Scheduling a Single System Task (at)

The following sections describe how to use the at command 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 that belong 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, which becomes the job's file name.

Description of the at Command

Submitting an at job file involves:

1. Invoking the at utility and 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 the output 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 Tue Jul 31 23:45:00 2001

Controlling access to the at Command

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 the at command, /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 the at.deny 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.

   $ at [-m] time [date]

   -m	Sends you email 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. Acceptable keywords are midnight, noon, and now. 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, type 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 the job's 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 p.m. She used the -m option so that she would receive an email message after her job completed.

$ at -m 1930
at> rm /home/jones/*.backup
at> Press Control-D
job 897355800.a at Thu Jul  12 19:30:00 2001

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 a.m. Saturday morning. The job output 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 that belong to jones have been submitted to the queue.

$ atq
Rank	  Execution Date     Owner     Job         Queue   Job Name
  1st   Jul 12, 2001 19:30   jones  897355800.a     a     stdin
  2nd   Jul 14, 2001 23:45   jones  897543900.a     a     stdin
  3rd   Jul 17, 2001 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]

Where the -l job-id option identifies the identification number of the job whose status you want to display.

Example—Displaying at Jobs

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

$ at -l 
897543900.a	Sat Jul 14 23:45:00 2001
897355800.a	Thu Jul 12 19:30:00 2001
897732000.a	Tue Jul 17 04:00:00 2001

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

$ at -l 897732000.a
897732000.a		Tue Jul 17 04:00:00 2001 

How to Remove at Jobs

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

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

   $ at -r [job-id]

   Where the -r job-id option specifies the identification number of the job you want to remove.

3. Verify that the at job is removed by using the at -l (or the atq) command.

   The at -l command displays 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 a.m. 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	Sat Jul 14 23:45:00 2001
897355800.a	Thu Jul 12 19:30:00 2001
897732000.a	Tue Jul 17 04:00:00 2001
$ at -r 897732000.a
$ at -l 897732000.a
at: 858142000.a: No such file or directory

How to Deny Access to the at Command

1. Become superuser.

2. Edit the /etc/cron.d/at.deny file and add the names of users, one per line, that will be prevented from using at commands.

   daemon bin smtp nuucp listen nobody noaccess username1 username2 username3 . . .

Example—Denying at Access

The following example shows an at.deny file that has been edited so that the users smith and jones cannot access the at command.

$ cat at.deny
daemon
bin
smtp
nuucp
listen
nobody
noaccess
jones
smith

How to Verify that at Access Is Denied

To verify that a username was added correctly to /etc/cron.d/at.deny, use the at -l command while logged in as the user. If the user cannot access the at command, the following message is displayed.

# su smith
Password:
$ at -l
at: you are not authorized to use at.  Sorry.

Likewise, if the user tries to submit an at job, the following message is displayed:

$ at 2:30pm
at: you are not authorized to use at.  Sorry.

This message confirms that the user is listed in the at.deny file.

If at access is allowed, the at -l command returns nothing.