Chapter 4: Linux Advanced Commands
4.13 Basic shell scripting
Ekdum simple aahe — a script is just the commands you already know, saved in a file. Let's write one together.
A shell script is a text file containing commands, run top to bottom. It is the first step towards automation (and towards EC2 user data, which runs a script at first boot).
#!/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="Asha", 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 |