Cron is a daemon that runs scheduled tasks based on input from the command crontab. It accomplishes this task by waking up every minute and checking to see if there are any cron-jobs to run in any of the user crontabs.

There are a few cron implementations for you to choose from in Portage. All of them offer a similar interface, namely the use of crontab or a similar command. There is also a related utility called Anacron which is meant to work with cron on systems that are not continuously running.

It is also worth noting that all of the available cron packages depend on sys-process/cronbase. This package is not technically depended on by any of the cron packages, but it does provide cron-like functionality that most users can appreciate.

Before we get started working with cron, you will have to choose which implementation you want to use. For your convenience, I have collected information about each one below.

Vixie cron is a full featured cron implementation based on SysV cron. Each user has his own crontab and is allowed to specify environment variables within that crontab. Unlike the other cron variants, it also offers support for SELinux and PAM. It supports fewer architectures than Dcron, but more than Fcron.

Features of sys-process/vixie-cron:

• Support for SELinux
• Support for PAM /etc/security/limits.conf
• Setting of environment variables in crontabs (PATH, SHELL, HOME, etc.)
• Each user can have his own crontab; access is controlled by cron.allow and cron.deny

Dcron aims to be a simple, elegant and secure implementation of cron. It does not allow the specification of environment variables in crontabs and all cron-jobs are run from /bin/sh. Like Vixie cron, each user has his own crontab.

Features of sys-process/dcron:

• Fast, simple and free of unnecessary features
• Access to crontab is limited to the cron group, i.e. it doesn't rely on any external faculties

Fcron aims at replacing Vixie cron and Anacron. It is designed to work on systems that are not continuously running and it is packed with extra features. It has job startup constraints, job serialization controls, the ability to assign nice values to jobs and the ability to schedule jobs to run at system startup. See fcron's home page for more information.

Features of sys-process/fcron:

• Designed to work on systems that are not continuously running, i.e. it can run a job after restarting if it was missed
• Setting of environment variables and many other options in crontabs
• Each user can have his own crontab, access is controlled by cron.allow and cron.deny
• Enhanced crontab syntax with support for many new features

bcron is a new cron system designed with secure operations in mind. To do this, the system is divided into several separate programs, each responsible for a separate task, with strictly controlled communications between them. The user interface is a drop-in replacement for similar systems (such as vixie-cron), but the internals differ greatly. For more information, see the bcron homepage at http://untroubled.org/bcron/.

Features of sys-process/bcron:

• Drop-in replacement for vixie-cron
• Multiprocess design
• Native daylight savings time support

Anacron is not a cron daemon, it is something that usually works in conjunction with one. It executes commands at intervals specified in days and it does not assume that the system is running continuously; it will run jobs that were missed while the system was down. Anacron usually relies on a cron daemon to run it each day.

Select the cron implementation you like the most, and then emerge it.

# emerge dcron
# /etc/init.d/dcron start
# rc-update add dcron default

Optionally, if you have not installed Fcron, you may also want to install Anacron.

# emerge anacron
# /etc/init.d/anacron start
# rc-update add anacron default

The post install messages from some of these cron packages tell you to run crontab /etc/crontab. The /etc/crontab file is your system crontab. A cron installation can use it in conjunction with sys-process/cronbase to run the scripts in /etc/cron.{daily,hourly,weekly,monthly}. Note that only Vixie-cron schedules jobs in /etc/crontab automatically. Dcron and Fcron users will need to run crontab /etc/crontab every time they make changes to /etc/crontab.

Please note that jobs scheduled in the system crontab might not show up in the list of cron-jobs displayed by crontab -l.

Of course, you can choose not to use any system crontab at all. If you chose Dcron or Fcron, do not run crontab /etc/crontab. If you chose vixie-cron or bcron, you should comment all lines in /etc/crontab.

# sed -i -e "s/^/#/" /etc/crontab

If you want users other than root to have access to the cron daemon, you'll want to read this section, otherwise, you can proceed to the next section, scheduling cron-jobs.

No matter which cron package you use, if you want to allow a user to use crontab, he will first have to be in the cron group. As an example, if you wanted to add the user wepy to the cron group you would run:

# gpasswd -a wepy cron

If you're using Dcron, that's all you have to do to give a user access to crontab. Dcron users may proceed to the next section scheduling cron-jobs, all others will want to keep reading.

If you're using Fcron, you'll want to edit /etc/fcron/fcron.deny and /etc/fcron/fcron.allow. The most secure way is to first deny everyone in /etc/fcron/fcron.deny, and then explicitly allow users in /etc/fcron/fcron.allow.

all

Now, say we have a user wepy who should be able to schedule his own cron-jobs. We would add him to /etc/fcron/fcron.allow as follows:

wepy

If you chose Vixie cron, you'll probably just want to edit /etc/cron.allow.

For example, if you wanted to allow access to the user wepy, you would add him to /etc/cron.allow as follows:

wepy

The process of editing crontabs is different for each package, but they all support the same basic set of commands: adding and replacing crontabs, editing crontabs, deleting crontabs, and listing cron-jobs in crontabs. The following list shows you how to run those commands for each package.

Before we can use any of these commands though, you first need to understand the crontab itself. Each line in a crontab needs to specify five time fields in the following order: the minutes (0-59), hours (0-23), days of the month (1-31), months (1-12), and days of the week (0-7, Monday is 1, Sunday is 0 and 7). The days of the weeks and months can be specified by three-letter abbreviations like mon, tue, jan, feb, etc. Each field can also specify a range of values (e.g. 1-5 or mon-fri), a comma separated list of values (e.g. 1,2,3 or mon,tue,wed) or a range of values with a step (e.g. 1-6/2 as 1,3,5).

