Scheduling III

Summary:

• anacron
• Interactive Schedulers

anacron

If you look at '/etc/crontab', the control file for the cron scheduling daemon, you will notice that by default all daily, weekly and monthly system tasks are run between 4 and 5 o'clock in the morning. If your computer isn't turned on during this time, the cron jobs won't be executed.
One solution to this problem would be changing the hour fields to a time when your system is most likely running (I used to change them to 2 pm.).
Another solution would be running the 'anacron' system service.

First you should make sure that 'anacron' is up and running, either via the Mandrake Control Center (System - Services), or by making a shortcut via the command line (as 'root'):

~# chkconfig ––list anacron

to check and

~# chkconfig anacron on

to enable the service.

Note that 'anacron' isn't meant as a replacement for 'cron', it's an add-on.

'anacron', the 'anachronistic command scheduler', uses relative time scales ('once a day / week / month') instead of absolute time scales ('4:15am, Feb 15'). So, even if you 'miss' a particular time or date for which a job is scheduled, it will be executed shortly after you've booted the system. It works like this:

• 'anacron' looks up the 'timestamps' for its jobs in '/var/spool/anacron'. * If according to a time stamp a job is due, 'anacron' will run it. * 'anacron' updates the time stamp for the job.

1 5 cron.daily run-parts /etc/cron.daily
7 10 cron.weekly run-parts /etc/cron.weekly
30 15 cron.monthly run-parts /etc/cron.monthly

• The first field denotes the period in days:

  1

  is daily,

  7

  every 7 days (weekly) etc. * The second field denotes the delay for jobs to be executed after anacron has started in minutes. So,

  5

  says 'run scheduled jobs 5 minutes after start'. This mechanism prevents queued jobs from being run all at the same time. * The third field contains an identifier. 'anacron' uses this identifier for its timestamps and in messages. * The fourth field finally lists the command to execute.

Caveats And Limitations

'anacron' does not replace 'cron' because due to its very nature, it can't execute minutely or hourly scheduled jobs. It doesn't provide a mechanism for running jobs under different users, i.e. all jobs are run as 'root'.

Example: Getting Your New Mail After Boot

You want your computer to get your new mail after having booted it in the morning. Since you boot your machine at different times, a cron job won't do.
You need a properly configured mail daemon, a mail retrieving utility like Fetchmail and a scripted dial-up connection.

Add this command (one line) to '/etc/anacrontab':

1 3 getmail pppd call ISP && fetchmail -s && killall pppd

Since 'anacron' executes all commands as 'root', the '.fetchmailrc' file must have the permissions -rw––- root root set and should ideally reside in 'root's' home directory. Another possibility would be putting the commands into a script and putting su user before the line which starts 'fetchmail'.

section index

Interactive Schedulers

'at', 'cron' and 'anacron' do not allow to start programs which require user input, like X applications. This is because they don't have a shell to start a program from. 'sleep' can do that, but only within a session.

Surprisingly enough there seems to be only one program currently which saves schedules to a file and thus allows to launch arbitrary interactive programs at a given time. This program is called 'rclock' and is part of the 'rxvt' package. If you find others, you are invited to tell me ;).

section index

Related Resources:

man anacron
man anacrontab
man rclock

Revision / Modified: Dec. 25, 2001
Author: Tom Berger

Legal: This page is covered by the GNU Free Documentation License. Standard disclaimers of warranty apply. Copyright LSTB and Mandrakesoft.