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. 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(8).

/usr/lib/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 dash 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 input. 

Here are a few example lines from /usr/lib/crontab, to give you a better sense of the file’s format:

0 0 ∗ ∗ ∗ calendar -
15 0 ∗ ∗ ∗ /usr/etc/sa -s >/dev/null
0,30 ∗ ∗ ∗ ∗ /etc/dmesg - >>/usr/adm/messages
15 4 ∗ ∗ ∗ find /usr/preserve -mtime +7 -a -exec rm -f {} ;
10 4 ∗ ∗ ∗ egrep ’SYSERR|refused|unreachable’ /usr/spool/log/syslog.0 |mail Postmaster

Cron examines /usr/lib/crontab under the following conditions:

•At least once per hour (on the hour). 

•When the next command is to be run — cron looks ahead until the next command and sleeps until then. 

•When cron’s process is sent a SIGHUP.  This means that someone who changes /usr/lib/crontab can get cron to look at it right away. 

You can also create the /usr/adm/cronlog file, if you wish.  If the file exists, cron logs to it each time it executes an instruction from /usr/lib/crontab.  You can thus use the cronlog file to make sure cron is running properly.  The lines in the file consist of a timestamp, and process statement.  Lines look something like this:

Thu Mar  8 10:20:00 1984: /etc/dmesg - >>/usr/adm/messages
Thu Mar  8 10:29:59 1984: /etc/atrun
Thu Mar  8 10:30:00 1984: /etc/dmesg - >>/usr/adm/messages
Thu Mar  8 10:39:59 1984: /etc/dmesg - >>/usr/adm/messages

FILES

/usr/lib/crontabInstruction file
/usr/adm/cronlogLog file

SEE ALSO

crontab(5)

Sun Release 3.2  —  Last change: 4 November 1985