Having cron troubles with percent (%) signs in crontabs?

I recently wrote a cronjob which would save off my Ubuntu home server's current packages list every night to a dated file so that I have a history of the changes that I've made to my system. The entry in my crontab looked something like this:

# package lists (5am)
0 5 * * * dpkg -l > /var/log/packages/`date +%F`.txt

After a couple of days I decided to take a look at the files generated so far but to my disappointment I found none. After a quick search for dpkg in /var/log I found my problem reported in syslog:

... /USR/SBIN/CRON[28508]: ... CMD (dpkg -l > /var/log/packages/`date +)

I noticed that the command recorded to the logs was up to '`date +' - the next character - the percent sign (%) - was causing some kind of issue. A quick search threw up the Wikipedia page for Crontab which mentioned:

Another common mistake is to use unescaped % in your command; you have to escape them

All I had to do was change % to be \%. My working crontab entry now looks like this:

# package lists (5am)
0 5 * * * dpkg -l > /var/log/packages/`date +\%F`.txt

The funny thing was that most of the 'proper' Linux documentation for cron and crontab do not mention anything about percent signs (%) - I'm finding that I'm going to Wikipedia for answers more and more these days...

Technorati Tags: Cron, Crontab, Percent, Linux, Andrew Beacock