How to install a specific version of a ruby gem?

Use the -v flag:

$ gem install fog -v 1.8

Use the --version parameter (shortcut -v):

$ gem install rails -v 0.14.1

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You can also use version comparators like >= or ~>

$ gem install rails -v '~> 0.14.0'

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Or with newer versions of gem even:

$ gem install rails:0.14.4 rubyzip:'< 1'
…
Successfully installed rails-0.14.4
Successfully installed rubyzip-0.9.9

For installing gem install gemname -v versionnumber

For uninstall gem uninstall gemname -v versionnumber