Bash Scripting for Beginners: Loops, Functions and Real Admin Scripts

For loops

A for loop repeats commands for each item in a list.

for file in *.log; do
  echo "$file"
done

Arrays are a clean way to store lists.

SERVICES=("nginx" "mysql" "sshd")

for SERVICE in "${SERVICES[@]}"; do
  echo "$SERVICE"
done

Read a file line by line with while

This pattern is useful when reading a list of domains, users or paths.

while IFS= read -r DOMAIN; do
  echo "Checking $DOMAIN"
done < domains.txt

Bash functions

Functions let you reuse blocks of logic.

check_service() {
  local service="$1"

  if systemctl is-active --quiet "$service"; then
    echo "[OK] $service is running"
  else
    echo "[WARN] $service is not running"
  fi
}

Add simple logging to scripts

log_msg() {
  echo "[$(date -Is)] $*"
}

log_msg "Starting checks"

To log to a file as well as the terminal, use tee.

log_msg "Backup completed" | tee -a /var/log/my-script.log

Using set -euo pipefail carefully

You will often see this near the top of Bash scripts:

set -euo pipefail

Script 1: check multiple services

#!/usr/bin/env bash

SERVICES=("nginx" "mysql" "sshd")

check_service() {
  local service="$1"

  if systemctl is-active --quiet "$service"; then
    echo "[OK] $service is running"
  else
    echo "[WARN] $service is not running"
  fi
}

for service in "${SERVICES[@]}"; do
  check_service "$service"
done

[OK] nginx is running
[WARN] mysql is not running
[OK] sshd is running

What the multiple services script is doing

Script 2: warn when disk usage is high

#!/usr/bin/env bash

THRESHOLD=85
MOUNT="/"

usage="$(df "$MOUNT" | awk 'NR==2 {gsub(/%/,"",$5); print $5}')"

if [ "$usage" -ge "$THRESHOLD" ]; then
  echo "[WARN] Disk usage on $MOUNT is ${usage}%"
  exit 1
else
  echo "[OK] Disk usage on $MOUNT is ${usage}%"
  exit 0
fi

[OK] Disk usage on / is 55%

For deeper disk checks, use the Linux High Disk Usage Troubleshooting Guide.

Script 3: check DNS for multiple domains

Create a file named domains.txt:

example.com
commandlinequiz.com
example.org

Then create the script:

#!/usr/bin/env bash

while IFS= read -r DOMAIN; do
  [ -z "$DOMAIN" ] && continue
  echo "== $DOMAIN =="
  dig +short "$DOMAIN"
  echo
done < domains.txt

== example.com ==
93.184.216.34

== commandlinequiz.com ==
104.21.10.123
172.67.150.45

== example.org ==
93.184.216.34

For more DNS examples, see the dig command guide.

Practice exercises

Exercise 1: add another service

Add redis or httpd to the SERVICES array and run the script again.

Exercise 2: write output to a log file

Update the service checker so the output is also appended to /tmp/service-check.log.

./check-services.sh | tee -a /tmp/service-check.log

Exercise 3: check domains from a file

Add more domains to domains.txt and confirm your DNS checker loops through each one.

Download the example scripts

Check scripts before using them on a server

bash -n check-services.sh
shellcheck check-services.sh

When the script changes files or restarts services, test it on a safe machine first. Production servers are not the place for “let’s see what happens”.

Where to go next

• Use the Bash Scripting Cheat Sheet as a quick reference.
• Test snippets with the Bash Checker.
• Build real troubleshooting scripts using the Linux Troubleshooting Hub.
• Practise Linux admin topics with the Linux+ Practice Quiz.