In this article, we will explore how to schedule and execute Linux tasks automatically in the background at predefined intervals using the Crontab command.

Performing repetitive jobs manually can be a tedious process for system administrators. These tasks can be automated using the cron daemon in Linux or Unix-like systems, allowing them to run without manual intervention.

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• How to Create and Manage Cron Jobs on Linux
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For example, you can automate system backups, schedule updates, file synchronization, and more using the Cron daemon. You can either run these tasks directly from the command line or use online tools to generate cron job entries.

The Cron service runs every minute and checks for scheduled jobs. The Crontab (CRON TABle) is where we define these recurring tasks.

Tips: Each user can manage their own crontab entries including creating, modifying, and deleting scheduled tasks. By default, all users have access to cron, but you can block specific users by adding them to the /etc/cron.deny file.

Crontab in Linux

A Crontab file contains one command per line and consists of six fields. The first five fields specify the time when the task should be executed, and the last field contains the actual command to run.

• Minute (values between 0–59)
• Hour (values between 0–23)
• Day of Month (values between 1–31)
• Month of Year (values between 1–12 or Jan–Dec, using the first three letters)
• Day of Week (values between 0–6 or Sun–Sat, again using the first three letters)
• Command – the /path/to/command or script you wish to schedule.

1. List Crontab Entries

Use the crontab -l command to list current scheduled tasks for the logged-in user.

<code><strong># crontab -l</strong>
<p>00 10 <em> </em> * /bin/ls >/ls.txt</p></code>

2. Edit Crontab Entries

To edit your crontab, run it with the -e option as shown below. This opens the file in the VI editor. Make changes and press :wq to save and exit.

<code><strong># crontab -e</strong></code>

3. View Scheduled Cron Jobs for a Specific User

To display cron jobs for a particular user like tecmint, use the -u (User) and -l (List) options.

<code><strong># crontab -u tecmint -l</strong><p>no crontab for tecmint</p></code>

Note: Only the root user has permission to view other users' crontab files. Regular users cannot access others’ schedules.

4. Delete Crontab Entry

Caution: Using crontab -r deletes all scheduled jobs without confirmation. Use the -i flag before deletion to confirm.

<code><strong># crontab -r</strong></code>

5. Confirm Before Deleting Crontab

Using the -i option with crontab prompts for confirmation before removing the user’s crontab.

<code><strong># crontab -i -r</strong><p>crontab: really delete root's crontab?</p></code>

6. Allowed Special Characters (, -, /, ?, #)

• Asterisk() – Matches any value in the field.
• Hyphen(-) – Defines a range.
• Slash (/) – Specifies increments (e.g., */10 means every ten minutes).
• Comma (,) – Separates multiple values.

7. System-Wide Cron Schedule

System-wide cron jobs can be placed in the following directories:

• /etc/cron.d
• /etc/cron.daily
• /etc/cron.hourly
• /etc/cron.monthly
• /etc/cron.weekly

8. Schedule a Job for a Specific Time

The following job removes empty files and folders from /tmp at 12:30 AM daily. You must specify the user who will execute the cron job. In this case, the root user is running it.

<code><strong># crontab -e</strong><p>30 0 <em> </em> *   root   find /tmp -type f -empty -delete</p></code>

9. Special Strings for Common Schedules

If you want to use these strings, replace the five time fields with the keyword.

10. Run Multiple Commands Using Double Ampersand(&&)

In the example below, both command1 and command2 are executed daily.

<code><strong># crontab -e</strong><p>@daily <command1> && <command2></command2></command1></p></code>

11. Turn Off Email Notifications

By default, cron sends an email to the user account executing the job. To stop receiving emails, add >/dev/null 2>&1 at the end of your cron entry to discard all output.

<code>[root@tecmint ~]# crontab -e<ul><li><ul><li><ul><li><ul><li><ul><li>>/dev/null 2>&1</li></ul></li></ul></li></ul></li></ul></li></ul></code>

Conclusion: Automating routine tasks improves efficiency, reduces errors, and ensures timely execution. For more information, refer to the manual page for crontab by typing ‘man crontab‘ in your terminal.

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