Elastic Compute Service:Cron jobs

What is cron?

Cron is a program used to execute jobs on schedule in Linux and Unix operating systems. Cron allows you to automatically run scripts, commands, or software on schedule. Cron is suitable for tasks that need to be performed on a regular basis, such as data backups and system updates.

Working mechanism of cron

Cron runs based on a crontab (cron table) file, which is a configuration file that contains a series of commands and the execution schedules of the commands. Each account can have their own crontab file in their home directory. The system-level crontab file is used for jobs executed at the administrator level.

The cron daemon periodically checks the crontab files, parses the schedules and commands defined in the files, and runs the commands on schedule.

Cron components

1. Cron daemon: This is a resident background program responsible for regularly checking, scheduling, and executing jobs specified in crontab files.

2. crontab files:

   • Account-level crontab files: Each account can edit their own cron jobs in the account-level crontab file by running the crontab -e command. The jobs are valid only for the current account.

   • System-level crontab file: is stored in the /etc/crontab directory. In most cases, the system-level crontab file is managed by a system administrator and used to execute system-level jobs.

3. crontab syntax: Each line in a crontab file consists of six fields that represent the minutes, hours, day of the month, month, day of the week, and command to run.

Cron expression

To help you better understand how cron works, the following sample code provides an example of a simple crontab file:

# .---------------- minute
# |  .------------- hour
# |  |  .---------- day of month 
# |  |  |  .------- month 
# |  |  |  |  .---- day of week (Sunday=0 or 7)
# |  |  |  |  |
# *  *  *  *  * user-name  command to be run // Specify the path of the command or script that the cron job runs.
  30 04 *  *  *   root     /path/to/daily_backup.sh

In this example, the job configuration indicates that the system runs the daily_backup.sh script as the root user at 04:30 every day.

Considerations

• Make sure that you specify the correct source and destination directories. In this example, /path/to/original/file.txt is used as the source directory and /path/to/backup/file_backup.txt is used as the destination directory.

• Check whether the specified destination directory exists. If the destination directory does not exist, you must first create the directory or add a command used to create the directory to the cron job.

• Make sure that your account has permissions to read the source file and write data to the destination directory.

• Periodically check backup files and logs to check whether the cron job runs as expected and generates valid backup files.

How do I resolve the cron job execution timeout issue?

• If the execution of a cron job times out, use the following solutions to resolve the issue:

  • Optimize the command logic to reduce the execution time.

  • If the cron job can be split, split the job into multiple smaller jobs.

  • Add the timeout command to the Command field to change the maximum execution time of the cron job. For example: add the 0 5 * * * timeout 300 /path/to/script.sh configuration to limit the maximum execution time of the job to 300 seconds.

How do I use simple test commands?

Before you configure a complex cron job, use a simple test command to check whether cron jobs can be triggered as expected. For example, run the following command to configure a cron job that writes the current time to a file every minute:

* * * * * date >> /path/to/date_output.txt

Open the date_output.txt file after a few minutes and check whether a new time record is written to the file every minute.

How do I redirect the output and errors to a log file?

Add a redirection configuration to a cron expression to redirect the output and errors to a log file. This way, you can check whether the cron job is executed as expected and view the output and errors that occurred during execution.

30 4 * * * /path/to/your-script.sh > /path/to/logfile.log 2>&1

Check the /path/to/logfile.log file to view the output and errors of the cron job during execution.

How do I configure the frequency of a cron job?

• Refer to the following suggestions to configure the frequency of a cron job:

  • To prevent a cron job from affecting system performance, we recommend that you do not specify an excessively high frequency for the job. Specify an appropriate frequency for the cron job based on the characteristics of the job.

  • Configure the frequency of a cron job based on your business requirements. Simple daily jobs, such as backups, may need to be executed only once every day, while specific monitoring jobs may need to be executed every hour or more frequently.

  • Consider the system load. Execute resource-intensive jobs during low-load periods.

What operations can I perform to facilitate the troubleshooting of cron issues?

• Redirect the output and errors of a cron job to a log file to facilitate issue tracking. Example: 30 4 * * * /path/to/job.sh > /path/to/job.log 2>&1.

• Configure the cron daemon to send notifications by email when a cron job encounters an exception or generates output. Make sure that the system email settings are correctly configured or explicitly configure email notifications for the cron job.

• In most cases, absolute paths are more reliable than relative paths. We recommend that you use absolute paths in cron jobs.

• To edit a cron job, run the crontab -e command. To view all cron jobs in your account, run the crontab -l command. You can check whether the cron jobs are correctly configured.

Why is a cron job not executed on schedule?

• Causes and solutions:

  • If an error exists in the cron expression, check the cron expression and correct the error. Make sure that the schedule settings are correct.

  • If your account does not have sufficient permissions to execute the cron job, grant your account the required permissions to execute the cron job by running the chmod +x /path/to/script.sh command.

  • If an environment issue occurs, the cron job may fail to load the complete environment configurations of your account. Make sure that you specify the full path of the command in the cron job or explicitly configure environment variables in the command.

