Cron is nice and all, but don't forget about its cousin at.

When I first started using Linux, it was like being tossed into the deep end of the UNIX pool. You were expected to use the command line heavily along with all the standard utilities and services that came with your distribution. At lot has changed since then, and nowadays, you can use a standard Linux desktop without ever having to open a terminal or use old UNIX services. Even as a sysadmin, these days, you often are a few layers of abstraction above some of these core services.

I say all of this to point out that for us old-timers, it's easy to take for granted that everyone around us innately knows about all the command-line tools we use. Yet, even though I've been using Linux for 20 years, I still learn about new (to me) command-line tools all the time. In this "Back to Basics" article series, I plan to cover some of the command-line tools that those new to Linux may never have used before. For those of you who are more advanced, I'll spread out this series, so you can expect future articles to be more technical. In this article, I describe how to use the at utility to schedule jobs to run at a later date.

at vs. Cron

at is one of those commands that isn't discussed very much. When people talk about scheduling commands, typically cron gets the most coverage. Cron allows you to schedule commands to be run on a periodic basis. With cron, you can run a command as frequently as every minute or as seldom as once a day, week, month or even year. You also can define more sophisticated rules, so commands run, for example, every five minutes, every weekday, every other hour and many other combinations. System administrators sometimes will use cron to schedule a local script to collect metrics every minute or to schedule backups.

On the other hand, although the at command also allows you to schedule commands, it serves a completely different purpose from cron. While cron lets you schedule commands to run periodically, at lets you schedule commands that run only once at a particular time in the future. This means that at fills a different and usually more immediate need from cron.

Using at

At one point, the at command came standard on most Linux distributions, but these days, even on servers, you may find yourself having to install the at package explicitly. Once installed, the easiest way to use at is to type it on the command line followed by the time you want the command to run:

$ at 18:00

The at command also can accept a number of different time formats. For instance, it understands AM and PM as well as words like "tomorrow", so you could replace the above command with the identical: