Ruby version management (rvm)

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# Set default ruby version
rvm --default use 1.9.3
ruby -v

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# Install a particular version of Ruby
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rvm install ruby-1.9.2-p320

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# To switch back to your system ruby:
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rvm use system
ruby -v

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# To switch at any time to the ruby you have selected as default:
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rvm default
ruby -v

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# To show what ruby is currently the selected default, if any, do:
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rvm list default

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# If you wish to switch back to your system ruby as default, 
# remember that RVM does not "manage" the system ruby and is "hands off".
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# This means to set the "system" ruby as default, you reset RVM's defaults as follows.
rvm reset
ruby -v