Objective

After completing this section, students should be able to schedule one-time tasks with at.

Scheduling future tasks

From time to time, an administrator (or end user) wants to run a command, or set of commands, at a set point in the future. Examples include the office worker who wants to schedule an email to his boss, as well as the system administrator working on a firewall configuration who puts a “safety” job in place to reset the firewall settings in ten minutes' time, unless he deactivates the job before then.

These scheduled commands are often called tasks or jobs.

Scheduling one-time tasks with at

One of the solutions available to users of a Red Hat Enterprise Linux system for scheduling future tasks is at. This is not a standalone tool, but rather a system daemon (atd), with a set of command-line tools to interact with the daemon (at, atq, and more). In a default Red Hat Enterprise Linux installation, the atd daemon will be installed and enabled automatically. The atd daemon can be found in the at package.

Users (including root) can queue up jobs for the atd daemon using the command-line tool at. The atd daemon provides 26 queues, a to z, with jobs in alphabetically later queues getting less system priority (higher nice levels, discussed in a later chapter).

Scheduling jobs

A new job can be scheduled by using the command at <TIMESPEC>. at will then read the commands to execute from stdin. For larger commands, and typo-sensitive commands, it is often easier to use input redirection from a script file, e.g., at now +5min < myscript, than typing all the commands by hand in a terminal window. When entering commands by hand, you can finish your input by pressing Ctrl+D.

The <TIMESPEC> allows for many powerful combinations, giving users an (almost) free-form way of describing exactly when a job should be run. Typically, they start with a time, e.g., 02:00pm, 15:59, or even teatime, followed by an optional date or number of days in the future.

Some examples of combinations that can be used are listed in the following text. For a complete list, see the timespec definition in the references.

Inspecting and managing jobs

Inspecting jobs

To get an overview of the pending jobs for your user, use the command atq or, alternatively, the alias at -l.

Running this command gives the following output:

[student@desktopX ~]$ atq
28  Mon Feb  2 05:13:00 2015 a student
29  Mon Feb  3 16:00:00 2014 h student
27  Tue Feb  4 12:00:00 2014 a student

This shows four columns for every job scheduled to run in the future:

To inspect the actual commands that will run when a job is executed, use the command at -c <JOBNUMBER>. This output will first show the environment for the job being set up to reflect the environment of the user who created the job at the time it was created, followed by the actual commands to be run.

Removing jobs

The atrm <JOBNUMBER> will remove a scheduled job. This is useful when a job is no longer needed; for example, when a remote firewall configuration succeeded, and does not need to be reset.