Crontab

Here is some information about crontab and the flags that a crontab command can accept.

crontab -e --> edits the crontab file so you can make entries of scheduled jobs to run.

crontab -l --> lists the crontab file, its like running a "cat" command on a text file.

crontab -r --> removes the crontab file. The most dangerous, make sure you have a backup of your crontab file before you run this command even be it for test reasons.

One has got to be very careful in dealing with the -r flag. if you inadvertently type -r for -e and hit enter, the next thing you know is your crontab file is gone. since E and R are located next to each other on a regular QWERTY keyboard and if you are not really good with the finger positioning of the keyboard, this is sure a nightmare.

This had once happened with me since I am not good at the finger positioning myself on a QWERTY keyboard.It is only after hitting the return key that I realized, I typed the wrong command. But a backup of the crontab should save the day, like it did to me.

Its always a good practice to have a backup of the crontab file. Use the command "crontab -l" to backup the crontab file as shown below.

00 01 * * * crontab -l > /export/oracle/crontab_backup.txt

The above line takes a backup of the crontab at 01:00 hours everyday.

Now, to restore a blank crontab, all you need to do is run the following command. sometimes this might require a bounce of the cron daemon

crontab < /export/oracle/crontab_backup.txt

Let's discuss about the conventions of making entries in a crontab file. There are six fields for an entry in a crontab, below is a depiction which explains about the fields. Fields are seperated by a white space or a tab.

*  *  *  *  *  COMMAND_OR_SCRIPT_LOCATION
--------------------------------------------------------------------
|  |  |  |  |   |_ Command or script location
|  |  |  |  |
|  |  |  |  |___   Day_Of_Week (0 - 6) (0=Sun, 1=Mon, 2=Tue,3=Wed,
|  |  |  |                                    4=Thu, 5=Fri, 6=Sat)
|  |  |  |_____    Month_Of_Year (1 - 12)
|  |  |
|  |  |_______     Day_Of_Month (1 - 31)
|  |
|  |_________      Hour (0 - 23)
|
|___________       Minutes (0 - 59)