6.11 Basic Shell Scripting

By Ravindra Bagale Last updated: 27 September 2026

Also in the AWS course: this lesson is the same as 4.13 Basic shell scripting in the AWS course, which is the main version of this page.

Script mhanje tumhala already yenare commands ek file madhe. Hech automation chi pahili payri – aani security tools pan asech lihile jatat.

#!/bin/bash
# file: sysinfo.sh  - print a quick health report
set -uo pipefail         # stop on undefined variables and pipe failures

NAME=$(hostname)
echo "Health report for $NAME at $(date)"
echo "---------------------------------"
echo "Uptime : $(uptime -p)"
echo "Disk / : $(df -h / | awk 'NR==2 {print $5 " used"}')"
echo "Memory : $(free -m | awk '/Mem/ {printf "%d/%d MB used", $3, $2}')"

# if-else
if systemctl is-active --quiet nginx; then
  echo "Nginx  : running"
else
  echo "Nginx  : NOT running"
fi

# loop
for svc in sshd crond; do
  echo "$svc -> $(systemctl is-active $svc 2>/dev/null)"
done

# function with argument
check_port() {
  if ss -tln | grep -q ":$1 "; then echo "Port $1 open"; else echo "Port $1 closed"; fi
}
check_port 22
check_port 80

nano sysinfo.sh        # paste the script, save with Ctrl+O, exit with Ctrl+X
chmod +x sysinfo.sh
./sysinfo.sh

Concept	Syntax
Variable	`NAME="Ruhi"`, use `$NAME` or `${NAME}` (no spaces around `=`)
Command output	`TODAY=$(date +%F)`
Arguments	`$1`, `$2`, all: `$@`, count: `$#`
Exit status of last command	`$?` (0 = success)
Test file exists	`if [ -f /etc/nginx/nginx.conf ]; then ... fi`
Test directory	`[ -d /var/www ]`
Compare numbers	`[ "$a" -gt 10 ]` (`-eq -ne -lt -le -ge`)
Compare strings	`[ "$a" = "yes" ]`
Read input	`read -p "Enter name: " NAME`

Ravindra Bagale's Tip

Many students write a variable with spaces, like NAME = "Raja" – bash thinks it's a command and gives an error. There must be no space on either side of =. And always use variables inside double quotes: "$NAME".

Ravindra Bagale's Tip – मराठी

Variable लिहिताना NAME = "Raja" असा space ठेवणारे बरेच students आहेत – bash ला ते command वाटतं आणि error येतो. = च्या दोन्ही बाजूला space नको. आणि variable नेहमी double quotes मध्ये वापरा: "$NAME".

Ravindra Bagale's Tip – हिंदी

बहुत से students variable लिखते समय NAME = "Raja" जैसा space रख देते हैं – bash को यह command लगता है और error आता है. = के दोनों तरफ़ space नहीं होना चाहिए. और variable हमेशा double quotes में इस्तेमाल करो: "$NAME".

Lab

Write sshwatch.sh that prints the number of failed SSH logins today and the top three source IPs (use journalctl or auth.log, grep, awk, sort, uniq -c). Make it executable and run it.

Thodkyaat sangaycha tar

Search: grep -i/-r/-v/-E, find by name, size, time, permission.
Process text: awk for columns, sed for replace (test before -i).
Combine with pipes; > overwrites, >> appends; sudo tee for root files.
Services: sudo service <name> start|stop|restart|reload|status; boot: sudo systemctl enable; logs: journalctl -u.
Network: ip a, ss -tulnp, curl -I, dig, nc -zv. Disk: df -h, du -sh, lsblk.
Cron schedules jobs (and attackers abuse it for persistence). ssh/scp/rsync for remote work.

Samjla ka? Linux aata tumcha haath aahe. Part 2 sampla – aata pudhe jaauya web servers kade!