cron

Understanding the cron Daemon

The cron daemon runs in the background, constantly checking a system-wide configuration file for scheduled jobs. These jobs, specified in the crontab (cron table), are executed at the designated times. Let’s look at the structure of a crontab entry:

* * * * * command_to_be_executed

This represents five fields, separated by spaces, determining when the command will run:

1. Minute (0-59): The minute the job should start.
2. Hour (0-23): The hour the job should start (0 is midnight).
3. Day of the month (1-31): The day of the month the job should start.
4. Month (1-12): The month the job should start (1 is January).
5. Day of the week (0-6): The day of the week the job should start (0 is Sunday).

Special Characters in crontab Entries

cron supports special characters to create flexible schedules:

• * (asterisk): Represents all possible values for the field. For example, * in the minute field means “every minute.”
• , (comma): Specifies a list of values. For example, 1,15,30 in the minute field means “at minutes 1, 15, and 30.”
• - (hyphen): Specifies a range of values. For example, 1-10 in the minute field means “every minute from 1 to 10.”
• / (slash): Specifies an increment. For example, */5 in the minute field means “every 5 minutes.”

Practical Examples

Let’s illustrate with some examples:

1. Running a script every hour:

This example runs the script /home/user/myscript.sh every hour, on the hour:

0 * * * * /home/user/myscript.sh

2. Running a command every day at 3 PM:

This example runs the command date >> /tmp/log.txt every day at 3 PM:

0 15 * * * date >> /tmp/log.txt

3. Running a command on specific days of the week:

This example runs the command backup_db.sh every Monday and Friday at 10 PM:

0 22 * * 1,5 /home/user/backup_db.sh

4. Running a command every 5 minutes:

This example runs the check_status.py script every 5 minutes:

*/5 * * * * /home/user/check_status.py

5. Running a command only during specific months:

This example runs seasonal_task.sh every day at 8 AM during July and August:

0 8 * 7,8 * /home/user/seasonal_task.sh

Managing Your crontab

To edit your crontab, use the command crontab -e. This will open your crontab file in a text editor (usually vi or nano). After making changes, save and close the file. cron will automatically reload the updated schedule. Remember to specify the full path to your scripts and commands. For enhanced error handling, redirect output to a log file. Improperly configured cron jobs can silently fail, so logging is essential.

Important Security Considerations

Avoid running commands with excessive privileges. Use the principle of least privilege; grant only the necessary permissions to the user running the cron job. Secure your scripts and avoid hardcoding sensitive information directly into crontab entries. Consider using environment variables or configuration files for sensitive data. Regularly review your crontab entries to ensure they are still necessary and appropriately configured.