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.
