Bash Script Automation and Scheduling

The real power of Bash scripting comes from automation — running tasks without manual effort. From automated backups to scheduled cleanups, Bash scripts combined with the Linux scheduler (cron) allow any repetitive task to run automatically at a chosen time.

What Is Automation?

Automation means writing a script once and letting the system run it on a schedule or trigger. Manual tasks that take time every day become zero-effort background jobs.

┌─────────────────────────────────────────────────────┐
│                                                     │
│  Without Automation:                                │
│  Developer → Opens terminal daily → Runs backup     │
│           → Waits for completion → Logs result      │
│                                                     │
│  With Automation:                                   │
│  Script + Cron Job → Runs at 2:00 AM every night    │
│                   → Backs up, logs, and exits       │
│                   → Developer sleeps peacefully     │
│                                                     │
└─────────────────────────────────────────────────────┘

Writing a Reusable Automation Script

Example – Automated Backup Script

#!/bin/bash
set -euo pipefail

# Configuration
SOURCE_DIR="/home/john/documents"
BACKUP_DIR="/home/john/backups"
DATE=$(date +%Y-%m-%d_%H-%M-%S)
BACKUP_FILE="backup_$DATE.tar.gz"
LOGFILE="/home/john/backups/backup.log"

# Create backup directory if it does not exist
mkdir -p "$BACKUP_DIR"

# Create compressed archive
tar -czf "$BACKUP_DIR/$BACKUP_FILE" "$SOURCE_DIR"

# Log result
echo "[$DATE] Backup created: $BACKUP_FILE" >> "$LOGFILE"
echo "Backup complete: $BACKUP_DIR/$BACKUP_FILE"

Making a Script Executable and Portable

chmod +x backup.sh

Run with full path (important for cron jobs):

/home/john/scripts/backup.sh

Always use full paths inside automation scripts. Cron jobs do not have the same PATH as an interactive shell, so commands like tar, grep, and python3 must use absolute paths like /bin/tar or /usr/bin/python3.

Introduction to Cron

cron is the Linux task scheduler. It runs commands or scripts automatically at scheduled times. The schedule is defined in a file called the crontab.

Open the Crontab Editor

crontab -e

View Current Cron Jobs

crontab -l

Remove All Cron Jobs

crontab -r

Crontab Syntax

┌────────────────────────────────────────────────────────────┐
│                                                            │
│  * * * * * /path/to/script.sh                              │
│  │ │ │ │ │                                                 │
│  │ │ │ │ └── Day of week (0–7, 0 and 7 = Sunday)           │
│  │ │ │ └──── Month (1–12)                                  │
│  │ │ └────── Day of month (1–31)                           │
│  │ └──────── Hour (0–23)                                   │
│  └────────── Minute (0–59)                                 │
│                                                            │
│  * means "every"                                           │
└────────────────────────────────────────────────────────────┘

Cron Schedule Examples

Crontab Entry Example – Nightly Backup

0 2 * * * /home/john/scripts/backup.sh >> /home/john/logs/cron.log 2>&1

This runs the backup script every night at 2:00 AM and logs all output (including errors) to cron.log.

Running Scripts at System Startup

@reboot /home/john/scripts/start_services.sh

Common Automation Script Patterns

Pattern 1 – Delete Old Log Files

#!/bin/bash
# Delete log files older than 30 days
find /var/log/myapp/ -name "*.log" -mtime +30 -delete
echo "Old logs cleaned up on $(date)"

Pattern 2 – Disk Space Alert

#!/bin/bash
THRESHOLD=80
USAGE=$(df / | tail -1 | awk '{print $5}' | sed 's/%//')

if [ $USAGE -gt $THRESHOLD ]; then
  echo "ALERT: Disk usage is at ${USAGE}% on $(hostname)" | mail -s "Disk Alert" admin@example.com
fi

Pattern 3 – Auto-Restart a Service if Down

#!/bin/bash
SERVICE="nginx"
if ! systemctl is-active --quiet $SERVICE; then
  systemctl start $SERVICE
  echo "$(date): $SERVICE was down and has been restarted." >> /var/log/service_monitor.log
fi

Environment Variables in Cron

Cron runs with a minimal environment. Commands that work in the terminal may fail in cron because the PATH variable is different. Set PATH explicitly at the top of crontab or the script.

# In crontab
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
0 2 * * * /home/john/scripts/backup.sh

at Command – Run Once at a Specific Time

The at command schedules a script to run once at a specific time — unlike cron which repeats.

at 10:30 PM tomorrow <<EOF
/home/john/scripts/send_report.sh
EOF

List pending at jobs:

atq

Remove a scheduled at job:

atrm job_number

Key Takeaways

• Cron is the Linux scheduler — use it to run scripts automatically at any time.
• Use crontab -e to add, edit, or remove cron jobs.
• Always use full paths for commands and files in cron jobs.
• Redirect cron output to a log file to capture success and error messages.
• Use @reboot to run a script once when the system starts.
• The at command schedules a one-time job at a specific time.