at Command Syntax
The table below show some (not all!) of the ways you can enter
dates and times for the at command.
Although different Unix systems may have slightly different
syntaxes (check the local man pages) in general the syntax is:
at TIME [ DATE ]
In all cases of specifying a time only, the command will run at that time today if the specified time is in the future, or tomorrow otherwise. For example if you specify 3:00 PM and it is currently only 1:00 PM, the at job will run in two hours from now. But if is is currently 3:30 PM, the job will run tomorrow at 3 PM.
(See your local man pages for details on other options to at.)
| Example | Meaning |
|---|---|
at noon |
12:00 PM September 18, 2001 |
at midnight |
12:00 AM September 19, 2001 |
at teatime |
4:00 PM September 18, 2001 |
at tomorrow |
10:00 AM September 19, 2001 |
at noon tomorrow |
12:00 PM September 19, 2001 |
at next week |
10:00 AM September 25, 2001 |
at next monday |
10:00 AM September 24, 2001 |
at fri |
10:00 AM September 21, 2001 |
at OCT |
10:00 AM October 18, 2001 |
at 9:00 AM |
9:00 AM September 19, 2001 |
at 2:30 PM |
2:30 PM September 18, 2001 |
at 1430 |
2:30 PM September 18, 2001 |
at 2:30 PM tomorrow |
2:30 PM September 19, 2001 |
at 2:30 PM next month |
2:30 PM October 18, 2001 |
at 2:30 PM Fri |
2:30 PM September 21, 2001 |
at 2:30 PM 9/21 |
2:30 PM September 21, 2001 |
at 2:30 PM Sept 21 |
2:30 PM September 21, 2001 |
at 2:30 PM 9/21/2010 |
2:30 PM September 21, 2010 |
at 2:30 PM 21.9.10 |
2:30 PM September 21, 2010 |
at now + 30 minutes |
10:30 AM September 18, 2001 |
at now + 1 hour |
11:00 AM September 18, 2001 |
at now + 2 days |
10:00 AM September 20, 2001 |
at 4 PM + 2 days |
4:00 PM September 20, 2001 |
at now + 3 weeks |
10:00 AM October 9, 2001 |
at now + 4 months |
10:00 AM January 18, 2002 |
at now + 5 years |
10:00 AM September 18, 2007 |
This is controlled by a pair of files called at.allow and
at.deny.
The location and even the exact use of these files vary from system
to system.
For Linux these file exists in /etc.
On Linux:
root
can use the at command.at.deny exists, then any user except those
listed in this file can use the at command.at.allow exists then only users listed in this file
can use the at command.
Note if this file exists then any at.deny file
is ignored.
To see a list of pending at jobs (the ones that haven't run yet), use the command "atq". This will show the job number and date-time for that job.
To see the contents of some at job, use the command "at -c jobnum". This shows the complete environment that gets set for the job as well; the actual commands of your job are at the bottom.
To delete an at job before it has run, use the command atrm jobnum".
An at job may be created in a particular queue, using the form "at -q queue date-time". The queue is a single letter. The default queue is "a". Queue "b" is reserved for batch jobs. Using higher lettered queues will run your at job with higher nice values.
at for a Reminder Message
Suppose today is Tuesday, September 18, 2001 at 4 PM.
Then the following will "pop-up" a reminder message on Friday,
September 21, 2001 at 3:00 PM for someone whose account name
is user:
/home/user> at 3 PM Fri at> echo "Leave now for 4 PM PTA meeting" | write user >/dev/null 2>&1 at> ^D warning: commands will be executed using /bin/sh job 1 at 2001-09-21 15:00 /home/user> _
Longer messages can be saved in a file.
In that case use cat file instead of
echo message.
The reason to use write is that at jobs
are not attached to your current (or any future) terminal session.
Without the write command,
the output of the echo command is
sent as email to the user.
Note that in the example above any output or errors
from the write command
are discarded to avoid sending email to the user in the event
user is not logged in at 3:00 PM on Friday.
If you would like such email then eliminate the I/O redirections to
/dev/null.
(In fact this is quite common.)