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I am using ruby 1.9.3 and I need to update Rails version or need to reinstall the latest version but in vain.

I've tried to install rails (sudo gem install rails), showing:

Building native extensions.  This could take a while...
ERROR:  Error installing rails:
    ERROR: **Failed to build gem native extension**.

        /usr/bin/ruby1.9.1 extconf.rb
/usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in **`require': cannot load such file -- mkmf (LoadError)**
    from /usr/local/lib/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
    from extconf.rb:1:in `<main>'

Gem files will remain installed in /usr/lib/ruby/gems/1.9.1/gems/json-1.7.6 for inspection.
Results logged to /usr/lib/ruby/gems/1.9.1/gems/json-1.7.6/ext/json/ext/generator/gem_make.out

Where to go from here? I am not able to remove rails because of installing rails with sudo command.

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"I am using ruby 1.9.3" The output shows you're running 1.9.1 instead. – gertvdijk Jan 24 '13 at 12:26

Based on this very similar question on StackOverflow you just need to install the -dev package of your Ruby version installed, e.g.:

sudo apt-get install ruby1.9.1-dev

And then re-run the gem installation.

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