View Source: CronNotes - Waikato Linux Users Group

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!!! Introduction

Scheduling under Linux is provided by the cron(8) daemon. It reads a file called a crontab(5), but you don't necessarily need to edit any cron table directly.

!!! Running commands at standard intervals

On most systems, the easiest thing you can do if you simply want your command to be run once an hour, or once a day, or once a week, etc, is to put a script in the corresponding one of the <tt>/etc/cron.{hourly,daily,weekly,monthly}</tt> etc directories. This is a simple [Shell] script, not a crontab(5) file. On most systems, all the “daily” scripts will be run sometime in the wee hours, such as 4am.

Something to note is that this is usually done by a script called <tt>run-parts</tt> which has strange restrictions on what it considers a valid filename. The Linux Standard Base seems to be to blame for this: [LSB: Cron jobs|http://www.linuxbase.org/spec/book/LSB-generic/LSB-generic/sysinit.html]. DebianLinux' version of <tt>run-parts</tt> is even worse, defaulting to their legacy scheme which only allows ~[A-Za-z0-9_-] chars! This means it will silently ignore any scripts that contain a dot in the name, which is a horrible bug.

!!! Running commands at custom times/intervals

If you need to run commands at custom times or intervals, you need to know the crontab(5) line format described in the ManPage. Read that now, if you don't know about it yet.

The main, system wide crontab(5) is <tt>/etc/crontab</tt>, but on most LinuxDistribution~s, there is a <tt>/etc/cron.d</tt> directory where you should put single line crontab files, eg:

<verbatim>
# /etc/cron.d/exim: crontab fragment for exim
# Run queue every 15 minutes
08,23,38,53 * * * *     mail   if [ -x /usr/sbin/exim -a -f /etc/exim/exim.conf ]; then /usr/sbin/exim -q ; fi
</verbatim>

This will be run every 15 minutes (8 past the hour, 23 past the hour etc) on every hour, every day, etc, as user mail. The initial fields are delimited by spaces, up to the the command (<tt>if ~[ ... ] ; then ... ; fi</tt>), which is taken as a whole to the end of the line. (The <tt>if</tt> bit is a sanity check to ensure a runnable [Exim] binary and an <tt>exim.conf</tt> file exist.)

Users can also have their own cron tables, with commands run under their respective [UID]. These should be manipulated using the crontab(1) command. <tt>crontab -e</tt> will launch your configured interactive TextEditor on your user crontab, which generally resides under <tt>/var/spool/cron</tt>.

!!! Running commands at standard intervals, revisited, and more

With Vixie Cron (used in [*BSD] and some [LinuxDistribution]s such as [Debian] and RedHat) you can use several special keywords instead of a time specification. This [FreeBSD 4.1 manpage | http://www.freebsd.org/cgi/man.cgi?query=crontab&apropos=0&sektion=5&manpath=FreeBSD+4.1-RELEASE&format=html], where this feature first appears, lists the following keywords:

<?plugin OldStyleTable
|^ string            |^ meaning
| <tt>@reboot</tt>   | Run once, at startup.
| <tt>@yearly</tt>   | Run once a year, <tt>0 0 1 1 *</tt>.
| <tt>@annually</tt> | (sames as @yearly)
| <tt>@monthly</tt>  | Run once a month, <tt>0 0 1 * *</tt>.
| <tt>@weekly</tt>   | Run once a week, <tt>0 0 * * 0</tt>.
| <tt>@daily</tt>    | Run once a day, <tt>0 0 * * *</tt>.
| <tt>@midnight</tt> | (same as @daily)
| <tt>@hourly</tt>   | Run once an hour, <tt>0 * * * *</tt>.
?>


<tt>@reboot</tt> is particularly interesting, as cron will run such a command at system startup. This allows regular users to start their own daemons such as fetchmail(1) at boot time.

!!!Troubleshooting
!!Command isn't being run properly
* cron uses a minimal PATH EnvironmentVariable. Explicitly set the PATH variable if you want anything not in /usr/bin and /bin (+ maybe /usr/local?)
* You have a "%" symbol in your command, and cron treats this in a special fashion. From the crontab(5) ManPage:
>     The  entire  command  portion  of the line, up to a newline or % character, will be executed by /bin/sh or by the shell specified in the SHELL  variable of the crontab file.  Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters,  and  all  data  after the first % will be sent to the command as standard input. There is no way to split a  single  command  line  onto multiple lines, ala the shell’s trailing "\".
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Last edited on Tuesday, December 20, 2005 7:34:28 am by JohnMcPherson
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