Cron is a system daemon used to execute desired tasks (in the background) at designated times. 

Commands that normally run with administrative privileges (i.e. they are generally run using sudo) should be added to the root crontab. To edit the root crontab enter:

 sudo crontab -e

Starting to Use Cron

To use cron for tasks meant to run only for your user profile, add entries to your own user's crontab file. To edit the crontab file enter:

crontab -e

Edit the crontab using the format described in the next sections. Save your changes. (Exiting without saving will leave your crontab unchanged.) To display the on-line help describing the format of the crontab file enter:

man 5 crontab

Commands that normally run with administrative privileges (i.e. they are generally run using sudo) should be added to the root crontab. To edit the root crontab enter:

 sudo crontab -e

Crontab Lines

Each line has five time-and-date fields, followed by a command, followed by a newline character ('\n'). The fields are separated by spaces. The five time-and-date fields cannot contain spaces. The five time-and-date fields are as follows: minute (0-59), hour (0-23, 0 = midnight), day (1-31), month (1-12), weekday (0-6, 0 = Sunday).

01 04 1 1 1 /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on January 1st plus every Monday in January.

An asterisk (*) can be used so that every instance (every hour, every weekday, every month, etc.) of a time period is used.

01 04 * * * /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on every day of every month.

Comma-separated values can be used to run more than one instance of a particular command within a time period. Dash-separated values can be used to run a command continuously.

01,31 04,05 1-15 1,6 * /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 01 and 31 past the hours of 4:00am and 5:00am on the 1st through the 15th of every January and June.

The "/usr/bin/somedirectory/somecommand" text in the above examples indicates the task which will be run at the specified times. It is recommended that you use the full path to the desired commands as shown in the above examples. Enter which somecommandin the terminal to find the full path to somecommand. The crontab will begin running as soon as it is properly edited and saved.

You may want to run a script some number of times per time unit. For example if you want to run it every 10 minutes use the following crontab entry (runs on minutes divisible by 10: 0, 10, 20, 30, etc.)

*/10 * * * * /usr/bin/somedirectory/somecommand

which is also equivalent to the more cumbersome

0,10,20,30,40,50 * * * * /usr/bin/somedirectory/somecommand

Cron also offers some special strings, which can be used in place of the five time-and-date fields:

• string	meaning
  @reboot	Run once, at startup.
  @yearly	Run once a year, "0 0 1 1 *".
  @annually	(same as @yearly)
  @monthly	Run once a month, "0 0 1 * *".
  @weekly	Run once a week, "0 0 * * 0".
  @daily	Run once a day, "0 0 * * *".
  @midnight	(same as @daily)
  @hourly	Run once an hour, "0 * * * *".

@reboot /path/to/execuable1

The above example will execute /path/to/executable1 when the system starts.

For more information on special strings enter "man 5 crontab".

Two Other Types of Crontab

The crontab files discussed above are user crontabs. Each of the above crontabs is associated with a user, even the root crontab, which is associated with the root user. There are two other types of crontab, with syntax as follows:

minute(s) hour(s) day(s)_of_month month(s) day(s)_of_week user command

Note that the only difference from the syntax of the user crontabs is that the line specifies the user to run the job as.

The first type is as follows. As mentioned above anacron uses the run-parts command and /etc/cron.hourly, /etc/cron.weekly, and /etc/cron.monthly directories. However anacron itself is invoked from the /etc/crontab file. This file could be used for other cron commands, but probably shouldn't be. Here's an example line from a fictitious /etc/crontab:

00 01 * * * rusty /home/rusty/rusty-list-files.sh

This would run Rusty's command script as user rusty from his home directory. However, it is not usual to add commands to this file. While an experienced user should know about it, it is not recommended that you add anything to /etc/crontab. Apart from anything else, this could cause a problem if the /etc/crontab file is affected by updates! Rusty could lose his command.

The second type is to be found in the directory /etc/cron.d. This directory can contain crontab files. The directory is often used by packages, and the crontab files allow a user to be associated with the commands in them.

Example: Instead of adding a line to /etc/crontab, which Rusty knows is not a good idea, he might well add a file to the directory /etc/cron.d with the name rusty, containing his cron line above. This would not be affected by updates but is a well known location.

When would you use these alternate crontab locations? Well, on a single user machine or a shared machine such as a school or college server, a user crontab would be the way to go. But in a large IT department, where several people might look after a server, then the directory /etc/cron.d is probably the best place to install crontabs - it's a central point and saves searching for them!

You may not need to look at /etc/crontab or /etc/cron.d, let alone edit them by hand. But an experienced user should perhaps know about them and that the packages that he/she installs may use these locations for their crontabs.

GUI Applications

It is possible to run gui applications via cronjobs. This can be done by telling cron which display to use.

00 06 * * * env DISPLAY=:0 gui_appname

The env DISPLAY=:0 portion will tell cron to use the current display (desktop) for the program "gui_appname".

And if you have multiple monitors, don't forget to specify on which one the program is to be run. For example, to run it on the first screen (default screen) use :

00 06 * * * env DISPLAY=:0.0 gui_appname

The env DISPLAY=:0.0 portion will tell cron to use the first screen of the current display for the program "gui_appname".

