Software & AppsOperating SystemLinux

How To Set a Cron Job for Running a Shell Script in Ubuntu

Ubuntu 7

Cron is a time-based job scheduler in Unix-like operating systems. Users can schedule jobs (commands or scripts) to run periodically at fixed times, dates, or intervals. This article will guide you on how to set a cron job for running a shell script in Ubuntu.

Quick Answer

To set a cron job for running a shell script in Ubuntu, you can use the crontab command to edit your crontab file and schedule the job using the appropriate syntax. Make sure to specify the full path to the shell script and set the necessary permissions for the script to be executed. You can also redirect the output of the cron job if needed.

What is a Cron Job?

A cron job is a task scheduler in Unix-like operating systems. It allows users to schedule jobs (commands or scripts) to run at specific times or on specific days. This is particularly useful for automating system maintenance or administration tasks that would otherwise require manual intervention.

Setting up a Cron Job

To set up a cron job, you will need to edit your crontab file. The crontab (short for “cron table”) is a list of commands that are scheduled to run at regular intervals. You can edit your crontab file by running the following command in your terminal:

crontab -e

This will open your crontab file in your default text editor. If this is your first time running the command, you may be prompted to select a text editor.

Scheduling a Job

In your crontab file, you can add a new line for each job you want to schedule. Each line follows the following format:

* * * * * command-to-be-executed

Each asterisk can be replaced with a specific value to schedule the job:

  • The first asterisk represents the minute (0 – 59)
  • The second asterisk represents the hour (0 – 23)
  • The third asterisk represents the day of the month (1 – 31)
  • The fourth asterisk represents the month (1 – 12)
  • The fifth asterisk represents the day of the week (0 – 7, where both 0 and 7 represent Sunday)

For example, if you want to schedule a job to run every day at 8 PM, you can use the following line:

0 20 * * * command-to-be-executed

Running a Shell Script

To run a shell script, you need to specify the full path to the script in your cron job. For example:

0 20 * * * /home/user/myscript.sh

This will run the myscript.sh script every day at 8 PM. However, you need to make sure that your script has the necessary permissions to be run as a script. You can set the executable permission using the chmod command:

chmod +x /home/user/myscript.sh

Redirecting Output

By default, the output of your cron jobs will be mailed to your user. If you want to prevent this, you can redirect the output to /dev/null:

0 20 * * * /home/user/myscript.sh > /dev/null 2>&1

This will discard both the standard output and error output.

Conclusion

Cron jobs are a powerful tool for automating tasks on your Ubuntu system. By scheduling a shell script to run at specific times, you can automate tasks without having to manually run them each time. Remember to check your script permissions, and redirect output if necessary, to ensure your cron jobs run smoothly.

For more information on setting up cron jobs, you can refer to the Ubuntu Community Help: CronHowto and The Geek Stuff: 15 Practical Cron Job Examples.

How do I view my current crontab file?

To view your current crontab file, you can use the following command in your terminal:

crontab -l

This will display the contents of your crontab file in the terminal.

How do I edit my crontab file?

To edit your crontab file, you can use the following command in your terminal:

crontab -e

This will open your crontab file in your default text editor, allowing you to make any necessary changes.

How do I remove a cron job from my crontab?

To remove a cron job from your crontab, you need to edit your crontab file. Use the command crontab -e to open your crontab file, then delete the line corresponding to the cron job you want to remove. Save and exit the file to apply the changes.

Can I schedule a cron job to run every minute?

Yes, you can schedule a cron job to run every minute by using an asterisk (*) for the minute field. For example:

* * * * * command-to-be-executed

This will run the command every minute.

How do I schedule a cron job to run on specific days of the week?

To schedule a cron job to run on specific days of the week, you can use the fifth field in the cron job line. The values range from 0 to 7, where both 0 and 7 represent Sunday. For example, to schedule a job to run every Monday at 9 AM, you can use the following line:

0 9 * * 1 command-to-be-executed
How do I schedule a cron job to run at a specific time every month?

To schedule a cron job to run at a specific time every month, you can use the third and fifth fields in the cron job line. For example, to schedule a job to run on the 15th day of every month at 3 PM, you can use the following line:

0 15 15 * * command-to-be-executed
How can I check if a cron job has been executed?

By default, cron jobs do not generate output unless there is an error. To check if a cron job has been executed, you can redirect the output to a log file. For example:

0 20 * * * /home/user/myscript.sh >> /path/to/logfile.log 2>&1

This will append the output (both standard output and error output) of the cron job to the specified log file.

How can I disable a cron job temporarily?

To disable a cron job temporarily, you can comment out the corresponding line in your crontab file by adding a # at the beginning of the line. This will prevent the cron job from being executed until you remove the comment.

How do I schedule a cron job to run only on weekdays?

To schedule a cron job to run only on weekdays (Monday to Friday), you can use the fifth field in the cron job line. You can set it to a range of values from 1 to 5. For example, to schedule a job to run every weekday at 8 AM, you can use the following line:

0 8 * * 1-5 command-to-be-executed
How can I receive the output of a cron job via email?

By default, the output of a cron job will be mailed to the user who owns the crontab. If you want to receive the output via email, make sure the sendmail or a similar email service is properly configured on your system. You can also redirect the output to a file and configure your cron job to send the file as an attachment in an email.

Leave a Comment

Your email address will not be published. Required fields are marked *