In this chapter you can learn about the Unix Cron and At facilities,
used for running commands on a regular schedule or once at some
Introduction to Cron jobs
A Cron job
is a Unix term for a command that is run on a regular
schedule by the cron daemon. Each job is owned by a Unix user, and
runs with the permissions of that user. Each has a set of minutes,
hours, days, months and days of weeks on which it runs, allowing
considerable flexibility in scheduling. For example, a job
may run every 10 minutes, or at 3 am every day, or at 5pm Monday to
Friday in January, February and march.
Cron jobs are very useful for performing regular system tasks,
such as cleaning up log files, synchronizing the system type,
backing up files and so on. Most Linux distributions will have
several Cron jobs that were setup by default as part of the operating
system install process for doing things like removing unneeded
kernel modules, updating the database used by the locate command
and rotating log files.
The actual Cron job configuration files are stored in different
places, depending on whether they are part of a package included
in your Linux distribution or created by a user. The /var/spool/cron
directory is for jobs created manually by users, and contains
one file per Unix user. The /etc/crontab file and the files under
the /etc/cron.d directory contain jobs that are part of packages,
such as those that are part of your distribution.
The Scheduled Cron Jobs module
The Webmin module for editing Cron jobs can be found under the
System category. When you enter it, the main page displays a table
of all the existing jobs on your system as shown below.
For each action, the owner, active status and command are listed.
If the command for a job is too long, it will be truncated for display
on the page.
The Scheduled Cron Jobs module
Creating a new Cron job
Using Webmin, you can easily create a new Cron job that will execute
as any Unix user on your system. To steps to follow to achieve this
- On the main page of the module, click on the Create a new scheduled cron job link above or below the list of existing jobs. This will take you to the job creation form shown below.
- In the Execute cron job as field, enter the name of the Unix user you want the job to execute as. The command executed by the job will run in the user's home directory with his full permissions.
- The Active? field can be set to No if you don't want this new job to be actually executed. This is useful for creating jobs to be enabled at a later date.
- In the Command field, enter the shell commands that you want the Cron job to run. Just as at the shell prompt, multiple commands can be entered separated by ;, and all the normal shell operators such as &tl, > and && can be used. By default, any output from the command will be emailed to the owner of the Cron job. If you don't want this to happen, make sure that output is redirected to a file or /dev/null.
- Anything that you enter into the Input to command field will be fed to the command as input when it is run. If for example your command was mail email@example.com , anything entered into the field would be sent to that email address.
- In the When to execute section, choose the times and dates on which you want the command to execute. For each of the Minutes, Hours, Days, Months and Weekdays options you can choose multiple times or dates, or select the All option. For example, to have a job executed at 3:15am every Monday and Friday, you should change the Minutes option to Selected and select 15, change the Hours option to Selected and select 3, and the Weekdays option to Selected and select Monday and Friday. The Days and Months options would remain on All.
- Click the Create button to add the new Cron job. Assuming there are no errors in your selections, you will be returned to the main page of the module and your new job should appear next to its owner.
The Cron job creation form
Editing a Cron job
Existing Cron jobs, including those created by users, through
Webmin or included with your operating system, can be edited
and re-scheduled using this module. Be careful when editing
jobs that came as part of your distribution though, as some perform
important tasks like truncating webserver, mail and login log
files so that they do not use up all of your disk space.
To edit an existing job, the steps to follow are :
- On the main page of the module, click on the command for the job that you want to edit. This will take you to the module editing form, which is similar to the image above.
- Change any of the details of the job, including the user, command, active status and execution times and dates.
- When done, click the Save button, and you will be returned to the main page of the module.
Existing Cron jobs can be deleted by following the steps above,
but clicking the Delete
button instead of Save
. You can also
force the immediately execution of a job by clicking the *Run
Now* button on the edit page, which will execute the command and
display any output that it produces.
Controlling users' access to Cron
The Scheduled Cron Jobs module can also be used to control access
to the crontab command by Unix users at the command line. This
can be useful if you all un-trusted users to login to your system,
and want to prevent some of them from setting up Cron jobs to run
commands and take up CPU time when they are not logged in. Usually
by default, all users will have the ability to create Cron jobs,
but to change that the steps to follow are :
- At the bottom of the module's main page, click on the Control user access to cron jobs link. This will take you to a form for entering the usernames of users who can or cannot use Cron.
