Scheduled Activity: cron and at

Your Linux computer can be made to do the right thing at the right time.
Using at

Use at when you want to execute a command or multiple commands once at some future time.

In Linux, the at command requires that the atrun command be started in root's crontab file. Many Linux distributions ship with at enabled, but some do not. To enable the at utility on your system, become superuser and edit root's crontab file:

$ su root
# crontab -e

and add the following line:

* * * * * directory/atrun

where directory is the location where the atrun executable is stored. On my system that's /usr/lib, so the entry is:

* * * * * /usr/lib/atrun

This causes atrun to be executed every minute. After a minute or so of adding the atrun line and saving the crontab file, any existing at commands are evaluated and executed if the time is right. (Before this, you may have been able to enter at commands, but they were never executed.)

To demonstrate the at command, let's have it print “hello” on your current terminal window after a few minutes. First, get the time and your current terminal device:

$ date
Tue Oct  3 15:33:37 PDT 1995
$ tty

Now run the at command. Specify a time in the command line, press Return, and then enter the command, followed by another Return and a Ctrl-D:

$ at 15:35
echo "hello" > /dev/ttyp2
Job c00ceb20b.00 will be executed using /bin/sh

The at command takes input up to the end-of-file character (generated by pressing ctrl-D while at the beginning of a line.) It reports the job number and informs you that it will use /bin/sh to execute the command. In two minutes, hello should appear on the display of /dev/ttyp2. Note that you can enter a series of commands, one per line—at will read each line up to the end-of-file and execute the file as a /bin/sh shell script at the specified time.

Suppose you want to set an alarm. One way to tell at to do something is to use the relative form of timing, specifying a time relative to now. If you want your computer to beep at you in 25 minutes, enter:

$ at now + 25 minutes
echo ^G > /dev/ttyp4
Job c00ceb7fb.00 will be executed using /bin/sh

and you are beeped in 25 minutes. There is a great deal of flexibility allowed in entering time specifications. For example, at recognizes military time, “am” and “pm”, month abbreviations, time notation that includes the year, and so on. My at man page even claims at accepts teatime, noon, and other constructs. Refer to the at man page for more examples of valid time specifications.

You must tell at your tty location, or it won't send output to your terminal window. If you prefer, you can receive mail:

$ at 4:55pm Friday
echo '5 p.m. meeting with Carol' | mail raithel
Job c00ceb7fb.01 will be executed using /bin/sh

To get a list of your pending at jobs, enter:

$ atq

If you are superuser, atq shows you the pending at jobs of all users. To delete a job, enter:

$ atrm job_number

where job_number is the job number returned by atq. The superuser can also remove other user's jobs.

A Reminder Script Using at

The following is a simple script that makes it easier for me to use at to send myself reminders. The script sends mail to the user containing the message line(s) entered at the prompt at the time specified. It also displays some syntax examples of how to specify time, which I find a useful memory refresher.

Notice the script, as written, requires you to have a Msgs directory in your home directory. I created $HOME/Msgs, rather than using something like /usr/tmp, so the messages are more private until they are deleted by the script.

echo "Enter your reminder message.
When finished, enter a period (.) at
the beginning of a line and press Enter.
(Or press Ctrl-C or DEL to exit.)"
while :
    read MESSAGE
    if [ "$MESSAGE" = "." ]
        echo $MESSAGE > $HOME/Msgs/message.$$
cat << !!
Enter time and day you want to receive
the message, for example:
      0815am Jan 24
      8:15am Jan 24
      now + 1 day
      5 pm Friday
Then press Enter.
read TIME
echo \
  "at $TIME mail $LOGNAME $HOME/Msgs/message.$$"
at $TIME  << !!
mail $LOGNAME < $HOME/Msgs/message.$$
rm -f $HOME/Msgs/message.$$
exit 0

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