Everyone who has ever worked on Linux, must have heard of cron expression, which is used to schedule tasks execution at certain intervals.
cron is a generic software service for scheduling tasks.
It comprises two key components:
cron daemon (crond) and cron configuration.
crond reads the cron configuration to determine when to run which task.
It iterates all the files under /var/spool/cron, /etc/crontab, and /etc/cron.d to execute the commands.
crontab is the command-line utility to manage all cron jobs.
The cron packages are available by default in all recent Linux variant systems. If not, we can install it using a package manager.
the expression syntax is as follows -
# [minute] [hours] [day of month] [month] [day of the week] command-to-execute
This allows us to execute commands at a specific time, but only once.
Examples -
$ date
Wed Aug 25 06:49:57 IST 2021
$ date > current-time.txt | at now
warning: commands will be executed using /bin/sh
job 9 at Wed Aug 25 06:50:21 2021
$ cat current-time.txt
Wed Aug 25 06:50:22 IST 2021in the above date > current-time.txt - this will create a new file named 'current-time.txt' and writes the current date into it at now.
to schedule a shell script, we can use like below:
$ at 09:00 -f /home/baeldung/one-time-env-setup.shthe batch command is an extended feature of the at command.
It executes a command based on CPU load and not on time parameters
We can restrict the users accessing the at and batch services using /etc/at.deny and /etc/at.allow files.
Now, let's block the user jack from accessing at and batch services by simply adding the username into the /etc/at.deny file:
root@sandbox1:~$ cat /etc/at.deny | grep jack
jack
jack@sandbox1:~$ at
You do not have permission to use at.
jack@sandbox1:~$ batch
You do not have permission to use at.Reference: