Being on hardy, I had developed this evil habit of “sudo apt-get update” which I ‘d do now and then, hoping for a better updated, greater hardy or hoping for some miraculous update which would fix all my annoying hardy bugs. 
Even otherwise, we do miss our nifty Windows task scheduler. Or how about shutting down your internet connection at 8 in morning to save bandwidth. So here is the linux cute thing.
Cron is a simple scheduler that runs by default on most Linux distros, including Ubuntu. The daemon “crond” is responsible for reading tasks from a simple user file “crontab” (stored in /var/spool/cron/crontabs).
So lets create some useful automated tasks. Creating a task is as simple as:
crontab -e -u
This will start editing, the user specific crontab file using system default editor which is nano in Ubuntu. You edit your own crontab, omit -u crontab -e. To run commands which might require sytem level permissions, edit root crontab,
sudo crontab -e -u root
or
su
crontab -e
To use a different editor you could simply use place export EDITOR=vim in your .bashrc. Or you could even do EDITOR=vim && crontab -e. You can also change system wide default editor you should update the link for /usr/bin/editor -> to whatever editor you want.
Ok now to add some tasks, The first line looks something like this:
# m h dom mon dow command
m -> minutes (0-59)
h -> hour (0-24)
dom -> date of month (1-31)
mon -> month (1-12)
dow -> day of week (0-6) [I think sunday is 0]
You can use ‘,’ ‘-’ or ‘*’ as
1,2,3,5
1-5 etc
You could use * as wild char for skipping an attribute. For e.g. I might add following line to stop my music player automatically at 3:00 am in morning every day.
0 3 * * * killall audacious
0 3 * * * amarok -s
To start a command on UI, add export DISPLAY=:0 to the command. Example :
export DISPLAY=:0 && amarok mms://some_radio_show : Starts some music show at some scheduled time automatically.
Some other useful commands ::
1. For updating database used by locate everynight at 3(should be added to root – sudo crontab -e, and no sudo in the command) : 0 3 * * * updatedb
2. For keeping your hardy packages updated and getting the update notifications as soon as possble (Finally I got rid of my habit where it all started 
). This would check for updates every hour at 10th minute. I know that’s kinda harsh but that’s how much I want to be updated : 10 * * * * apt-get update
3. For shutting down internet connection at 7:15 in morning everyday for bandwidth limitations.
15 7 * * * ifconfig eth1 down
4. For synchronising my ipod (my favourite) with my music:
15 7 * * * rsync -van --exclude=*.mp4 --delete --max-size=20m iPod_Control/Music/ /media/Vimpudeo/iPod_Control/Music/
5. Or running root-kit scan at 6 in morning(To be added to root crontab):
01 6 * * * chkrootkit
You can have your own complex scripts and run them at scheduled times. For e.g. backup scripts, synchronization scripts.
So happy time scheduling.
