cron
searches
/var/cron/tabs
for crontab files which are named after accounts in
/etc/passwd
.
Crontabs found are loaded into memory.
cron
also searches for
/etc/crontab
which is in a different format (see
crontab(5)).
cron
then wakes up every minute, examining all stored crontabs, checking each
command to see if it should be run in the current minute.
When executing commands, any output is mailed to the owner of the
crontab (or to the user named in the
MAILTO
environment variable in the crontab, if such exists).
Events such as
START
and
FINISH
are recorded in the
/var/log/cron
log file with date and time details.
This information is useful for a number of reasons, such as
determining the amount of time required to run a particular job.
By default, root has an hourly job that rotates these log files
with compression to preserve disk space.
Additionally,
cron
checks each minute to see if its spool directory's modtime (or the modtime
on
/etc/crontab
)
has changed, and if it has,
cron
will then examine the modtime on all crontabs and reload those which have
changed.
Thus
cron
need not be restarted whenever a crontab file is modified.
Note that the
crontab(1)
command updates the modtime of the spool directory whenever it changes a
crontab.
The -x flag turns on some debugging flags. debugflags is comma-separated list of debugging flags to turn on. If a flag is turned on, cron writes some additional debugging information to system log during its work. Available debugging flags are:
/var/cron/tabs
/etc/crontab
/var/log/cron