Error when installing nokogiri on mac with"sudo gem install nokogiri" - ruby-on-rails

Question

I was trying to install nokogiri because it is required for rails to be started
$ rails s
/usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/bundler-1.1.5/lib/bundler/spec_set.rb:90:in `block in materialize': Could not find nokogiri-1.5.5 in any of the sources (Bundler::GemNotFound)
from /usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/bundler-1.1.5/lib/bundler/spec_set.rb:83:in `map!'
from /usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/bundler-1.1.5/lib/bundler/spec_set.rb:83:in `materialize'
from /usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/bundler-1.1.5/lib/bundler/definition.rb:127:in `specs'
from /usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/bundler-1.1.5/lib/bundler/environment.rb:27:in `specs'
from /usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/rubygems-bundler-1.0.3/lib/rubygems-bundler/noexec.rb:41:in `candidate?'
from /usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/rubygems-bundler-1.0.3/lib/rubygems-bundler/noexec.rb:60:in `setup'
from /usr/local/rvm/gems/ruby-1.9.3-p194#global/gems/rubygems-bundler-1.0.3/lib/rubygems-bundler/noexec.rb:75:in `<top (required)>'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `require'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:60:in `rescue in require'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:35:in `require'
from /usr/local/rvm/gems/ruby-1.9.3-p194/bin/ruby_noexec_wrapper:9:in `<main>'
However, it failed when I tried to install nokogiri with the command
$ sudo gem install nokogiri
Here is the error.
$ sudo gem install nokogiri
Building native extensions. This could take a while...
ERROR: Error installing nokogiri:
ERROR: Failed to build gem native extension.
/usr/local/rvm/rubies/ruby-1.9.3-p194/bin/ruby extconf.rb
checking for libxml/parser.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
Provided configuration options:
--with-opt-dir
--with-opt-include
--without-opt-include=${opt-dir}/include
--with-opt-lib
--without-opt-lib=${opt-dir}/lib
--with-make-prog
--without-make-prog
--srcdir=.
--curdir
--ruby=/usr/local/rvm/rubies/ruby-1.9.3-p194/bin/ruby
--with-zlib-dir
--without-zlib-dir
--with-zlib-include
--without-zlib-include=${zlib-dir}/include
--with-zlib-lib
--without-zlib-lib=${zlib-dir}/lib
--with-iconv-dir
--without-iconv-dir
--with-iconv-include
--without-iconv-include=${iconv-dir}/include
--with-iconv-lib
--without-iconv-lib=${iconv-dir}/lib
--with-xml2-dir
--without-xml2-dir
--with-xml2-include
--without-xml2-include=${xml2-dir}/include
--with-xml2-lib
--without-xml2-lib=${xml2-dir}/lib
--with-xslt-dir
--without-xslt-dir
--with-xslt-include
--without-xslt-include=${xslt-dir}/include
--with-xslt-lib
--without-xslt-lib=${xslt-dir}/lib
--with-libxslt-config
--without-libxslt-config
--with-pkg-config
--without-pkg-config
--with-libxml-2.0-config
--without-libxml-2.0-config
--with-libiconv-config
--without-libiconv-config
/usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:381:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:506:in `try_cpp'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:970:in `block in find_header'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:790:in `block in checking_for'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:284:in `block (2 levels) in postpone'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:284:in `block in postpone'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:254:in `open'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:280:in `postpone'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:789:in `checking_for'
from /usr/local/rvm/rubies/ruby-1.9.3-p194/lib/ruby/1.9.1/mkmf.rb:969:in `find_header'
from extconf.rb:114:in `<main>'
Gem files will remain installed in /usr/local/rvm/gems/ruby-1.9.3-p194/gems/nokogiri-1.5.5 for inspection.
Results logged to /usr/local/rvm/gems/ruby-1.9.3-p194/gems/nokogiri-1.5.5/ext/nokogiri/gem_make.out

Answer 1

It looks like you need to install Xcode.

Answer 2

The reason you're getting this error is because the nokogiri gem includes C/C++ extensions that need to be compiled on your machine before they can run.
Just installing XCode won't fix it--you need to specifically install the XCode Command Line Tools, which is a C/C++ compiler/linker toolchain that isn't installed by default when you install XCode.
There's an option to install them within a menu inside the XCode GUI, see this thread for a screenshot:
Xcode 4.4 and later install Command Line Tools
If you've already done that, then you may need to create a symbolic link to your compiler so your OS knows where to find it:
Installing nokogiri Mac OS X 10.8.2 XCode installed