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.
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:
HH:MM format — For example, 04:00 specifies
4:00 a.m. If the time is already past, it is executed at the
specified time the next day.
midnight — Specifies 12:00 a.m.
noon — Specifies 12:00 p.m.
teatime — Specifies 4:00 p.m.
month-name day year format — For example, January 15
2002 specifies the 15th day of January in the year 2002. The year is
optional.
MMDDYY, MM/DD/YY, or MM.DD.YY formats — For example,
011502 for the 15th day of January in the year 2002.
now + time — time is in minutes, hours, days, or
weeks. For example, now + 5 days specifies that the command should
be executed at the same time five days from now.
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 35.2.3 Viewing Pending Jobs for more information.
Usage of the at command can be restricted. For more
information, refer to
Section 35.2.5 Controlling Access to At and Batch for details.
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 35.2.3 Viewing Pending Jobs for more information.
Usage of the batch command can be restricted. For
more information, refer to
Section 35.2.5 Controlling Access to At and Batch for details.
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.
Additional command line options for at and
batch include:
| Option | Description |
|---|
| -f | Read the commands or shell script from a file instead of
specifying them at the prompt. |
| -m | Send email to the user when the job has been completed. |
| -v | Display the time that the job is executed. |
Table 35-1. at and batch Command Line Options
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.
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 20 Controlling Access to Services for details on starting the cron service
automatically at boot time.
