Linux Job Scheduling

These time specifications may be optionally followed by a date specification. Date specifications come in a number of forms, including:

today tomorrow

These mean what you would expect. “at teatime tomorrow” will run the commands at 4 p.m. the following day. Note that if you specify a time already passed (as in “at noon today” when it is 3 p.m.), the job will be run at once. You do not get an error. At first you might think this a bad thing, but look at it this way. What if the system had been down since 10 a.m. and was only being restarted now at 3 p.m.? Would you want a critical job skipped, or would you want it to run as soon as possible? The at system takes the conservative view and assumes you will want the job run.

<month_name> <day> [<year>]

MM/DD/YYYY YYYY-MM-DD

Another way you can modify a time specification is to apply a relative time to it. The format of a relative time specification is + <count> <time units>, where “count” is simply a number and “time units” is one of “minutes”, “hours”, “days” or “weeks”.

So, you can say:

at 7pm + 2 weeks

and the programs will be scheduled for two weeks from today at 7 p.m. local time.

One of the most common forms is this:

at now + x units

to specify a program or programs to be run so many units from now. Something I often use this for is in shutting down my home machine's dial-up connection from work. I dial in before I leave for work, and then I kill it before my wife gets home (I'm too cheap to buy a second line). I use ssh to log in from work, and I like to close all my windows cleanly, so I frequently do something like this:

# ps fax | grep wvdial
  599 ?        S      0:00      \_ wvdial
  875 pts/2    S      0:00      \_ grep wvdial
# at now + 10 minutes
at> kill 599
at>
warning: commands will be executed using /bin/sh
job 9 at 2000-04-17 16:30
# exit
$ exit

Note that the plain old Bourne shell is used for all commands run by at. (Also note: I had to type ctrl-d, the *nix EOF character to close the interactive at session. More on this in the section on the at command line. This is just one factor affecting the behavior of at scheduled commands. Here are some other facts to bear in mind. The present working directory, environment variables (with three exceptions, see below), the current userid and the umask that were in effect when the at command was issued are retained and will be used when the commands are executed. The three environment variable exceptions are TERM, DISPLAY and “_” (which usually contains the last command executed in the shell). The output of the commands is mailed to the user who issued the at command. If the at command is issued in an su shell (meaning, if you “became” another user), the output mail will be sent to the login user, but the programs will run under the su user.

The ability to use at is controlled by two files: /etc/at.deny and /etc/at.allow.

The /etc/at.allow file is checked first. If it exists, only user names in this file are allowed to run at. If the /etc/at.allow file does not exist, then the /etc/at.deny file is checked. All user names not mentioned in that file may run at.

If neither file exists, only the superuser may run at.