While working on Linux systems we preferred crontab for scheduling jobs generally. There are another utility
- Read This => Examples to schedule recurring tasks with crontab
At command can be useful for shutdown system at specified time, Taking one time backup, sending email as reminder at specified time etc. This article will help you to understand the working of at command with useful examples.
Commands used with at:
- at : execute commands at specified time.
- atq : lists the pending jobs of users.
- atrm : delete jobs by their job number.
1. Schedule first job using at command
Below example will schedule “sh backup.sh” command to be executed on next 9:00 AM once.
# at 9:00 AM at> sh backup.sh at> ^d job 3 at 2013-03-23 09:00
Use ^d to exit from at prompt.
You can also use following option to schedule job. The below command will run “sh backup.sh” at 9:00 in the morning.
# echo "sh backup.sh" | at 9:00 AM
2. List the scheduled jobs using atq
When we list jobs by root account using atq , it shows all users jobs in result. But if we execute it from non root account, it will show only that users jobs.
# atq 3 2013-03-23 09:00 a root 5 2013-03-23 10:00 a rahul 1 2013-03-23 12:00 a root
Fields description:
First filed: job id
Second filed: Job execution date
third filed: Job execution time
Last field: User name, under which job is scheduled.
3. Remove scheduled job using atrm
You can remove any at job using atrm with their job id.
# atrm 3 # atq 5 2013-03-23 10:00 a rahul 1 2013-03-23 12:00 a root
4. Check the content of scheduled at job
atq command only shows the list of jobs but if you want to check what script/commands are scheduled with that task, below example will help you.
# at -c 5
In above example 5 is the job id.
Examples of at Command:
Example 1: Schedule task at coming 10:00 AM.
# at 10:00 AM
Example 2: Schedule task at 10:00 AM on coming Sunday.
# at 10:00 AM Sun
Example 3: Schedule task at 10:00 AM on coming 25’th July.
# at 10:00 AM July 25
Example 4: Schedule task at 10:00 AM on coming 22’nd June 2015.
# at 10:00 AM 6/22/2015 # at 10:00 AM 6.22.2015
Example 5: Schedule task at 10:00 AM on same date at next month.
# at 10:00 AM next month
Example 6: Schedule task at 10:00 AM tomorrow.
# at 10:00 AM tomorrow
Example 7: Schedule task at 10:00 AM tomorrow.
# at 10:00 AM tomorrow
Example 8: Schedule task to execute just after 1 hour.
# at now + 1 hour
Example 9: Schedule task to execute just after 30 minutes.
# at now + 30 minutes
Example 10: Schedule task to execute just after 1 and 2 weeks.
# at now + 1 week # at now + 2 weeks
Example 11: Schedule task to execute just after 1 and 2 years.
# at now + 1 year # at now + 2 years
Example 12: Schedule task to execute at mid night.
# at midnight
Above job will execute on next 12:00 AM
Thanks for reading this article, I hope you will understand to how to use ‘at’ command in Linux.
Great article Rahul!
I just wanted to extend it a bit with some useful additions.
1) You can use -f option to point “at” to the script you need to run:
at -f /path/to/the/script time_spec
2) One can use “at” to start a process in background without nohup, etc. As easy as
at -f /a/command now
or
echo “/a/command” | at now
3) You can use “at” to run a command repeatedly, but unlike cron you can use “at” to run commands with some period between runs, for example after 3 minutes after previous run was completed. This allow you to avoid various checks preventing next run to start before previous is finished.
Moreover you can define this period as random value. Examples:
The script (lets name it /home/user1/at_run.sh):
————————————-
#!/bin/bash
/the/command/you/need
# fixed period between runs
period=3
# or random period. RANDOM is a bash’s random number from 0 to 32767
period=$[ ($RANDOM % 20) + 15 ]
at -f /home/user1/at_run.sh now + $period minutes
————————————-
run /home/user1/at_run.sh and all next runs will be scheduled automatically, so your /the/command/you/need will run repeatedly forever. Sure, you can break the next run with atq/atrm.
Thx Rahul and Sergey.
@Sergey, in your last point you are basically using a wrapper with a random pause and re-scheduling of the at job.
I think this is also achievable with cron, without the “various checks preventing next run to start before previous is finished”.
You can just replace the last line of your script with this:
sleep $(($period * 60)) && exec /home/user1/at_run.sh
Then set a crontab for /home/user1/at_run.sh .
Note: the ‘exec’ bash builtin will prevent calling bash recursively (nested bash’s) and spare memory.
You will need to kill the process to end it.
If the job is not supposed to stay permanently (boot safe), I would prefer to use an interactive bash inside a ‘screen’ command and just do a loop:
while : ; do
/the/command/you/need
sleep 60
done
So you can follow in “live” the output of the script.
hello!
i am running centos. when i submit an at nothing happens. i can call it up using “atq #”, however, it doesn’t execute?
thanks for the help!!!
What are the differences between at-command and chrontab?
at command is used for one-time task scheduling. crontab is used for repeated tasks.
Hi,
I need to run a script at particular time-stamp which internally calls one more script.
But I am getting an error :
at `./radConnect.sh` 09:00
sh: netstat: command not found
sh: /sbin/ifconfig: No such file or directory
Radskman rc:[0] [Request successfully completed]
syntax error. Last token seen: h
Garbled time
Hi Shitanshu,
Schedule job like following
Hi Rahul,
Thanks for the quick response, the use case which I am trying is a bit different.
I have created some containers using a script and a script internally calls another one which needs to run at a particular timestamp. So the command prompt for giving the script won’t be visible. So is there any alternative to it. Or can we do it using crontab?
Hi Shitanshu,
Try following command.
What would be the command for future date? midnight on 30 October 2017
echo “sh radConnect.sh” | at ?
i think it should be
echo “sh radConnect.sh” | at 0 0 30 10 * *
this would repeat every year on 30th October.
I think i’ve mixed up with crontab function command.
Correction of the previous command:
echo “sh radConnect.sh” | at 12:00 AM 30.10.17
Hi Nabil.. This will work
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