Contents
Crontab Basics
crontab is most powerful yet very simple.this little tool is used to set or schedule periodic or one time jobs in linux..
it is not that complicated..
Here is a summary of commands which are used to configure jobs with crontab
crontab -e (edit user’s crontab)
crontab -l (list user’s crontab)
crontab -r (delete user’s crontab)
crontab -i (prompt before deleting user’s crontab)
to create a new periodic or schedule task you need to edit the crontab file .
enter
$crontab -e
the configuration file will be opened in nano
ok these are the fields you have to fill.. any way
# m h dom mon dow command
m —— minute (0-59)
h —— hour (0 to 23)
dom —— day of month (1to31)
mon —— month (1-12)
dow —— day of week (0 to 6 , 0 –> sunday )
command ——- program to be run
note:- all fields are mandatory
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Alarm
lets write a script that plays an audio file on evoking it..
like below
#!/bin/sh
mplayer audiofile.mp3
lets save this as file alarm
as owner of file you can make the file executable by entering
$ chmod a+x alarm
ps: both file alarm & audiofile.mp3 must be in same directory
ok now you want it to ring at every morning 7:45
Then here is the line here you have to enter
# m h dom mon dow command
45 7 * * * ~/alarm
you have an alarm now free of cost!!
Other utilities?
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Upgrading System
take this script for example;
#!/bin/sh
apt-get update
apt-get -y upgrade
apt-get clean
apt-get autoclean
save above script in auto-upgrade.sh make it an executable like before and save it in /root directory
now i set my pc’s root crontab as
# m h dom mon dow command
00 00 * * 6 /root/auto-upgrade.sh
You can see my auto upgrade program runs every saturday at midnight and keep my system uptodate!!
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if you can mix shell scripting and crontab you do a lot of cool things..
Website Backup
you can also take a periodic backup of your website using wget and crontab.
#!/bin/sh
dir=`date -d now +%d-%m-%y`
mkdir $dir
cd $dir
wget -r http://link-to-your-website
save it as websitebackup.sh
and set it in crontab to every sunday night
# m h dom mon dow command
00 00 * * 0 /home/myuserid/websitebackup.sh
well you have your website weekly backup now for you.!!
any way you can see now how powerful this simple tool can be.
You can also schedule a periodic back up of your important files in the above manner.
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References
crontab command manual
wget command manual
date command manual
apt-get manual