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Automating system tasks with Cron

Cron is a time-based job scheduler in Unix-like operating systems. It allows users to schedule tasks (known as "cron jobs") to run at specific times or intervals. These tasks are defined in a crontab (cron table) file.

This guide will walk you through the basics of using cron, setting up cron jobs, and providing examples.

Introduction​

Key Components​

  • Cron Daemon (crond): A background process that continuously checks the schedule and executes tasks.
  • Crontab File: A configuration file where users define tasks to run and their schedule.

Cron Time Syntax​

The time format for cron jobs is:

* * * * * /path/to/command
- - - - -
| | | | |
| | | | +--- 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)
  • Use * to represent "any value."
  • Use commas (,) to specify multiple values (e.g., 1,15,30).
  • Use hyphens (-) to specify ranges (e.g., 9-17 for 9 AM to 5 PM).
  • Use / for intervals (e.g., */5 for every 5 minutes).

Accessing Cron​

  • To edit the crontab file: 
    crontab -e
  • To list existing cron jobs: 
    crontab -l
  • To remove all cron jobs: 
    crontab -r

Setting Up a Cron Job​

  1. Open the crontab editor:

    crontab -e

  2. Add a line for your task. For example:

    30 9 * * 1 /path/to/backup.sh

    This runs the script backup.sh at 9:30 AM every Monday.

  3. Save the file and exit. Cron will automatically load the new configuration.

Examples of Cron Jobs​

Here are several example cron job entries:

  1. Running a Script Every Hour
0 * * * * /path/to/hourly_script.sh

This runs hourly_script.sh at the start of every hour.

  1. Running a Command Every Day at 6 AM
0 6 * * * /path/to/daily_task.sh

This runs daily_task.sh at 6:00 AM daily.

  1. Deleting Temporary Files Every Week
0 3 * * 0 rm -rf /tmp/*

This deletes all files in the /tmp directory every Sunday at 3:00 AM.

  1. Running a Task on Specific Days
0 7 1,15 * * /path/to/fortnightly_task.sh

This runs fortnightly_task.sh at 7:00 AM on the 1st and 15th of every month.

  1. Running a Task During Work Hours (9 AM to 5 PM)
*/30 9-17 * * 1-5 /path/to/work_task.sh

This runs work_task.sh every 30 minutes from 9:00 AM to 5:00 PM, Monday through Friday.

  1. Running a Task Every 10 Minutes
*/10 * * * * /path/to/ten_minute_task.sh

This runs ten_minute_task.sh every 10 minutes.

  1. Backing Up a Database Daily at Midnight
0 0 * * * mysqldump -u user -p'password' database_name > /backup/db_backup.sql

This creates a backup of a MySQL database every day at midnight.

  1. Logging System Information Hourly
0 * * * * echo "$(date): $(uptime)" >> /var/log/system_info.log

This appends the current date and system uptime to a log file every hour.

  1. Restarting a Service Weekly
0 2 * * 1 systemctl restart myservice

This restarts a service named myservice every Monday at 2:00 AM.

  1. Running a Python Script
15 14 * * * /usr/bin/python3 /path/to/script.py

This runs a Python script at 2:15 PM daily.

Advanced Usage​

Redirecting Output to a File​

To log the output of a cron job, redirect it to a file:

0 4 * * * /path/to/task.sh > /var/log/task_output.log 2>&1

Environment Variables​

You can set environment variables in the crontab:

SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin

0 5 * * * /path/to/script.sh

Email Notifications​

Cron can send email notifications of job output. To enable this, set the MAILTO variable in your crontab:

MAILTO=your-email@example.com
0 6 * * * /path/to/task.sh

Viewing Logs​

Cron logs are usually stored in /var/log/syslog or /var/log/cron. To view the logs, run:

grep CRON /var/log/syslog

Notes​

  1. Ensure the user running the cron job has the necessary permissions to execute the task.
  2. Always test your commands/scripts before scheduling them with cron.
  3. Use absolute paths for scripts and executables to avoid issues with cron’s limited environment.

For more information, refer to the Cron Man Page.

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