  • If other issues occur, check cron logs to identify the issues. The location of cron logs varies based on the system configuration. In most cases, cron logs are stored in the /var/log/cron directory. If you redirect the output and errors of the cron job to a specific file, view the logs in the file to check whether the cron job is triggered as expected and whether error messages are returned.

How do I view all cron jobs in my account?

Run the crontab -l command to view all cron jobs in your account. This is a common command in Linux and Unix. Cron is a time-based job scheduler that allows you to automatically run scripts or commands on schedule.

• Take note of the following items:

  • The preceding command displays only cron jobs in the current account. To view the cron jobs in another account, you must obtain the required system permissions.

  • If no cron job is configured in your account, the crontab -l command may not display information.

How do I view cron jobs that can be accessed by my account?

To view cron jobs that can be accessed by your account, you can run the crontab -l command. If your account does not have root permissions or permissions to view and edit the cron jobs of other accounts, the preceding command displays only cron jobs that belong to your account. If your account has root permissions or permissions to view and edit the cron jobs of other accounts, you can run the preceding command and specify the username of an account in the command to view all cron jobs that belong to the specified account.

• Take note of the following items:

  • You can use a regular account to view and edit only cron jobs that belong to the account. You can use the root account or an account to which sufficient permissions are granted to view and edit the cron jobs of all accounts.

  • Commands in cron jobs defined in the crontab file are run in a simplified environment. By default, a number of environment variables may not be loaded. We recommend that you use full paths in the commands.

How do I edit a crontab file to delete a cron job?

1. Run the crontab -e command to open the crontab file of your account in the default editor. The default editor can be vi or nano.

2. In the editor, find the line that contains the cron job that you want to delete, and then delete the line. In the vi or vim editor, move the pointer to the line and press dd to delete the line. In the nano editor, move the pointer to the line by using the arrow keys, and then press the Delete key to delete the line or press Ctrl+K to cut the line.

3. Save and close the crontab file.

   • In the vi or vim editor, enter :wq and press the Enter key to save the changes and close the file.

   • In the nano editor, press Ctrl+X, enter Y to confirm and save the changes, and then press the Enter key to close the file.

4. If you want to delete all cron jobs from your account, run the following command.

crontab -r

• Take note of the following items:

  • Before you edit or delete a cron job, we recommend that you back up your crontab file. You can run the crontab -l > crontab_backup.txt command to back up the cron jobs in your crontab file to another file.

  • When you delete or edit cron jobs in your crontab file, the operating system or applications in the operating system may be affected. Before you delete or edit a cron job, make sure that you are fully aware of the potential impact of the operation.

What do I do if the "You (*) are not allowed to use this program (crontab)" error message appears when I run the crontab command as a non-root user in Linux?

1. Connect to the Linux ECS instance as the root user.

   For more information, see Use Workbench to connect to a Linux instance over SSH.

2. Run the following commands to check whether the cron.allow and cron.deny files exist:

   find /etc/cron.allow
   find /etc/cron.deny

   The following table describes the relationship between the existence of the cron.allow and cron.deny files and user permissions to run the crontab command.

   Whether the cron.allow file exists	Whether the cron.deny file exists	Users who can run the crontab command
   No	No	Only the root user can run the crontab command.
   Yes	No	Only users who are specified in the cron.allow file can run the crontab command.
   No	Yes	Users who are not specified in the cron.deny file can run the crontab command. Note If the cron.deny file is empty, all users can run the crontab command.
   Yes	Yes	Only users who are specified in the cron.allow file can run the crontab command. Note The cron.allow file takes precedence over the cron.deny file. In this case, the cron.deny file does not take effect.

3. Modify the cron.allow or cron.deny file based on your business scenario:

   • If the cron.allow file does not exist and the cron.deny file contains the username of the non-root user, delete the username, and then save the cron.deny file.

   • If the cron.allow file exists, add the non-root user to the cron.allow file, and then save the file.

4. Run the following command to restart the cron service:

   systemctl restart crond.service

5. Switch to the non-root user and run the crontab command again.

What do I do if the "errors in crontab file, can't install" error message appears when I run the crontab -e command on a Linux instance?

To resolve this issue, you can log on to the instance and change the cron expression format in the crontab file or resize the disk on which the file is stored.

1. Connect to the Linux ECS instance.

   For more information, see Methods for connecting to an ECS instance.

2. Check whether the format of each cron expression of the job in the crontab file is correct.

   • If the format of each cron expression of the job is correct, proceed to Step 3.

   • If the format of a cron job is incorrect, correct the format of the cron expression of the job. The following figure shows the correct format of a corn expression of the job.

     

3. Run the following command to query the usage of disk space:

   df -h

   A command output similar to the following one is displayed. The values in the Use% column indicate the space usage of each file system. In this example, the space usage of the /dev/xvda1 partition is 15%.

   If the disk usage is close to 100%, the disk space is insufficient. We recommend that you resize the disk before you run the crontab -e command. For information about how to resize a disk, see Overview.