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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