This is not the first time that I've had to set-up cron jobs [I've got several that date back a few years.] I think that having the ability to set-up scheduled tasks is really quite important. Therefore I thought I'd blog about it for future reference.

What is cron?

Cron is a daemon/service that executes shell commands periodically on a given schedule. Cron is driven by a crontab, a configuration file that holds details of what commands are to be run along with a timetable of when to run them.

Creating a crontab file

You can create a crontab file by entering the following terminal command:

crontab -e

Entering the above command will open a terminal editor [Ubuntu uses Nano by default] with a new blank crontab file [or it will open an existing crontab if you already have one.] You can now enter the commands to be executed [see syntax below] before saving the file and exiting the editor. As long as your entries were entered correctly your commands should now be executed at the times/dates you specified. You can see a list of active crontab entries by entering the following terminal command:

crontab -l

Crontab syntax

A crontab file has six fields for specifying minute, hour, day of month, month, day of week and the command to be run at that interval. See below:

*     *     *     *     *  command to be executed
-     -     -     -     -
|     |     |     |     |
|     |     |     |     +----- day of week (0 - 6) (Sunday=0)
|     |     |     +------- month (1 - 12)
|     |     +--------- day of month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)

Crontab examples

Writing a crontab file can be a somewhat confusing for first time users [and the above table probably doesn't help much!] Therefore I've listed below some crontab examples:

* * * * * <command> #Runs every minute
30 * * * * <command> #Runs at 30 minutes past the hour
45 6 * * * <command> #Runs at 6:45 am every day
45 18 * * * <command> #Runs at 6:45 pm every day
00 1 * * 0 <command> #Runs at 1:00 am every Sunday
00 1 * * 7 <command> #Runs at 1:00 am every Sunday
00 1 * * Sun <command> #Runs at 1:00 am every Sunday
30 8 1 * * <command> #Runs at 8:30 am on the first day of every month
00 0-23/2 02 07 * <command> #Runs every other hour on the 2nd of July

As well as the above there are also special strings that can be used:

@reboot <command> #Runs at boot
@yearly <command> #Runs once a year [0 0 1 1 *]
@annually <command> #Runs once a year [0 0 1 1 *]
@monthly <command> #Runs once a month [0 0 1 * *]
@weekly <command> #Runs once a week [0 0 * * 0]
@daily <command> #Runs once a day [0 0 * * *]
@midnight <command> #Runs once a day [0 0 * * *]
@hourly <command> #Runs once an hour [0 * * * *]

Multiple commands

A double-ampersand "&&" can be used to run multiple commands consecutively. The following example would run command_01 and then command_02 once a day:

@daily <command_01> && <command_02>

Disabling email notifications

By default a cron job will send an email to the user account executing the cronjob. If this is not needed put the following command at the end of the cron job line:

>/dev/null 2>&1

Specifying a crontab file to use

As mentioned at the top of this post, you can create a new crontab file with the "crontab -e" command. However, you may already have a crontab file, if you do you can set it to be used with the following command:

crontab -u <username> <crontab file>

Therefore the following command…

crontab -u tux ~/crontab

…would set Tux's crontab file to that of the file named "crontab" residing in Tux's home directory.

Removing a crontab file

To remove your crontab file simply enter the following terminal command:

crontab -r

Further information

Refer to the man page for further information about crontab. Enter the terminal command:

man crontab

External links

Some external links for your browsing pleasure:

• Cron article on Wikipedia
• Ubuntu Documentation - CronHowto
• Debian Help - Scheduling Backup Jobs using at and crontab in debian
• Matt Cutts - How to fetch a url with curl or wget silently
• Sitepoint - Getting PHP and Cron to Work Together

I think that pretty much covers the subject of cron jobs and crontab. Please feel free to comment if I've missed anything/made any boobs.

Tags: code, crontab, linux, php, ubuntu