SRJahirTech

Linux Full Tutorial -- Part 7: Shell Scripting

By Suraj Ahir 2025-12-09 11 min read

Shell scripting is the automation superpower of Linux. Every task you do repeatedly in the terminal is a candidate for a script. Backup files, deploy applications, restart services, clean up logs, check disk space and alert if low -- all of these are 10-50 line bash scripts that run automatically and save hours of manual work every week.

I still remember the first time I wrote a script that automated our deployment process. What took 15 manual steps and 20 minutes of careful attention became a 30-second automated process. That script ran thousands of times without a single manual mistake. That is the power of shell scripting.

Script Basics

#!/bin/bash
# The shebang line tells the OS which interpreter to use

# Comments explain what the script does

echo "Script started at $(date)"

# Variables
APP_NAME="myapp"
DEPLOY_DIR="/home/ubuntu/${APP_NAME}"
LOG_FILE="/var/log/${APP_NAME}/deploy.log"

echo "Deploying ${APP_NAME} to ${DEPLOY_DIR}"

chmod +x deploy.sh     # Make executable
./deploy.sh            # Run from current directory
bash deploy.sh         # Run with bash explicitly
/home/ubuntu/deploy.sh # Run with full path

Variables and Input

#!/bin/bash

# Positional arguments
ENVIRONMENT=$1       # First argument
VERSION=$2           # Second argument

# Special variables
echo "Script name: $0"
echo "All args: $@"
echo "Arg count: $#"
echo "Last exit code: $?"
echo "Process ID: $$"

# Command substitution
CURRENT_DATE=$(date +%Y-%m-%d)
DISK_USAGE=$(df -h / | tail -1 | awk "{print $5}")

# Read user input
read -p "Enter deployment environment: " ENV
read -sp "Enter password: " PASSWORD  # -s = silent

Conditionals

#!/bin/bash

ENV=$1

if [ -z "$ENV" ]; then
    echo "Error: Environment required"
    echo "Usage: $0 "
    exit 1
fi

if [ "$ENV" == "production" ]; then
    echo "Deploying to production"
    DEPLOY_HOST="prod.example.com"
elif [ "$ENV" == "staging" ]; then
    echo "Deploying to staging"
    DEPLOY_HOST="staging.example.com"
else
    echo "Unknown environment: $ENV"
    exit 1
fi

# File checks
[ -f "/etc/nginx/nginx.conf" ] && echo "Nginx config exists"
[ -d "/var/log/app" ] || mkdir -p /var/log/app
[ -r "$FILE" ] && echo "File is readable"

# Numeric comparison
DISK_PCT=85
if [ $DISK_PCT -gt 90 ]; then
    echo "CRITICAL: Disk almost full"
elif [ $DISK_PCT -gt 80 ]; then
    echo "WARNING: Disk usage high"
fi

Loops

# for loop over list
for server in web1 web2 web3; do
    echo "Deploying to $server"
    ssh ubuntu@$server "sudo systemctl restart myapp"
done

# for loop with range
for i in $(seq 1 5); do
    echo "Attempt $i"
done

# for loop over files
for logfile in /var/log/nginx/*.log; do
    echo "Processing: $logfile"
    gzip "$logfile"
done

# while loop
ATTEMPT=0
while [ $ATTEMPT -lt 3 ]; do
    curl -sf http://localhost/health && break
    ATTEMPT=$((ATTEMPT + 1))
    echo "Health check failed, attempt $ATTEMPT"
    sleep 5
done

# Read file line by line
while IFS= read -r line; do
    echo "Processing: $line"
done < servers.txt

Functions

#!/bin/bash

# Function definitions
log() {
    echo "[$(date +%H:%M:%S)] $1" | tee -a /var/log/deploy.log
}

check_health() {
    local url=$1
    local max_attempts=5
    local attempt=0
    
    while [ $attempt -lt $max_attempts ]; do
        if curl -sf "$url/health" > /dev/null 2>&1; then
            log "Health check passed"
            return 0
        fi
        attempt=$((attempt + 1))
        log "Health check failed ($attempt/$max_attempts)"
        sleep 10
    done
    return 1
}

# Call functions
log "Starting deployment"
if ! check_health "http://localhost:8000"; then
    log "ERROR: Service unhealthy after deployment"
    exit 1
fi

Error Handling

#!/bin/bash
set -e          # Exit on any error
set -u          # Error on undefined variables
set -o pipefail # Pipes fail if any command fails

# Trap for cleanup on exit
cleanup() {
    echo "Cleaning up..."
    rm -f /tmp/deploy.lock
}
trap cleanup EXIT
trap "echo ERROR: Script failed on line $LINENO; exit 1" ERR

# Check prerequisites
command -v docker >> /dev/null || { echo "Docker not found"; exit 1; }

# Safe variable expansion
echo "${VARIABLE:-default_value}"  # Use default if unset

Frequently Asked Questions

In Part 8, we cover cron jobs -- scheduling your scripts to run automatically at specific times.

Written by

Suraj Ahir

Cloud & DevOps engineer running four live production services on my own AWS infrastructure. I write everything on this site myself — no ghostwriters, no AI filler.

More about me → GitHub LinkedIn