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27.2. At and Batch

While cron is used to schedule recurring tasks, the at command is used to schedule a one-time task at a specific time and the batch command is used to schedule a one-time task to be executed when the systems load average drops below 0.8.

To use at or batch, the at RPM package must be installed, and the atd service must be running. To determine if the package is installed, use the rpm -q at command. To determine if the service is running, use the command /sbin/service atd status.

27.2.1. Configuring At Jobs

To schedule a one-time job at a specific time, type the command at time, where time is the time to execute the command.

The argument time can be one of the following:

The time must be specified first, followed by the optional date. For more information about the time format, read the /usr/share/doc/at-<version>/timespec text file.

After typing the at command with the time argument, the at> prompt is displayed. Type the command to execute, press [Enter], and type [Ctrl]-[D]. Multiple commands can be specified by typing each command followed by the [Enter] key. After typing all the commands, press [Enter] to go to a blank line and type [Ctrl]-[D]. Alternatively, a shell script can be entered at the prompt, pressing [Enter] after each line in the script, and typing [Ctrl]-[D] on a blank line to exit. If a script is entered, the shell used is the shell set in the user's SHELL environment, the user's login shell, or /bin/sh (whichever is found first).

If the set of commands or script tries to display information to standard out, the output is emailed to the user.

Use the command atq to view pending jobs. Refer to Section 27.2.3 Viewing Pending Jobs for more information.

Usage of the at command can be restricted. For more information, refer to Section 27.2.5 Controlling Access to At and Batch for details.

27.2.2. Configuring Batch Jobs

To execute a one-time task when the load average is below 0.8, use the batch command.

After typing the batch command, the at> prompt is displayed. Type the command to execute, press [Enter], and type [Ctrl]-[D]. Multiple commands can be specified by typing each command followed by the [Enter] key. After typing all the commands, press [Enter] to go to a blank line and type [Ctrl]-[D]. Alternatively, a shell script can be entered at the prompt, pressing [Enter] after each line in the script, and typing [Ctrl]-[D] on a blank line to exit. If a script is entered, the shell used is the shell set in the user's SHELL environment, the user's login shell, or /bin/sh (whichever is found first). As soon as the load average is below 0.8, the set of commands or script is executed.

If the set of commands or script tries to display information to standard out, the output is emailed to the user.

Use the command atq to view pending jobs. Refer to Section 27.2.3 Viewing Pending Jobs for more information.

Usage of the batch command can be restricted. For more information, refer to Section 27.2.5 Controlling Access to At and Batch for details.

27.2.3. Viewing Pending Jobs

To view pending at and batch jobs, use the atq command. The atq command displays a list of pending jobs, with each job on a line. Each line follows the job number, date, hour, job class, and username format. Users can only view their own jobs. If the root user executes the atq command, all jobs for all users are displayed.

27.2.4. Additional Command Line Options

Additional command line options for at and batch include:

OptionDescription
-fRead the commands or shell script from a file instead of specifying them at the prompt.
-mSend email to the user when the job has been completed.
-vDisplay the time that the job is executed.

Table 27-1. at and batch Command Line Options

27.2.5. Controlling Access to At and Batch

The /etc/at.allow and /etc/at.deny files can be used to restrict access to the at and batch commands. The format of both access control files is one username on each line. Whitespace is not permitted in either file. The at daemon (atd) does not have to be restarted if the access control files are modified. The access control files are read each time a user tries to execute the at or batch commands.

The root user can always execute at and batch commands, regardless of the access control files.

If the file at.allow exists, only users listed in it are allowed to use at or batch, and the at.deny file is ignored.

If at.allow does not exist, users listed in at.deny are not allowed to use at or batch.

27.2.6. Starting and Stopping the Service

To start the at service, use the command /sbin/service atd start. To stop the service, use the command /sbin/service atd stop. It is recommended that you start the service at boot time. Refer to Chapter 12 Controlling Access to Services for details on starting the cron service automatically at boot time.