scripts/cleanup.sh >> /var/log/cleanup.log 2>&1
