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Linux Cron Jobs and Job Scheduling

Cron Jobs​

Cron is a time-based job scheduler in Unix-like operating systems. It allows users to schedule scripts or commands to run automatically at specified times or intervals.

How Cron Works:​

  • The cron daemon runs in the background and checks for scheduled jobs.
  • Users define their jobs in a crontab (cron table) file.
  • Cron executes jobs based on the schedule defined in crontab.

Understanding Crontab Syntax​

Each cron job follows this syntax:

* * * * * command-to-execute
| | | | |
| | | | +---- Day of the week (0 - 7, Sunday = 0 or 7)
| | | +------ Month (1 - 12)
| | +-------- Day of the month (1 - 31)
| +---------- Hour (0 - 23)
+------------ Minute (0 - 59)

Examples:

  • Run a script every day at midnight: 
    0 0 * * * /path/to/script.sh
  • Run a command every Monday at 9 AM: 
    0 9 * * 1 /usr/bin/python3 /path/to/script.py
  • Run a backup script every 6 hours: 
    0 */6 * * * /usr/local/bin/backup.sh

Managing Cron Jobs​

View Current Cron Jobs:​

crontab -l

Edit Cron Jobs:​

crontab -e

Remove All Cron Jobs:​

crontab -r

Using at for One-Time Jobs​

The at command schedules a task to run once at a specific time.

Example:

echo "/path/to/script.sh" | at 14:30

This runs the script at 2:30 PM today.

Running Jobs in the Background​

Use & to run a job in the background:

/path/to/script.sh &

Use nohup to keep the process running even after logging out:

nohup /path/to/script.sh &

Systemd Timers (Alternative to Cron)​

Systemd timers offer better logging and reliability compared to cron. Here's how you can set systemd timers

  1. Define the service
/etc/systemd/system/backup.service
[Unit]
Description=Run backup script

[Service]
ExecStart=/path/to/backup.sh
  1. Define the timer
/etc/systemd/system/backup.timer
[Unit]
Description=Run backup script every day

[Timer]
OnCalendar=daily
Persistent=true

[Install]
WantedBy=timers.target
  1. Enable and start the timer
sudo systemctl enable --now backup.timer