run a cron job only ONCE!

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

SYNOPSIS
       at [-V] [-q queue] [-f file] [-mldbv] TIME
       at [-V] [-q queue] [-f file] [-mldbv] -t time_arg
       at -c job [job...]
       atq [-V] [-q queue]
       atrm [-V] job [job...]
       batch [-V] [-q queue] [-f file] [-mv] [TIME]

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

       at      executes commands at a specified time.

       atq     lists the userâs pending jobs, unless the user is  the  superuser;  in
               that  case,  everybodyâs  jobs  are  listed.  The format of the output
               lines (one for each job) is: Job number, date, hour, job class.

       atrm    deletes jobs, identified by their job number.

       batch   executes commands when system load levels permit; in other words, when
               the  load average drops below 0.8, or the value specified in the invo-
               cation of atrun.

       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
       midnight,  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 MMDDYY or  MM/DD/YY  or  DD.MM.YY.
       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.

       For  example, to run a job at 4pm three days from now, you would do at 4pm + 3
       days, to run a job at 10:00am on July 31, you would do at 10am Jul 31  and  to
       run a job at 1am tomorrow, you would do at 1am tomorrow.

       The   exact   definition   of   the   time   specification  can  be  found  in
       /usr/share/doc/at-3.1.8/timespec.

       For both at and batch, commands are read from standard input or the file spec-
       ified with the -f option and executed.  The working directory, the environment
       (except for the variables TERM, DISPLAY and _) and the umask are retained from
       the time of invocation.  An at - or batch - command invoked from a su(1) shell
       will retain the current userid.  The user will be mailed  standard  error  and
       standard  output  from his commands, if any.  Mail will be sent using the com-
       mand /usr/sbin/sendmail.  If at is executed from a su(1) shell, the  owner  of
       the login shell will receive the mail.

       The superuser may use these commands in any case.  For other users, permission
       to use at is determined by the files /etc/at.allow and /etc/at.deny.

       If the file /etc/at.allow exists, only usernames mentioned in it  are  allowed
       to use at.

       If  /etc/at.allow  does not exist, /etc/at.deny is checked, every username not
       mentioned in it is then allowed to use at.