$ ruby --version ruby 2.0.0p247 (2013-06-27 revision 41674) [universal.x86_64-darwin13] $ gem install rails Fetching: i18n-0.6.9.gem (100%) ERROR: While executing gem ... (Gem::FilePermissionError) You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory. sudoしていいの?と思ったけど一応調べた。 http://stackoverflow.com/questions/14607193/installing-gem-or-updating-rubygems-fails-with-permissions