Linux with Operating System Concepts

military notation
. Thus, 
12:00 PM and 12:00 are treated the same but 1:00 PM would be equal to 13:00 and 11:59 PM 
would be indicated as 23:59. For 12:00 AM, the military time is indicated as 0:00. If you use 
AM or PM, you can use either uppercase or lowercase letters. You can also specify the time 
as 
noon
, 
midnight,
and 
teatime
(4 PM).
The date is specified using any one of the following four formats. Each of these is fol-
lowed by an example for November 12, 2013.
• MMDDYY as in 111213
• MM/DD/YY as in 11/12/13
• DD.MM.YY as in 12.11.13
• YYYY-MM-DD as in 2013-11-12
If you do not specify a date, the event is scheduled to take place at the next occurrence 
of the specified time. If it is 3:35 p.m. and you schedule for 14:30 (2:30 PM), then the event 

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Linux with Operating System Concepts
is scheduled to take place at 2:30 PM of the next day. You can also use the words 
today
or 
tomorrow
for the date, as in 
2:30 PM tomorrow
or 
23:59 today
.
The alternative for the time/date is to specify a time/date relative to the time that the at 
command is submitted. This is specified using the notation: 
now 
+
count unit
, where 
count
is an integer and 
unit
is a temporal unit such as minutes, hours, or days. Here are 
some examples:
• now 
+
5 minutes
• now 
+
2 hours
• now 
+
7 days
• now 
+
4 weeks
You can combine relative and absolute methods but it may not make much sense. For 
instance, 
now 
+
12:00 PM
makes no sense at all but 
now 
+
3 days
would schedule the 
event at the current time in 3 days. Errors in the time/date specification will yield the mes-
sage 
Garbled time
. Note that at schedules events to the minute, not to the second. You 
would never specify the seconds unit for either an absolute or a relative time specification.
The event to be scheduled can be either specified in a separate file or through the com-
mand line. As an example, you have written the activities to be scheduled in the shell script 
my_event.sh
. You would then schedule this using
at 
time/date specifier
–f ./my_event.sh
where 
time/date specifier
is as explained above. The –f option indicates that the event(s) to 
take place is(are) stored in the accompanying file. The file does not have to be a script. It 
could comprise Linux commands or invoke a script.
The alternate approach to specifying the event(s) in a file is to omit the –f option. In this 
case, upon hitting 
<
enter 
>
, you are dropped into an 
at
>
prompt. You now enter your 
commands one line at a time, pressing the 
<
enter 
>
key to end each line. To exit the 
at
>
prompt, press 
control
+
d
. For instance, you might specify
at now 
+
1 minute
Since you did not include –f filename, you now see
at
>
You might then enter
at
>
mount 10.11.12.13:/home/backup/mnt/backup
at
>
tar –xzf /home/* /mnt/backup/home-backup.tar
at
>
umount /mnt/backup
at
>
control
+
d

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Maintaining and Troubleshooting Linux
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581
Thus, in 1 minute time, you will use tar to create a backup on a mounted file server and 
then unmount the file server when done.
Once events are scheduled, you can examine them using 
atq
. This lists each waiting 
event scheduled through at or batch. You can remove any scheduled item using 
atrm
. If 
there is only one item in the event queue, 
atrm
will delete it. Otherwise, when you use 
atq
, each event is numbered and you will have to specify the number of the event with 
atrm as in 
atrm 2
to remove the second item in the scheduled list.
The batch command is identical to the at command except that you do not specify a 
time. If you specify batch by itself, you are dropped into the at
>
prompt; otherwise, you 
would specify 
batch –f