I had issues installing ruby using rbenv on the raspberry pi. Half way through the install, the pi would power down with no trace of what caused the error. I guessed that it was due to /tmp space issues.
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Follow instructions to install ruby using 'rbenv' (except for the steps to increase your /tmp size): http://blog.pedrocarrico.net/post/29478085586/compiling-and-installing-ruby-on-the-raspberry-pi-using
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Increase your /tmp size (requires reboot)