Linux job scheduling with at

Being able to execute a Linux command at a later time is very important feature for systems Administrator. Job scheduling is taken care by two daemons: atd and crond. Atd manages jobs that run only ones while crond runs jobs at predefined times.

AT

Used to schedule one-time job execution at predefined time.

Lets check if at command already installed on our system. (We are using CentOS 7.0)

 

#yum list installed at
Loaded plugins: fastestmirror
Repodata is over 2 weeks old. Install yum-cron? Or run: yum makecache fast
Loading mirror speeds from cached hostfile
 * base: centos.mirror.iweb.ca
 * extras: centos.mirror.rafal.ca
 * updates: centos.mirror.rafal.ca
Error: No matching Packages to list

As we can see at command is not installed. We will install it using yum package manager

#yum install at -y

We will also have to start at service

#service atd start

We can now schedule shell script test.sh to run 5 min from now

#at -f test.sh now + 5 min

Ways to express time with at

at 2:20 am – will execute at 2:20 am

at 16:30 tomorrow – will execute at 16:30 next day

at now + 5 hours – will execute 5 hours from now

at now + 5 min – will execute 5 minutes from now

at 4:00 6/14/16 – will execute on this time on this day
In this example we will create at job that will run test.sh 20 min from now. We will list it and then remove it. You will need to node the job number highlighted in red.

# at -f test.sh now + 20 min 
job 5 at Tue Mar  8 08:47:00 2016
# at -l
5	Tue Mar  8 08:47:00 2016 a root
# ls /var/spool/at/
a000050172a5bb  spool
# at -d 5
# at -l
# ls /var/spool/at/

User Access

All users allowed to use at command by default. This behaviour can be adjusted using files at.allow and at.deny. All we need to do is put username – one per line in allow or deny file.

Log file

Log for all at activities located under /var/log/cron