Cron vs. Crontab

Cron is a daemon that runs scheduled tasks based on input from the command crontab.

Crontab is the list of jobs and times at which they have to execute. To run cron, jobs need to be entered in the crontab.

Formating Crontab for a Schedule job

1. Crontab Format: Crontab Format: each entry in Crontab has at least 6 fields separated by a single space. Details description of each field is expressed in the following table.

 

Field	Definition	Range of Values
1	Minute	00-59
2	Hour	00-23
3	Day	1-31
4	Month	1-12
5	Day of week ( Sunday being 0)	0-6
6	Command to Execute	Full Path of Executable program

2. Entries for Crontab:

Example 1: Entry of crontab to run something.sh under /home/user every 1pm.

0 13 * * * /home/user/something.sh

The asterisk (*) is used to indicate as wild card that every instance of the particular time period will be used (i.e. every minutes, every hour, every weekday, etc.).

Full path to the script /home/user/backup.sh should be used instead of just using backup.sh. This is because cron runs as root.

Example 2: Following entry is cron to run every minute the program something.sh under /home/user/something.sh

* * * * * /home/user/something.sh

Example 3: how to make a crontab entry should be to run a script backup.sh every day at 6:00pm. The entry would look like this:

0 18 * * * /home/user/backup.sh

More Advanced Example

· Let's run the script printinvoices.sh every sunday at 12:45pm.
45 12 * * 0 /home/account/printinvoices.sh

· How about clearaccount.sh every month beginning at 1:32am ?

32 1 1 * * /home/accont/clearaccount.sh

· Let's see how to schedule a task to run only on weekdays(monday to friday)
0 10 * * 1-5 /home/account/cleartemp.sh

3. Managing Ouput from cron:

 

Usually the output of cron gets mailed to the owner of the process or the person or email id specified in the MAILTO variable. To set the MAILTO variable, you'll have to add the following command to the top of your crontab:

MAILTO="your_email_address@domain.com"

If you have a command that is run frequently, and you don't want the output to be emailed each time, you can redirect the output to a log file

.

0 18 * * * /home/user/backup.sh>>log.file

If you don't want any output at all, you can redirect the output to a null file

0 18 * * * /home/user/backup.sh>>/dev/null