Cron Jobs

What are Cron Jobs?

• Cron is a time-based job scheduling service in Unix-like operating systems.

• Allows users to schedule jobs (commands or scripts) to run periodically at fixed times, dates, or intervals.

The Cron Daemon

• The cron daemon is the background service responsible for launching the scheduled cron jobs.

• On most Linux distributions, the cron daemon runs automatically from /etc/init.d or another initialization system.

Crontab Files

• Crontab (CRON TABle) is a file that contains the schedule of cron entries for a user.

• Every user can have their own crontab, and though these are files in /var/spool/, they are not intended to be edited directly. Instead, users should use the crontab command.

Basic Syntax

• A typical crontab line has five time and date fields, followed by a command:

  bash

• m h dom mon dow command

  • m: Minute (0-59)

  • h: Hour (0-23)

  • dom: Day of the month (1-31)

  • mon: Month (1-12)

  • dow: Day of the week (0-7, where both 0 and 7 represent Sunday)

Common Commands

• crontab -e: Edit the current user's crontab.

• crontab -l: Display the current user's crontab.

• crontab -r: Remove the current user's crontab.

• crontab -u [username]: Used by superusers to edit or display the crontab of a specified user.

Special Strings

• There are special shorthand strings that can be used to represent common patterns:

  • @reboot: Run once at startup.

  • @yearly or @annually: Run once a year, equivalent to "0 0 1 1 *".

  • @monthly: Run once a month, equivalent to "0 0 1 * *".

  • @weekly: Run once a week, equivalent to "0 0 * * 0".

  • @daily or @midnight: Run once a day, equivalent to "0 0 * * *".

  • @hourly: Run once an hour, equivalent to "0 * * * *".

Logging

• By default, the output of a cron job is emailed to the job's owner.

• To redirect the output to a log file, use the standard redirection syntax, e.g., mycommand > /path/to/logfile 2>&1.

Common Gotchas

1. Environment: Cron runs under a different environment than the user's terminal session, so always use full paths for commands and files.

2. Permissions: Ensure that the scripts or commands being run have the necessary permissions to execute.

3. % Character: In the crontab, the % character is a newline delimiter. If you need to use it in a command, escape it with a backslash (\%).

Security Considerations

• Limit the number of users who have access to create or edit cron jobs.

• Be cautious when using the cron system to run scripts from untrusted sources or locations.

• Ensure that the cron daemon and related tools are regularly updated to patch known vulnerabilities.