Scheduling jobs using crontab and at in Debian linux

Scheduling job with crontab command

The crontab commands work by reading a crontab file. A crontab file is a plain text file that list all the job you have to scheduled. Each line of the file consists of command with all its options and arguments, date and time to run it.

There are two files to control access to crontab

/etc/cron.allow : only user listed can access the crontab

/etc/cron.deny : listed users are denied for crontab

Format of crontab file:

***** Command to be executed

first field is for Min (0-59)

second field for Hour (0-23)

third field for Day of month (1-31)

Fourth field for month (1-12)

Fift field for Day of week (0-6, where 0 is Sunday)

The last field is command to be executed

Edit the crontab file

To edit crontab file, use crontab -e

Consider the following example

Suppose we want to run alaram script everyday at noon, according to the task we have to edit crontab file

Follow the steps

# crontab -e

It will directly open the file in vi editor. Edit the following to run alaram script everyday at noon

0 12 *** /usr/share/bin/alaram

save and quit

This will run alaram script evryday at 12 noon.

Scheduling task with at command

We can use at command to schedule a command or script to run at single time

Root user can use two files to allow or not allow the user , at command

/etc/at.allow – will allow user to run the at command

/etc/at.deny – will deny user to use at command

Command syntax

at TIME [DATE]

at command options

-c task : Display the listed task

-d task : Delete the specific task

-f file : Read task from file entered

-l : list schedule task for current user

at command time format

day – set the day of week

hh:mm – sets the time

mmddyy – sets the date

now – runs the command immediately

tomorrow – Runs the command tomorrow

Example of at command

# at -f mycrontest.sh  10pm tomorrow

This command will run the script mycrontest.sh tomorrow at 10 pm