topic Re: crontab entry automation in Operating System - Linux

crontab entry automation

Pat Peter — Tue, 02 May 2006 10:08:54 GMT

Hi,

I need to automate updating the cron entry in my server.

Is there any way in which I can add an entry to the end of the crontab without actually opening the cron file.

-Pat

Re: crontab entry automation

Patrick Wallek — Tue, 02 May 2006 10:13:08 GMT

You could do something like:

echo "5 4 * * 1 /dir/command_to_run" >> /var/spool/cron/crontabs/root

If you do that, you will then have to stop and restart the cron daemon for the change to take effect. Otherwise your job will not run.

Re: crontab entry automation

Steven E. Protter — Tue, 02 May 2006 10:13:48 GMT

Shalom Pat,

Yes,

You can echo directly to /var/spool/cron/crontab.root

This is not recommended by me but quite possible

echo "10 16 * * * /usr/local/bin/job 2>&1" >> /var/spool/cron/crontab.root

I don't do this, and you may need to start and stop the cron daemon.

Really the best way to go is to edit the file or a copy of he file.

Some will not run crontab -e.

I once had an operator come to me with apologies because a PC power supply failure wile editing a regular users crontab file caused it to be empty.

It was restored from backup, but messing with the file directly does risk that outcome.

SEP

Re: crontab entry automation

Peter Nikitka — Tue, 02 May 2006 10:16:58 GMT

Hi,

if you want a change without changing - that is not possible :-) .
If you mean a 'batch change' without opening an editor:

crontab -l >/tmp/crontab.save
cp /tmp/crontab.save /tmp/crontab.new
print '1 2 3 4 * new-cronjob' >>/tmp/crontab.new
crontab /tmp/crontab.new

You have '/tmp/crontab.save' as a backup if something goes wrong ...

Perhaps you better check if 'new-cronjob' does not already exists.

mfG Peter

Re: crontab entry automation

A. Clay Stephenson — Tue, 02 May 2006 10:32:10 GMT

Never, ever edit the crontab directly. If you do, cron won't have a clue that that has been done until cron is restarted or receives a SIGHUP. The correct procedure is to use the crontab command to extract the current cron entries and then to read them back in. Crontab also sends a SIGHUP (kill -1) to cron to trigger a re-read.

Something like this should work:

#!/usr/bin/sh

typeset TDIR=${TMPDIR:-/var/tmp}
typeset C1=${TDIR}/X${$}_1.cron

crontab -l > ${C1}
echo "15 23 * * * /new/command/goes_here # comment for new command" >> ${C1}
crontab < ${C1}

rm -f ${C1}

You should add a little error checking and probably get the new entry from the command line but that's the gist of it.

Re: crontab entry automation

Patrice Le Guyader — Tue, 02 May 2006 11:39:42 GMT

Demat Pat,

Another way to do it is to use dsau(Distributed Systems Administration Utility) if you have several servers. If you just have one it's not a good idea and should be a waste of time to configure it.

http://docs.hp.com/en/B2355-90950/ch03s04.html

If you can't use it directly due to some OS restriction, go directly to the site of the GNU product cfengine(http://www.cfengine.org/). This product deserve to work on it,it should verify and edit (if it need) the cron for you and restart or not the daemon. Cfengine can do much more, take a look.

Hope this helps
Kenavo
Pat