6.11 Basic Shell Scripting

By Ravindra Bagale Last updated: 27 September 2026

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!