Skip to main content

Automating system tasks with Cron

· 4 min read
Hirusha Adikari
Hirusha Adikari
Student interested in Tech

This guide covers the basics of using Cron, a time-based job scheduler in Unix-like systems. You'll learn how to set up and manage cron jobs, understand cron syntax, and explore practical examples for automating system tasks efficiently.

alt text

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.

Loading Comments...