To diagnose and solve, here’s what worked for me.
To find out what failed, go to your ruby gems directory.
For example:
$ cd <MY RUBY DIRECTORY>/lib/ruby/gems/2.0.0/gems
If you don’t know your gem directory, try this:
$ echo $GEM_HOME
/opt/gems/2.0.0
$ cd /opt/gems/2.0.0/gems
What version of nokogiri am I installing?
$ ls -ladg nokogiri-*
nokogiri-1.5.5
Go to the installer directory:
$ cd nokogiri-1.5.5/ext/nokogiri
Try installing manually:
$ ruby extconf.rb
Result:
checking for libxml/parser.h... *** extconf.rb failed ***
...
I’m using Ubuntu so I search for any similar packages:
$ aptitude search libxml
Results:
p libxml2 - GNOME XML library
p libxml2-dev - Development files for the GNOME XML library
...
I believe that libxml2 will work fine.
$ apt-get install libxml2
Ruby native gems often need the *-dev packages for headers so install them:
$ apt-get install libxml2-dev
Now do I have the parser file?
$ find / | grep libxml/parser.h
Yes, the result shows the parser:
/usr/include/libxml2/libxml/parser.h
Now try installing again, this time providing the libxml2 path:
$ gem install nokogiri -- --with-xml2-dir=/usr/include/libxml2
It still fails, so read the mkmf error log:
$ more mkmf.log
The error log shows what failed and has these lines that look promising:
package configuration for libxslt is not found
Is there a package for it?
$ aptitude search libxslt
Results:
v libxslt-dev
i libxslt-ruby
...
Install the dev package:
$ apt-get install libxslt-dev
Now try installing again, and also put xslt on the path:
$ gem install nokogiri -- \
--with-xml2-dir=/usr/include/libxml2 \
--with-xslt-dir=/usr/include/libxslt
Success!