Ygtjki

Here's what I did on Debian Jessie:

• install cron via apt-get install cron
  


• put a backup_crontab file in /etc/cron.d/
  

However the task is never running.

Here are some outputs:

/# crontab -l
no crontab for root

/# cd /etc/cron.d && ls
backup_crontab

/etc/cron.d# cat backup_crontab
0,15,30,45 * * * * /backup.sh >/dev/null 2>&1

Is there something to do to activate a particular crontab, or to activate the cron "service" in itself?

Files in /etc/cron.d need to also list the user that the job is to be run under.

i.e.

0,15,30,45 * * * * root /backup.sh >/dev/null 2>&1

You should also ensure the permissions and owner:group are set correctly (-rw-r--r-- and owned by root:root)

I think you probably just missing a necessary blank line from the end of your cron file. I had the same issue, but after checking everything listed here (user permissions, filename, cron version etc.), I realized that I did not have line break after the last entry in my /etc/cron.d/own_cron and that causes the entire file being ignored.

If you're the only user on this computer, you might want to use just crontab -e. You'll be prompted to select an editor the first time you run the command. Then you can add this to it:

0,15,30,45 * * * * /backup.sh >/dev/null 2>&1

If you change to a normal user account, you'll need to use sudo crontab -e to configure the scripts you want scheduled to run as root.

crontab -l only displays the current crontab, once you set one up using crontab -e. If you have a cron file in /etc/cron.d/, it will not be displayed with crontab -l.

You will also need to verify that your script is executable with: chmod +x /backup.sh.

Another thing I've observed is that the file in /etc/cron.d cannot have an extension. In my particular case, I had a symbolic link:

# my-job.crontab
* * * * * root echo "my job is running!" >> /tmp/my-job.log

$: ln -sf /home/me/my-job.crontab /etc/cron.d/
# This did not work -> job would not run

$: ln -sf /home/me/my-job.crontab /etc/cron.d/my-job
# This did work -> job ran fine

File name restriction are documented at run-part man page: http://manpages.ubuntu.com/manpages/xenial/man8/run-parts.8.html, one can pass
an --regex option to override the file format.

The default cron behavior however stayed without extensions, see comments under: https://bugs.launchpad.net/ubuntu/+source/debianutils/+bug/38022

Check your version of cron.

It seems that if you are using Dillon's crond, you don't need the user in a /etc/cron.d entry.

I figured this out after nearly pulling out my remaining hair.

I have a handful of entries that have been dropped in /etc/cron.d by various installs. After some investigation, I found one of them was working. It didn't have the user. So I took the user out of the others. And they began working.

For Cron from *bian distros (like Raspbian) you need to enable the -l parameter of the Cron daemon. That is advisable to do using /etc/default/cron config file, enabling the EXTRA_OPTS.