CRON(8) System Manager's Manual CRON(8)
NAME
cron - clock daemon
SYNOPSIS
cron [-d[level]]
DESCRIPTION
The cron daemon executes tasks that must be repeated every now and then
(cron jobs), and tasks that must be run just once (at jobs). It is
normally used to run daily or weekly system maintenance scripts. What
it needs to run and when is specified in a number of "cron tables", or
crontab files for short. These tables are:
/usr/lib/crontab
/usr/local/lib/crontab
/var/lib/crontab
/var/opt/name/lib/crontab (Minix-vmd only)
/usr/spool/crontabs/user
These files follow the usual pattern: One for the standard things, one
for local tasks per organization, one for tasks per system, and one
crontab per installed package. (Cron reads /usr/lib/packages to find
names of installed packages, it doesn't just grab everything in
/var/opt.) The last set of files fall outside the normal pattern, they
are per user crontabs that one can create with the crontab(1) command.
The file names in /usr/spool/crontabs/ are login names of the file own-
ers.
The format of a crontab file is described in crontab(5).
AT jobs
Cron also takes care of the execution of jobs issued by at(1) that are
found in /usr/spool/at/. Cron simply runs the AT job as if there were
an "sh at-job" as a cron job at the appropriate time under the user-id
of the owner of the script. The script takes care of the rest. See
at(1) for the details.
Job I/O
Standard input, output and error are the same as cron's if the job is
started by the system crontabs or from package crontabs. This means
that output from system jobs usually ends up on the console and in the
log file. Output from personal cron jobs or at jobs are mailed to the
owner of the job. No mail is sent if the job is silent.
OPTIONS
[-d[level]]
Set the debug level, by default 1. Makes cron print info on what
it happens to be doing. Level 1 just tells about sleep times and
what job is executed, level 2 also shows the internal crontab data
on a table load. (With time fields translated to match those of
struct tm, see ctime(3).)
SIGNALS
Cron takes the following actions when sent one of the following sig-
nals:
SIGHUP Reload the crontab tables if they changed since the last
time they were loaded, and reexamine the AT job spool.
Used by at(1) and crontab(1).
SIGUSR1 Increase the debug level by 1.
SIGUSR2 Turn debugging off.
ENVIRONMENT
Cron sets the environment variables USER, LOGNAME, HOME, and SHELL to
the user's login name (2x), home directory, and shell if a job is exe-
cuted for a given user. The working directory is set to the user's
home directory. Everything else is inherited from cron, exactly as
cron got it when it started. Note that commands are always passed to
/bin/sh, not to the user's shell.
System cron jobs are in principle executed with cron's environment, use
-u root or the crontab file /usr/spool/crontabs/root if you want to
give root the same treatment as ordinary users.
FILES
/usr/lib/crontab Main MINIX 3 crontab file.
/usr/local/lib/crontab Local jobs for all systems in an organization.
/var/lib/crontab System specific jobs.
/var/opt/name/lib/crontab
Per package jobs for Minix-vmd.
/usr/lib/packages List of installed packages.
/usr/spool/crontabs/user Per user jobs.
/usr/spool/at/* Jobs issued by at(1).
/usr/run/cron.pid Process id of cron when cron is running. Used
by at(1) and crontab(1) to send cron a hangup
signal.
SEE ALSO
at(1), crontab(1).
NOTES
A job is not reissued until a previous instance of it has exited. The
next time to execute is computed from the previous time it ran. If job
issuing lags behind on the system time then the next time to run it is
computed from the current system time.
BUGS
Cron doesn't like it if the system time is changed. If set forward
then cron will react when it next wakes up by running all jobs within
the skipped time once or twice before it catches up. Setting the clock
backwards makes cron play dead until the system time passes the old
time. (Changing the system time is bad idea anyway, and not just be-
cause of cron.)
Jobs that fall in the missing hour in a change to Daylight Saving Time
are skipped. Nothing is done in the extra hour on the change out of
DST.
AUTHOR
Kees J. Bot (
[email protected])
CRON(8)