- To grant access to all users, select the Allow all users option. To grant access to only some users, select the Allow only listed users option and enter their usernames into the text field. To give access to all except some users, select the Deny only listed users option and enter the usernames of the people that you want to deny access to into the text field.
- When done, click the Save button.
If a user has been denied access to Cron, you will no longer be able
to use the module to create, edit or delete jobs belonging to him.
However, existing jobs may continue to execute!
Module access control options
As described in WebminUsers
, it is possible to use the Webmin Users
module to control which Unix users a Webmin user can edit Cron
jobs for. To set this up, you must first grant the user access to
the module, then follow these steps :
- In the Webmin Users module, click on Scheduled Cron Jobs next to the name of the user that you want to restrict.
- Change the Can edit module configuration? field to No, so that the user cannot edit the commands that Webmin calls to create and edit jobs.
- Switch the Can edit cron jobs for field from All users to one of the other options. The most commonly used is Users with UID in range, which allows you to enter a minimum and maximum UID into the fields next to it. Never allow an un-trusted user access to the Cron jobs of system users like root or bin, as this will clearly give him full access to your system and so defeat any other Webmin access control.
- Set the Can control user access to cron? *field to *No, so that the Webmin user cannot stop users outside his control using Cron.
- Click the Save button at the bottom of the page to make the access control active.
Configuring the Scheduled Cron Jobs module
Most of the module settings that you can view by clicking on the
link on the main page are set by default to match
the installed operating system, and vary rarely need to be changed.
However, there is one field that effects the module's user interface,
shown in the table below :
Other operating systems
Cron is available on almost all Unix systems, with very similar
capabilities. That means that this module appears almost identical
on all operating systems, with only a couple of minor differences.
On some, there is no Input to command
field available for when
creating or editing a job. On others, when controlling which
users have access to Cron the default Allow all users
will be replaced with Allow all users except root
or *Deny all
Internally, other operating systems use different directories
for storing Cron jobs - Solaris for example uses
on Linux. Most other systems do not
However, when using Webmin you do not have to bother about these
differences, as it knows about the paths used by other Unix variants
and displays all Cron jobs using the same interface, no matter
which file they are stored in.
The Scheduled Commands module
At jobs (called Scheduled Commands by Webmin) are similar to
Cron jobs, but instead of executing repeatedly on a schedule
run only once at a specified date and time. Unlike Cron jobs, they
can be configured to execute in a specific directory instead
of the user's home directory. Scheduled commands also keep track
of the environment variables that were set when created, and
make them available to the command when it runs.
Normally the at command is used to create At jobs, the atq command
to list them and the atrm command to remove them. On Linux, the
is used to store jobs, one per file.
The daemon process atd which runs all the time in background checks
these files and runs them at the appropriate times. After a job
is run, it is automatically deleted as it is no longer needed.
The Webmin module for creating and deleting At jobs is called
Scheduled Commands, and can be found under the System category.
When you enter it, the main page will display a list of commands
that are waiting to be run (assuming there are any), and a form
for adding a new command. The image below shows an example.
The Scheduled Commands module
Any of the commands shown on the main page can be viewed in more
detail by clicking on its Job ID
. This will take you to a page
that shows all the full shell script that will be run when the command
executes, including all environment variables. For this page
you can cancel the command before it gets a chance to run by clicking
the Cancel this command
Creating a new scheduled command
A new command that executes at the time and as the user of your choice
can be created by following these steps :
- On the main page of the module in the New scheduled command form, enter the name of the user that you want the command to run as into the Run as user field.
- Fill in the Run on date and Run at time fields with the date and time that the command is to run at.
- Set the Run in directory field to whatever directory you want the command to run in.
- In the Commands to execute text box, enter as many shell commands as you want, one per line.
- When done, click the Create button. The page will be refreshed and your new command will appear on the list at the top of the page.
Scheduled commands created from within Webmin will use environment
variables set by Webmin itself, which are not be the same as the
variables that would have been set if the command was created
by its owner at the shell prompt.