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Cron Tips

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Number of fields specifying time to run (5) is the same as number of fingers on the hand ;-)

Crontab tips

Command output

Can be mailed to the crontab owner, can be mailed to a person other than the crontab owner or the feature can be turned off and no mail will be sent at all

You can hardlink at.allow to cron.allow and at.deny to cron.deny to avoid synchronization problem.

When you want to test that your cron job works as intended you can set invocation schedule a couple of minutes in the future and then check the results.

Use @reboot  to notify you when your server gets rebooted...

It is possible to run Gui applications via cronjobs.

This can be done by telling cron which display to use.

00 06 * * * env DISPLAY=:0 gui_appname

The env DISPLAY=:0  portion will tell cron to use the current display (desktop) for the program "gui_appname".

And if you have multiple monitors, don't forget to specify on which one the program is to be run. For example, to run it on the first screen (default screen) use :

00 06 * * * env DISPLAY=:0.0 gui_appname

The env DISPLAY=:0.0  portion will tell cron to use the first screen of the current display for the program "gui_appname".

How to Use percentage sign (%) in a crontab entry

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. Hat tip to
... A trivial example is:
* * * * * cat - % another minute has passed
This would output the text
another minute has passed
After the first %, all other %s in a crontab entry indicate a new line. So a slightly different trivial example is:
* * * * * cat - % another % minute % has % passed 
This would output the text
Note how the % has been used to indicate a new line.

The problem is how to use a % in a crontab line to as a % and not as a new line. Many manuals will say escape it with a \. This certainly stops its interpretation as a new line but the shell running the cron job can leave the \ in. For example:

* * * * * echo '\% another \% minute \% has \% passed'
would output the text
\% another \% minute \% has \% passed
Clearly, not what was intended.

A solution is to pass the text through sed. The crontab example now becomes:

* * * * * echo '\% another \% minute \% has \% passed'| sed -e 's|\\||g'
This would output the text
% another % minute % has % passed
which is what was intended.

This technique is very useful when using a MySQL command within a crontab. MySQL command can often have a % in them. Some example are:

SET @monyy=DATE_FORMAT(NOW(),"%M %Y")
SELECT * FROM table WHERE name LIKE 'fred%'
So, to have a crontab entry to run the MySQL command
mysql -vv -e "SELECT * FROM table WHERE name LIKE Fred%'" member_list
would have to appear in the crontab as
echo "SELECT * FROM table WHERE name LIKE 'Fred\%'" | sed -e 's|\\||g' | mysql -vv member_list
Pulling the crontab entry apart there is:
the echo command sends the MySQL command to STDOUT where it is piped into
sed which removes any back slashes before sending the output to STDOUT where it is piped into
the mysql command processor which reads its commands from STDIN

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Re cron usage and squid daily log rotate

This is all documented very nicely in the cron manpage:

$man cron
Cron should be started from /etc/rc.d/init.d or /etc/init.d

Cron  searches  /var/spool/cron for crontab files which are named after
accounts in crontabs found are loaded into memory.  
Cron also  searches for /etc/crontab and the files in the directory,
which are in a different format (see crontab(5) ).  Cron then wakes up
every minute, examining  all  stored crontabs, checking each command to
see if it should be run in the current minute.  When  executing
commands, any  output  is mailed  to the owner of the crontab (or to the
user named in the MAILTO environment variable in the crontab, if such

If we look at /etc/crontab we see how each of the
cron.{hourly,daily,weekly,monthly} crontabs are executed:

$cat /etc/crontab

# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

And if we look at the file in /etc/cron.daily we see a crontab for

Because cron.daily is run every morning at 4:02am we can expect
logrotate to run around that time.

The logrotate script say to move the squid log files out of the way
/var/log/squid/access.log {
    rotate 5
... <snipped other logfile entries>

/var/log/squid/store.log {
    rotate 5
# This script asks squid to rotate its logs on its own.
# Restarting squid is a long process and it is not worth
# doing it just to rotate logs
      /usr/sbin/squid -k rotate

The final logrotate statement above tells squid to rotate the log files:
/usr/sbin/squid -k rotate

For other ways of rotating the logs the Squid FAQ is indespensible:

and/or here:



On Mon, 2009-03-02 at 10:24 +0200, a bv wrote:
> Hi all,
> I would like to daily logrotate the squid log files at everyday at
> 00:00(midnight) at some RHEL 5.x systems. I havent used cron things
> for a long time so i forgot all about it. Also i have the
> logrotate.conf  and /etc/logrotate.d/squid file.
> i have tried to add a /usr../sbin/rotate -k thing as a cron job but it
> seem to be unsuccessfull.  Googling makes some mind mixing samples
> etc. also using crontab -e command brings an empty file, other than
> the one i find crontab.
> So is there someone who will shortly explain the cron related files
> and usage , also making the squid rotate its daily logs easily.   Most
> of the resources (including books) makes confusing the minds at these
> simple subjects. Couldnt see  a step by step guide.
> Regards



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