Sometimes you'll be able to execute a Linux shell command fine from the command prompt, but fail when you use the same command from Cron. You might even have remembered to address the script/whatever correctly, and run Cron from the appropriate user, yet it might complain about the syntax of the actual command.
I've found that this is generally down to different shell interpreters being used. Although the documentation strongly implies that when a SHELL directive is not specified in a user-level Crontab, it will use the one specified in /etc/passwd, this doesn't seem to be the case.
I have a user with /bin/bash set-up, but it's clear in the output mailed by Cron that the SHELL is /bin/sh.
So you might have to explicitly set a SHELL= environment in your crontab.
(I don't think it helps when Ubuntu distro's jump between bash and dash, but that's probably another tale...)
Addendum
Another important note for CRONTAB entries. The CRON program treats '%' as a line terminator. If your commands are failing, and the email you get seems to show only part of the command line, you probably have a '%' in the command. You need to either escape the % with a preceding '\' (e.g. "date +\%e") or pipe the command through sed.
