Bash Scripting Essentials

Write robust bash scripts for automation and system administration.

Script Structure

#!/bin/bash
set -euo pipefail  # exit on error, undefined vars, pipe failures
IFS=$'\n\t'       # safer word splitting

# Variables
NAME="world"
echo "Hello, $NAME"

Conditionals

if [[ -f /etc/passwd ]]; then
  echo "File exists"
elif [[ -d /tmp ]]; then
  echo "Directory exists"
else
  echo "Neither"
fi

# String comparison
if [[ "$USER" == "root" ]]; then
  echo "Running as root"
fi

# Numeric comparison
COUNT=5
if (( COUNT > 3 )); then
  echo "Count is greater than 3"
fi

Loops

# For loop over list
for server in web01 web02 db01; do
  echo "Checking $server..."
  ping -c 1 "$server" &>/dev/null && echo "  UP" || echo "  DOWN"
done

# While loop
COUNT=0
while (( COUNT < 5 )); do
  echo "Count: $COUNT"
  (( COUNT++ ))
done

# Loop over files
for f in /etc/*.conf; do
  echo "Config: $f"
done

Functions

log() {
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a /var/log/myscript.log
}

check_service() {
  local svc="$1"
  systemctl is-active --quiet "$svc" && log "$svc is running" || log "WARNING: $svc is down"
}

check_service nginx
check_service mysql

Error Handling

cleanup() {
  echo "Cleaning up..."
  rm -f /tmp/myscript.lock
}
trap cleanup EXIT ERR

# Lock file to prevent duplicate runs
if [[ -f /tmp/myscript.lock ]]; then
  echo "Script already running"
  exit 1
fi
touch /tmp/myscript.lock

Script Arguments

#!/bin/bash
# Usage: ./deploy.sh staging|production
ENV="${1:-staging}"  # default to staging

case "$ENV" in
  staging)    HOST="staging.example.com" ;;
  production) HOST="prod.example.com" ;;
  *)          echo "Usage: $0 staging|production"; exit 1 ;;
esac

echo "Deploying to $HOST..."

FAQ

What should every Bash script start with?

A shebang line (#!/usr/bin/env bash) and set -euo pipefail so the script stops on errors and unset variables.

How do I run a script?

Make it executable with chmod +x script.sh, then run ./script.sh.