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Tomas.R
Asked: 2020-12-10 04:32:01 +0800 CST

root cron job doesn't work (18.04 LTS SERVER)

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As a root I run three dd processes in the background

dd if=/dev/urandom of=/dev/null &
dd if=/dev/urandom of=/dev/null &
dd if=/dev/urandom of=/dev/null &

As a root I edit a crontab and put a job to kill all dd processes that were run by root every minute

crontab -e 

* * * * * pkill -u root dd

But when I use top to monitor processes the dd processes don't get killed. Why ?

Messages from /var/log/syslog :

Dec  9 12:33:01 champion CRON[3395]: Authentication failure
Dec  9 12:33:29 champion crontab[3383]: (root) END EDIT (root)
Dec  9 12:34:01 champion cron[1432]: Authentication failure
Dec  9 12:34:01 champion CRON[3398]: Authentication failure

Tried these suggestions but it didn't help :

  1. Invoke the crontab with :

    crontab -e -u root

  2. Make sure that you have an empty line at the end of the cronjob file, meaning that every line ends with a newline.

cron dd

2 Answers

  1. Best Answer

    Based on the following diagnostic output from pwck -r:

    no matching password file entry in /etc/shadow add user 'root' in /etc/shadow?

    you can see that the problem is the same as described here:

    You can fix it by running pwck again, this time in read-write mode

    sudo pwck

    and accepting the suggested action to add the appropriate shadow file entry for user root.

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  2. Have you made sure you're writing the crontab with root?

    sudo crontab -e

    A non-root account would not be able to kill processes owned by root.

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