cron - clock daemon
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:
/var/opt/name/lib/crontab (Minix-vmd only)
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
The format of a crontab file is described in crontab(5).
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.
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.
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).)
Cron takes the following actions when sent one of the following signals:
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.
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 executed
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.
/usr/lib/crontab Main Minix crontab file.
/usr/local/lib/crontab Local jobs for all systems in an organization.
/var/lib/crontab System specific jobs.
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
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.
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 because of
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.
Kees J. Bot (email@example.com)