Why isnt my cron job working?

November 16, 2016 4k views
Linux Commands Ubuntu 16.04
MikeChops
By:
MikeChops

For some reason my cron entry isn't running at all. I'm trying to make a job that executes ZNC after reboots and crashes.

The commands I tried:

*/10 * * * * /usr/local/bin/znc >/dev/null 2>&1

Also tried adding the user:

*/10 * * * * znc-admin /usr/local/bin/znc >/dev/null 2>&1

Neither one is working. These were added using the "crontab -e" command, under root user.

These were tested by rebooting and waiting 10 minutes for the command to work.

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3 Answers
ioanmoldovan199 November 16, 2016

The problem with the cronjob is that it has one extra star (which causes a syntax error). Replace it with this one:

*/10 * * * /usr/local/bin/znc >/dev/null 2>&1
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xMudrii November 16, 2016

@MikeChops
Sorry, but I don't agree @ioanmoldovan199, you have enough stars.

*/10 * * * * part is OK.
General syntax is OK - * * * * * * where from left to right * means:
1 - Minute (0-59)
2 - Hour (0-23)
3 - Day of Month (1-31)
4 - Month (0-7)
5 - Day of Week (0-6)
6 - Command
So, your syntax means execute command every 10 minutes. */10 on minute part, means it'll be repetitive - it'll execute every 10 minutes.

Even if you look at ZNC wiki, you'll see that same line:

*/10 * * * * /usr/local/bin/znc >/dev/null 2>&1

My guess is that command is invalid.
What could be a problem? Well, maybe ZNC is not located at /usr/local/bin/znc.
Verify location of it using which command. which will return you where it is stored. Execute this:

  • which znc

Sample output of 'which'
/usr/bin/znc

If you installed using apt-get, you'll probably have it in /usr/bin/znc instead of /usr/local/bin/znc, so line for crontab would be:

*/10 * * * * /usr/bin/znc >/dev/null 2>&1

If output of which is something other, use it.

To verify is it problem, you can try executing location of it - /usr/local/bin/znc.
If you type this in terminal:

  • /usr/local/bin/znc

In case it is under that location you'll see something along:

Sample output
[ .. ] Checking for list of available modules...
[ >> ] ok
...

(or any valid ZNC output).
Well, if it isn't under that location, you'll see this:

Sample output
-bash: /usr/local/bin/znc: No such file or directory
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  • MikeChops November 16, 2016

    I'll give this a try.
    Is putting "znc-admin" after the stars to run it as that user a valid command as well?

    edit: the "which" command returns /usr/local/bin/znc

    • xMudrii November 16, 2016

      I saw your edit, maybe it is not working because you're running it as root. If you try to run it as root, following will arise:

      • [ .. ] Checking for list of available modules...

      • [ >> ] ok

      • [ ** ] You are running ZNC as root! Don't do that! There are not many valid

      • [ ** ] reasons for this and it can, in theory, cause great damage!

      You can't put user with crontab -e or sudo crontab -e.
      crontab -e will let you change cronjobs for current user.
      sudo crontab -e will let you change cronjobs for root user.

      You can one of these:

      1. Use sudo crontab -u znc-admin -e. With this, you should be editing cronjobs for znc-admin user. There, at the end add same line as you would: */10 * * * * /usr/local/bin/znc >/dev/null 2>&1 If this is not working for you - you can login as that user. Make sure you only follow this part or above part with sudo crontab -u znc-admin -e. For example on your root user you can execute: command su - znc-admin That will login you to znc-admin user. Execute crontab -e and add needed line (same as above).

      Now there is second possibility. That's to use /etc/crontab file. In that file you can supply user for command to be executed.
      Open that file with your favorite text editor:

      • sudo nano /etc/crontab

      At the end add following line:

      */10 * * * * znc-admin /usr/local/bin/znc >/dev/null 2>&1

      Save it and exit editor.

      Optional, to be sure it'll affect changes, you can restart cron service:

      • sudo systemctl restart cron

      I hope it'll work for you. :)

      • MikeChops November 16, 2016

        Okay. I finally got it, turns out that I had to add the cron job to the users crontab using:

        crontab -u znc-admin -e

        Now to figure out how to replace the hair that fell out caused by the frustration.

        Thanks for your help as well!

Giand1941 October 24, 2017

Thank you for your answer. I had the same problem. Now it's okay.
If you also need help I work for this service and I can help you.

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