Why do we need the "at" command in Linux?

Bernhard's reply is correct: in multi-user systems, the ability to execute heavy programs at some ungodly hours of the night is especially convenient, for both the person submitting the job, and his coworkers. It is part of "playing nice".

I did most of my Ph.D. computations this way, combining the script with the nice command which demoted the priority of my work whenever other people were keeping the machine busy, while leaving intact its ability to hog all the system resources at night.

I used the very same command to check whether my program was running, and to restart it if necessary.

Also, you should keep in mind that at was written way before screen, tmux, and so on, so that it was a simple way to have a detached shell, i.e., one that would not die once you logged off the system.

Lastly, you should also notice that it is different from cron, which also has been around for a long time. The difference lies in the fact that at is occasional, while cron, being so repetitive, is more suited for system jobs which really need to be executed forever at fixed intervals: in fact, at gives you your own environment, with your own settings (and choices) of environment variable, while cron uses a minimal set of environment variables (just check the difference in PATH, as an example).

I use the at command when I need to do some heavy processing on data, which I want to have executed during the night, when I am not behind my computer. Of course I could start the process just after I leave, but this is something I tend to forget.

The result of the command is not different from regularly execution of the script or command.

When you have questions such as this always consult the man pages. They can be very enlightening.

What it does

excerpt from at man page

NAME
       at, batch, atq, atrm - queue, examine or delete jobs for later execution

DESCRIPTION
       at  and  batch  read  commands  from  standard  input or a specified file
       which are to be executed at a later time, using /bin/sh.

Usage

The usage of the tools:

Usage: at [-V] [-q x] [-f file] [-mldbv] timespec ...
       at [-V] [-q x] [-f file] [-mldbv] -t time
       at -c job ...
       atq [-V] [-q x]
       atrm [-V] job ...
       batch

at includes 4 commands (at, atq, atrm, and batch). You use at and batch to schedule the jobs, atq to see what's scheduled, and atrm to remove a job prior to it running.

$ at -f <cmd> timspec

Timespec

The time to run the at job can be specified in different ways.

excerpt form at man page

At allows fairly complex time specifications, extending the POSIX.2 standard. It accepts times of the form HH:MM to run a job at a specific time of day. (If that time is already past, the next day is assumed.) You may also specify mid‐ night, noon, or teatime (4pm) and you can have a time-of-day suffixed with AM or PM for running in the morning or the evening. You can also say what day the job will be run, by giving a date in the form month-name day with an optional year, or giving a date of the form MMDD[CC]YY, MM/DD/[CC]YY, DD.MM.[CC]YY or [CC]YY-MM-DD. The specification of a date must follow the specification of the time of day. You can also give times like now + count time-units, where the time- units can be minutes, hours, days, or weeks and you can tell at to run the job today by suffixing the time with today and to run the job tomorrow by suffixing the time with tomorrow.

Examples

Say you have this shell script.

$ cat mycrontest.sh

#!/bin/bash
 echo "It is now $(date +%T) on $(date +%A)"

Sample run:

$ ./mycrontest.sh
It is now 18:37:42 on Friday

Sample at job submissions:

$ at -f mycrontest.sh  10pm tomorrow
job 14 at Sun Jul  8 22:00:00 2007

$ at -f mycrontest.sh 2:00 tuesday
job 15 at Tue Jul 10 02:00:00 2007

$ at -f mycrontest.sh 2:00 july 11
job 16 at Wed Jul 11 02:00:00 2007

$ at -f mycrontest.sh 2:00 next week
job 17 at Sat Jul 14 02:00:00 2007

References

• Linux tip: Job scheduling with cron and at