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


     cron - clock daemon




     Cron  executes  commands  at  specified  dates   and   times

     according  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

     contain  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 executed by the Shell at the specified times.

     A percent character in this field is translated  to  a  new-

     line  character.   Only  the first line (up to a % or end of

     line) of the command field is executed by  the  Shell.   The

     other  lines  are  made available to the command as standard


     Crontab is examined by cron every hour.  Thus it could  take

     up  to  an  hour  for  entries  to  become effective.  If it

     receives a hangup signal, however,  the  table  is  examined

     immediately; so `kill -1 ...' can be used.




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


     None - illegal lines in crontab are ignored.


     A more efficient algorithm could be used.  The  overhead  in

     running  cron is about one percent of the machine, exclusive

     of any commands executed.