I'm trying to get a ruby script to run via cron. The cron job looks like this:

* * * * * /usr/local/rvm/rubies/ruby-2.0.0-p353/bin/ruby /usr/share/adafruit/webide/repositories/shed_watcher/lib/shed_watcher.rb >> /tmp/cron_shed_watcher.log 2>&1

I've encountered a couple of problems. The first was that the environment cron runs in could find one of the required gems:

require "rest_client"

This was solved by setting the GEM_PATH in the crontab:


The second problem is that the script, running on Occidentals on a Raspberry Pi, makes use of a temperature probe and needs to call:

`modprobe w1-gpio`
`modprobe w1-therm`

i.e. modprobe commands in backticks. Cron cannot execute this, instead I get an "operation not permitted" message. modprobe is found in /sbin so I added a path entry to my crontab:


But I get the same error (not surprising as it was a "not permitted" message, not a "not found" message). What do I need to do to get this working?

  • What if you just added your modprobes to rc.local or similar to run it once at boot time. – UVV Jan 12 '14 at 12:06
  • That's a good idea, although I'd prefer the script to not rely on another file – barry Jan 12 '14 at 12:27

You should add your job to the root's crontab, not the user's one. User's crontabs always executed with uid/gid of the user when root crontab can contain additional field:

* * * * * username /path/to/the/command

Then command will be executed with username privileges/permissions.

  • Thanks, that worked although I didn't need to add a username – barry Jan 12 '14 at 20:32
  • It works as root if no username placed, cause root hasn't restrictions. But for security reasons you have to set appropriate uid - possibly www. – Kondybas Jan 12 '14 at 21:37
  • Hmmm.... I didn't set a uid – barry Jan 13 '14 at 0:08
  • Crontab owner always assumed when username omitted. Root can change to privileges to some other user while nonprivileged users can't. – Kondybas Jan 13 '14 at 10:21

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