NAME

cron − clock daemon

SYNOPSIS

/etc/cron

DESCRIPTION

Cron executes commands at specified dates and times according to the instructions in the file /usr/lib/crontab.  Because cron never exits, it should be executed only once.  This is best done by running cron from the initialization process through the file /etc/rc (see init(1M)).

The file crontab consists of lines of six fields each.  The fields are separated by spaces or tabs.  The first five are integer patterns that specify in order:

minute (0-59),
hour (0-23),
day of the month (1-31),
month of the year (1-12),
and day of the week (0-6, with 0=Sunday).

Each of these patterns may contain:

a number in the (respective) range indicated above;
two numbers separated by a minus (indicating an inclusive range);
a list of numbers separated by commas (meaning all of these numbers); or
an asterisk (meaning all legal values).

The sixth field is a string that is executed by the shell at the specified time(s).  A % in this field is translated into a new-line character.  Only the first line (up to a % or the end of line) of the command field is executed by the shell.  The other lines are made available to the command as standard input.  A \% yields %; \\ yields \.  A newline preceded with a backslash and any following blanks or tabs are stripped out.  Other occurences of \ are removed. 

Cron examines crontab once a minute to see if it has changed; if it has, cron reads it.  Thus it takes only a minute for entries to become effective. 

EXAMPLE

If the shell file /etc/rc contains the command line

/etc/cron

the clock daemon will be started every time /etc/rc is invoked, i.e., each time the system goes into multi-user mode after booting. 

FILES

/usr/lib/crontab
/usr/adm/cronlog

SEE ALSO

init(1M), sh(1). 

DIAGNOSTICS

A history of all actions by cron are recorded in /usr/adm/cronlog. 

BUGS

Cron reads crontab only when it has changed, but it reads the in-core version of that table once a minute.  A more efficient algorithm could be used.  The overhead in running cron is about one percent of the CPU, exclusive of any commands executed by cron. Revision 1.0  85/05/01  16:33:03  robinf Initial Revision
 

Version 2.3  —  July 04, 1985