That sounds a little confusing, but with a few examples, you will see that it is not as complicated as it sounds.

# Run /bin/false every minute year round
*     *     *     *     *        /bin/false

# Run /bin/false at 1:35 on the mon,tue,wed and the 4th of every month
35    1     4     *     mon-wed  /bin/false

# Run /bin/true at 22:25 on the 2nd of March
25    22    2     3     *        /bin/true

# Run /bin/false at 2:00 every Monday, Wednesday and Friday
0     2     *     *     1-5/2    /bin/false

To test what we have just learned, let's go through the steps of actually inputting a few cron-jobs. First, create a file called crons.cron and make it look like the this:

$ nano crons.cron
#Mins  Hours  Days   Months  Day of the week
10     3      1      1       *       /bin/echo "I don't really like cron"
30     16     *      1,2     *       /bin/echo "I like cron a little"
*      *      *      1-12/2  *       /bin/echo "I really like cron"

Now we can add that crontab to the system with the "new command" from the table above.

# crontab crons.cron

To verify the cron-jobs you scheduled, we'll use the proper list command from the table above.

# crontab -l

You should see a list resembling crons.cron, if not, maybe you used the wrong command to input your new crontab.

This crontab should echo "I really like cron" every minute of every hour of every day every other month. Obviously you would only do that if you really liked cron. The crontab will also echo "I like cron a little" at 16:30 every day in January and February. It will also echo "I don't really like cron" at 3:10 on the January 1st.

If you are using Anacron, you should keep reading this section. Otherwise, proceed to the next section on editing crontabs.

Anacron users will want to edit /etc/anacrontab. This file has four fields: the number of days between each run, the delay in minutes after which it runs, the name of the job, and the command to run.

For example, to have it run echo "I like anacron" every 5 days, 10 minutes after Anacron is started, you would have:

5 10 wasting-time /bin/echo "I like anacron"

Anacron exits after all of the jobs in anacrontab are done, so if we want it to check these jobs every day, we will need to use cron. The instructions at the end of the next section tell you how to do that.

Let's be realistic though, you don't want your system telling you how much you like cron every minute. As a step forward, let's remove that crontab using the corresponding remove command from the table above. We will also list the cron-jobs after, just to make sure it worked.

# crontab -d
# crontab -l

You should see no cron-jobs in the output from crontab -l. If you do see jobs listed, that means we failed to remove the crontab, and that you should make sure that you used the correct remove command for your cron package.

Now that we have a clean slate, let's put something useful into the root crontab. Most people will want to run updatedb on a weekly basis to make sure that mlocate works properly. To add that to your crontab, let's first edit crons.cron again so that it looks like the following:

22 2 * * 1    /usr/bin/updatedb

That would make cron run updatedb at 2:22 A.M. on Monday morning every week. You should now input the crontab with the proper new command from the table above, and check the list again.

# crontab crons.cron
# crontab -l

Now let's say that you also want to add emerge --sync to your daily schedule. You could do this by first editing crons.cron and then using crontab crons.cron just as we did before, or you could use the proper edit command from the table above. This gives you a way to edit your user's crontab in situ, without depending on external files like crons.cron.

# crontab -e

That should open your user's crontab with an editor. We want to have emerge --sync run every day at 6:30 A.M., so we'll make it look something like this:

22 2 * * 1    /usr/bin/updatedb
30 6 * * *    /usr/bin/emerge --sync
(if you're using anacron, add this line)
30 7 * * *    /usr/sbin/anacron -s

Again, check the cron-jobs list as we did in the previous examples to make sure the jobs are scheduled. If they are all there, then you're all set.

As mentioned earlier, all of the available cron packages depend on sys-process/cronbase. The cronbase package creates /etc/cron.{hourly,daily,weekly,monthly}, and a script called run-crons. You might have noticed that the default /etc/crontab contains something like this:

*/15 * * * *     test -x /usr/sbin/run-crons && /usr/sbin/run-crons
0  *  * * *      rm -f /var/spool/cron/lastrun/cron.hourly
0  3  * * *      rm -f /var/spool/cron/lastrun/cron.daily
15 4  * * 6      rm -f /var/spool/cron/lastrun/cron.weekly
30 5  1 * *      rm -f /var/spool/cron/lastrun/cron.monthly

To avoid going into much detail, we can just assume that these commands will effectively run your hourly, daily, weekly and monthly scripts. This method of scheduling cron-jobs has some important advantages:

• They will run even if your computer was off when they were scheduled to run
• It is easy for package maintainers to place scripts in those well defined places
• You know exactly where your cron-jobs and your crontab are stored, making it easy for you to backup and restore this part of your system

If you're having problems getting cron to work properly, you might want to go through this quick checklist.

• Is cron running? Run ps ax | grep cron and make sure it shows up!
• Is cron working? Try: * * * * * /bin/echo "foobar" >> /file_you_own and make sure it works
• Is your command working? Try: * * * * * /bin/foobar > /file_you_own 2>&1 and look for errors in /file_you_own
• Can cron run your job? Check the cron log, usually /var/log/cron.log or /var/log/messages for errors
• Are there any dead.letters? cron usually sends mail when there's a problem; check your mail and also look for ~/dead.letter.

Remember, each cron package is different and the range of features varies greatly. Be sure to consult the man pages for crontab, fcrontab or anacrontab, depending on what you use.

Good luck!