Scientific Operations



CRON(VIII)                   10/25/74                  CRON(VIII)



NAME
     cron - clock daemon

SYNOPSIS
     /etc/cron

DESCRIPTION
     Cron  executes commands at specified dates and times accord-
     ing to the instructions in the file /usr/lib/crontab.  Since
     cron  never exits, it should only be executed once.  This is
     best done by running cron from  the  initialization  process
     through the file /etc/rc; see init(VIII).

     Crontab  consists  of  lines of six fields each.  The fields
     are separated by spaces or tabs.  The first five are integer
     patterns  to  specify the minute (0-59), hour (0-23), day of
     the month (1-31), month of the year (1-12), and day  of  the
     week  (1-7  with 1=monday).  Each of these patterns may con-
     tain a number in the range above; two numbers separated by a
     minus meaning a range inclusive; a list of numbers separated
     by commas meaning any of the numbers; or an asterisk meaning
     all  legal values.  The sixth field is a string that is exe-
     cuted by the Shell at the specified times.  A percent  char-
     acter  in  this field is translated to a new-line character.
     Only the first line (up to a % or end of line) of  the  com-
     mand  field  is  executed by the Shell.  The other lines are
     made available to the command as standard input.

     Crontab is examined by cron every hour.  Thus it could  take
     up  to  an  hour for entries to become effective.  If it re-
     ceives a hangup signal, however, the table is examined imme-
     diately; so `kill -1 ...' can be used.

FILES
     /usr/lib/crontab

SEE ALSO
     init(VIII), sh(I), kill(I)

DIAGNOSTICS
     None - illegal lines in crontab are ignored.

BUGS
     A  more  efficient algorithm could be used.  The overhead in
     running cron is about one percent of the machine,  exclusive
     of any commands executed.











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