Ubuntu Rails5 nokogiri 1.8.2 インストール エラー時の解決メモ ref: https://qiita.com/yowayowa/items/828d9fb91317f8db72ae

file0.txt

Installing nokogiri 1.8.2 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /var/lib/gems/2.5.0/gems/nokogiri-1.8.2/ext/nokogiri
/usr/bin/ruby2.5 -r ./siteconf20180609-8585-1wf897h.rb extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /var/lib/gems/2.5.0/gems/nokogiri-1.8.2 for inspection.
Results logged to /var/lib/gems/2.5.0/extensions/x86_64-linux/2.5.0/nokogiri-1.8.2/gem_make.out

An error occurred while installing nokogiri (1.8.2), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.8.2' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  rails was resolved to 5.1.4, which depends on
    actioncable was resolved to 5.1.4, which depends on
      actionpack was resolved to 5.1.4, which depends on
        actionview was resolved to 5.1.4, which depends on
          rails-dom-testing was resolved to 2.0.3, which depends on
            nokogiri

file1.txt

 sudo apt-get install build-essential liblzma-dev patch ruby-dev zlib1g-dev

file2.txt

libgmp-dev libgmpxx4ldbl liblzma-dev ruby-dev ruby2.5-dev ruby2.5-doc