Scheduling a Single System Event (at)

The following sections describe how to use at(1) to schedule jobs (commands and scripts) for execution at a later time, how to display and remove these jobs, and how to control access to the at command.

By default, users can create, display, and remove their own at job files. To access at files belonging to root or other users, you must have superuser privileges.

When you submit an at job, it is assigned a job identification number along with the .a extension that becomes its file name.

at Command Description

Submitting an at job file includes:

1. Invoking the at utility, specifying a command execution time.

2. Entering a command or script to execute later.

   ---

   Note -

   If output from this command or script is important, be sure to direct it to a file for later examination.

   ---

For example, the following at job removes core files from the user account smith near midnight on the last day of July.

$ at 11:45pm July 31
at> rm /home/smith/*core*
at> Press Control-d
commands will be executed using /bin/csh
job 933486300.a at Sat Jul 31 23:45:00 1999

at Command Security

You can set up a file to control access to the at command, permitting only specified users to create, remove, or display queue information about their at jobs. The file that controls access to at, /etc/cron.d/at.deny, consists of a list of user names, one per line. The users listed in this file cannot access at commands.

The at.deny file, created during SunOS software installation, contains the following user names:

daemon
bin
smtp
nuucp
listen
nobody
noaccess

With superuser privileges, you can edit this file to add other user names whose at access you want to restrict.

How to Create an at Job

1. Start the at utility, specifying the time you want your job executed, and press Return.

   $ at [-m] time [date]

   -m	Sends you mail after the job is completed.
   time	Hour that you want to schedule the job. Add am or pm if you do not specify the hours according to a 24-hour clock. midnight, noon, and now are acceptable keywords. Minutes are optional.
   date	First three or more letters of a month, a day of the week, or the keywords today or tomorrow.

2. At the at prompt, enter the commands or scripts you want to execute, one per line. You may enter more than one command by pressing Return at the end of each line.

3. Exit the at utility and save the at job by pressing Control-d.

   Your at job is assigned a queue number, which is also its file name. This number is displayed when you exit the at utility.

Examples--Creating an at Job

The following example shows the at job that user jones created to remove her backup files at 7:30 at night. She used the -m option so that she would receive a mail message after her job completed.

$ at -m 1930
at> rm /home/jones/*.backup
at> Press Control-d
job 897355800.a at Mon Jul  12 19:30:00 1999

She received a mail message which confirmed the execution of her at job.

Your "at" job "rm /home/jones/*.backup"
completed.

The following example shows how jones scheduled a large at job for 4:00 Saturday morning. The output of which was directed to big.file.

$ at 4 am Saturday
at> sort -r /usr/dict/words > /export/home/jones/big.file

How to Display the at Queue

To check your jobs that are waiting in the at queue, use the atq command. This command displays status information about the at jobs that you created.

$ atq

How to Verify an at Job

To verify that you have created an at job, use the atq command. The atq command confirms that at jobs belonging to jones have been submitted to the queue.

$ atq
Rank	  Execution Date     Owner     Job         Queue   Job Name
  1st   Jul 12, 1999 19:30   jones  897355800.a     a     stdin
  2nd   Jul 14, 1999 23:45   jones  897543900.a     a     stdin
  3rd   Jul 17, 1999 04:00   jones  897732000.a     a     stdin

How to Display at Jobs

To display information about the execution times of your at jobs, use the at -l command.

$ at -l [job-id]

Example--Displaying at Jobs

The following example shows output from the at -l command, used to get status information on all jobs submitted by a user.

$ at -l 
897543900.a	Wed Jul 14 23:45:00 1999
897355800.a	Mon Jul 12 19:30:00 1999
897732000.a	Sat Jul 17 04:00:00 1999

The following example shows output displayed when a single job is specified with the at -l command.

$ at -l 897732000.a
897732000.a		Sat Jul 17 04:00:00 1999 

How to Remove at Jobs

1. (Optional) Become superuser to remove an at job belonging to root or another user.

2. Remove the at job from the queue before it is executed.

   $ at -r [job-id]

   -r job-id

   Identification number of the job you want to remove.

3. Verify the at job is removed by using the at -l (or the atq) command to display the jobs remaining in the at queue. The job whose identification number you specified should not appear.

   $ at -l [job-id]

Example--Removing at Jobs

In the following example, a user wants to remove an at job that was scheduled to execute at 4 am on July 17th. First, the user displays the at queue to locate the job identification number. Next, the user removes this job from the at queue. Finally, the user verifies that this job has been removed from the queue.

$ at -l
897543900.a	Wed Jul 14 23:45:00 1999
897355800.a	Mon Jul 12 19:30:00 1999
897732000.a	Sat Jul 17 04:00:00 1999
$ at -r 897732000.a
$ at -l 897732000.a
at: 858142000.a: No such file or directory