Ttdfjt

I've understood that the entries in /etc/cron.d/ are executed automatically. But I have also found these in /etc/

/etc/cron.daily/
/etc/cron.hourly/
/etc/cron.monthly/
/etc/cron.weekly/

In /etc/cron.d/ I found 0hourly which has this content:

01 * * * * root run-parts /etc/cron.hourly

There are no files called *daily, *monthly or *weekly.

Does this mean that if I add a script in /etc/cron.hourly it will get executed automatically? And that this will NOT happen for scripts in /etc/cron.daily, /etc/cron.monthly/ and /etc/cron.weekly/?

EDIT:

My /etc/crontab is empty, except for initializing the variables SHELL, PATH and MAILTO.

In /etc/cron.hourly/ I found the script 0anacron which seems to check if cron.daily have been run today or not. I also found /etc/anacron which contains this:

# /etc/anacrontab: configuration file for anacron

# See anacron(8) and anacrontab(5) for details.

SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22

#period in days delay in minutes job-identifier command
1 5 cron.daily nice run-parts /etc/cron.daily
7 25 cron.weekly nice run-parts /etc/cron.weekly
@monthly 45 cron.monthly nice run-parts /etc/cron.monthly

I guess it gave me some reading to do. Especially anacron(8) and anacrontab(5).

It seems CentOS is like Ubuntu in this aspect, just configured slightly differently. Ubuntu uses anacron to run the daily/weekly/monthly jobs, and they're configured in /etc/crontab and /etc/anacrontab.

With CentOS, first we have:

# cat /etc/cron.hourly/0anacron
#!/bin/sh
# Check whether 0anacron was run today already
if test -r /var/spool/anacron/cron.daily; then
 day=`cat /var/spool/anacron/cron.daily`
fi
if [ `date +%Y%m%d` = "$day" ]; then
 exit 0;
fi

# Do not run jobs when on battery power
if test -x /usr/bin/on_ac_power; then
 /usr/bin/on_ac_power >/dev/null 2>&1
 if test $? -eq 1; then
 exit 0
 fi
fi
/usr/sbin/anacron -s

which checks/runs anacron once a day, and then:

# cat /etc/anacrontab
# /etc/anacrontab: configuration file for anacron

# See anacron(8) and anacrontab(5) for details.

SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22

#period in days delay in minutes job-identifier command
1 5 cron.daily nice run-parts /etc/cron.daily
7 25 cron.weekly nice run-parts /etc/cron.weekly
@monthly 45 cron.monthly nice run-parts /etc/cron.monthly

where the daily, weekly and monthly crontabs are configured.