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All Unix systems have a mechanism for running certain applications
regularly. The mechanism is through a daemon called crond.
Debian uses cron in a very neat and stylised fashion. Scripts to be
run regularly are maintained in one of /etc/cron.daily/,
/etc/cron.weekly/, or /etc/cron.monthly/. The entries in
/etc/crontab then simply run any scripts in these directories
on a regular basis. The daily scripts, by default, are run at
6:25am each day. The weekly scripts are run each Sunday at
6:47am and the monthly scripts are run at 6:52am on the first
day of each month. The default /etc/crontab essentially
contains just the following lines:
25 6 * * * root test -e /usr/sbin/anacron || run-parts --report /etc/cron.daily
47 6 * * 7 root test -e /usr/sbin/anacron || run-parts --report /etc/cron.weekly
52 6 1 * * root test -e /usr/sbin/anacron || run-parts --report /etc/cron.monthly
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The first line is read as: 25 minutes past 6am on every day of the
month and every month of the year and every day of the week as user
root run the rest of the line. The ``rest of the line'' tests
to see whether anacron is installed and if not runs the
scripts found in /etc/cron.daily/. If anacron is
installed then it is used instead to ensure the scripts are run. The
anacron package is a replacement for cron that
does not assume the local host is always powered on (Unix and
GNU/Linux systems rarely require rebooting and are happy to remain
powered on indefinitely) and will ensure the regular scripts still get
run appropriately when the local host is powered on.
If the default times for cron don't suit (e.g., you like to
work early in the morning and get annoyed by the sudden disk whirling
each morning at 6:25 am) then simply change the times in
/etc/crontab. Unlike traditional versions of cron,
the newer versions do not require the running of the
crontab command to install the new version when you edit
/etc/crontab.
A user can easily add entries to their own crontab (stored in
/var/spool/cron/crontabs through the crontab
command. Use the -l option to list the current contents and
the -e option to edit the crontab file.
Copyright © 1995-2006 [email protected]
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