Cron Jobs Explained
A cron job is a scheduled task in Unix-like operating systems that runs automatically at specified intervals. These jobs are managed by the cron daemon, which executes commands according to instructions in configuration files called “crontabs.”
Key Components
- Cron daemon: Background service that checks for scheduled tasks
- Crontab file: Configuration file containing scheduled commands
- Cron expression: Timing syntax that defines when jobs run (minute, hour, day, month, weekday)
Cron Expression Format
* * * * * command_to_execute
↑ ↑ ↑ ↑ ↑
│ │ │ │ └─── Day of week (0-7, where 0 and 7 are Sunday)
│ │ │ └───── Month (1-12)
│ │ └─────── Day of month (1-31)
│ └───────── Hour (0-23)
└─────────── Minute (0-59)
Examples of Cron Jobs
-
Daily Database Backup at 2 AM:
0 2 * * * /usr/local/bin/backup_database.sh -
System Update Every Monday at 3 AM:
0 3 * * 1 apt-get update && apt-get -y upgrade -
Log Rotation Every Hour:
0 * * * * /usr/sbin/logrotate /etc/logrotate.conf -
Website Monitoring Every 5 Minutes:
*/5 * * * * curl -s https://example.com > /dev/null || mail -s "Website Down" admin@example.com -
Monthly Sales Report on 1st Day:
0 7 1 * * /usr/local/bin/generate_monthly_report.py -
Clear Temporary Files Every Sunday:
0 0 * * 0 find /tmp -type f -atime +7 -delete -
Run Scripts at System Reboot:
@reboot /path/to/startup_script.sh
Cron jobs are essential for automating routine maintenance tasks, scheduled backups, report generation, and system monitoring without requiring manual intervention.