I've setup a cron job that greps a file every five minutes and then writes (appends) the grep output/result to another file:

grep "monkey" zoo.log | tail -1 >> cron-zoo-log

Is there any way I can add the date and time (timestamp) to the cron-zoo-log file for each time a new line was added?

At the moment the cron-zoo-log just looks like this:

monkey house
monkey car

Could I get it to be:

monkey house 2009-01-01:16:54:00
monkey car 2009-01-01:17:01:00

Thank you.

Excellent.

That is putting the date before the grep result - is there a way to switch it around and maybe separate them by a comma?

sure:
I'll let you change the format of 'date' yourself.

Thanks but that is now doing:

,Thu Jun 25 13:04:59 BST 2009count: 3

Where 'count 3' should have four words infront of it!

I've tried changing the order of sed "s/$/,$(date)/" around but that doesn't seem to help.

edit: doesnt seem to be writing the date when its done as a cronjob. Just has $(date),monkey

hm....... strange
try: sed "s/\$/,$(date)/"
'crontab' runs in Bourne shell - the '$(date)' is ksh/bash specific. Change '$(date)' to '`date`' in crontab - or write your own ksh/bash wrapper and call it from cron.