Note: GUI users may prefer to use gnome-schedule (aka "Scheduled tasks") to configure GUI cron jobs. In gnome-schedule, when editing a GUI task, you have to select "X application" in a dropdown next to the command field.

Note: In Karmic(9.10), you have to enable X ACL for localhost to connect to for GUI applications to work.

 ~$ xhost +local:
non-network local connections being added to access control list
 ~$ xhost
access control enabled, only authorized clients can connect
LOCAL:
...

Crontab Options

• The -l option causes the current crontab to be displayed on standard output.
• The -r option causes the current crontab to be removed.
• The -e option is used to edit the current crontab using the editor specified by the EDITOR environment variable.

After you exit from the editor, the modified crontab is checked for errors and, if there are no errors, it is installed automatically. The file is stored in /var/spool/cron/crontabs but should only be edited using the crontab command.

Allowing/Denying User-Level Cron

If the /etc/cron.allow file exists, then users must be listed in it in order to be allowed to run the crontab command. If the/etc/cron.allow file does not exist but the /etc/cron.deny file does, then users must not be listed in the /etc/cron.deny file in order to run crontab.

In the case where neither file exists, the default on current Ubuntu (and Debian, but not some other Linux and UNIX systems) is to allow all users to run jobs with crontab.

No cron.allow or cron.deny files exist in a standard Ubuntu install, so all users should have cron available by default, until one of those files is created. If a blank cron.deny file has been created, that will change to the standard behavior users of other operating systems might expect: cron only available to root or users in cron.allow.

Note, userids on your system which do not appear in /etc/shadow will NOT have operational crontabs, if you desire to enter a user in /etc/passwd, but NOT /etc/shadow that user's crontab will never run. Place an entry in /etc/shadow for the user with a * for the password crypt, eg:

joeuser:*:15169::::::

Further Considerations

Crontab commands are generally stored in the crontab file belonging to your user account (and executed with your user's level of permissions). If you want to regularly run a command requiring administrative permissions, edit the root crontab file:

sudo crontab -e

Depending on the commands being run, you may need to expand the root users PATH variable by putting the following line at the top of the root crontab file:

PATH=/usr/sbin:/usr/bin:/sbin:/bin

crontab -e uses the EDITOR environment variable. To change the editor to your own choice, just set that variable. You may want to set EDITOR in your .bashrc because many commands use this variable. For example, in order to set the editor to be nano (a very easy editor to use) add this line to .bashrc:

export EDITOR=nano

It is sensible to test that your cron jobs work as intended. One method for doing this is to set up the job to run a couple of minutes in the future and then check the results before finalising the timing. You may also find it useful to put the commands into script files that log their success or failure, for example:

echo "Nightly Backup Successful: $(date)" >> /tmp/mybackup.log

If your machine is regularly switched off, you may also be interested in at and anacron, which provide other approaches to scheduled tasks. For example, anacron offers simple system-wide directories for running commands hourly, daily, weekly, and monthly. Scripts to be executed in said times can be placed in /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/, and/etc/cron.monthly/. All scripts in each directory are run as root, and a specific order to running the scripts can be specified by prefixing the scripts' filenames with numbers (see the man page for run-parts for more details). Although the directories contain periods in their names, run-parts will not accept a file name containing a period and will fail silently when encountering them (bug #38022). Either rename the file or use a symlink (without a period) to it instead (see, for example, python + cron without login? andProblems with Hourly Cron Job).

Common Problems

Edits to a user's crontab and the cron jobs run are all logged by default to /var/log/syslog and that's the first place to check if things are not running as you expect.

If a user was not allowed to execute jobs when their crontab was last edited, just adding them to the allow list won't do anything. The user needs to re-edit their crontab after being added to cron.allow before their jobs will run.

Note that user-specific crontabs (including the root crontab) do not specify the user name after the date/time fields. If you accidentally include the user name in a user-specific crontab, the system will try to run the user name as a command.

Cron jobs may not run with the environment, in particular the PATH, that you expect. Try using full paths to files and programs if they're not being located as you expect.

The "%" character is used as newline delimiter in cron commands. If you need to pass that character into a script, you need to escape it as "\%".

If you're having trouble running a GUI application using cron, see the GUI Applications section below.

Crontab Example

Below is an example of how to setup a crontab to run updatedb, which updates the slocate database: Open a terminal, type "crontab -e" (without the double quotes) and press enter. Type the following line, substituting the full path of the application you wish to run for the one shown below, into the editor:

45 04 * * * /usr/bin/updatedb

Save your changes and exit the editor.

Crontab will let you know if you made any mistakes. The crontab will be installed and begin running if there are no errors. That's it. You now have a cronjob setup to run updatedb, which updates the slocate database, every morning at 4:45.

Note: The double-ampersand (&&) can also be used in the "command" section to run multiple commands consecutively, but only if the previous command exits successfully. A string of commands joined by the double-ampersand will only get to the last command if all the previous commands are run successfully. If exit error-checking is not required, string commands together, separated with a semi-colon (;).

45 04 * * * /usr/sbin/chkrootkit && /usr/bin/updatedb

The above example will run chkrootkit followed by updatedb at 4:45am daily - providing you have all listed apps installed. If chkrootkit fails, updatedb will NOT be run.

Source