Rubyの環境には2種類の状態しかない。

壊れているか、これから壊れるかだ。

というわけでいつの間にか、gemを実行すると以下のエラーが出るようになってしまった。

hichon-no-MacBook-Pro:~ hichon$ gem list
/usr/local/Cellar/ruby/2.4.3_1/lib/ruby/2.4.0/yaml.rb:5:in `<top (required)>':
It seems your ruby installation is missing psych (for YAML output).
To eliminate this warning, please install libyaml and reinstall your ruby.
/usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': dlopen(/usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.bundle, 9): Library not loaded: /usr/local/opt/ruby/lib/libruby.2.4.1.dylib (LoadError)
  Referenced from: /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.bundle
  Reason: image not found - /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.bundle
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.rb:8:in `<top (required)>'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/local/Cellar/ruby/2.4.3_1/lib/ruby/2.4.0/yaml.rb:6:in `<top (required)>'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems.rb:701:in `load_yaml'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/config_file.rb:340:in `load_file'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/config_file.rb:202:in `initialize'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/gem_runner.rb:75:in `new'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/gem_runner.rb:75:in `do_configuration'
    from /usr/local/lib/ruby/site_ruby/2.4.0/rubygems/gem_runner.rb:40:in `run'
    from /usr/local/bin/gem:21:in `<main>'

復旧方法

問題のファイルを手動で削除して、該当のパッケージを再インストールして復旧した。

$ rm /usr/local/lib/ruby/gems/2.4.0/gems/psych-2.2.4/lib/psych.bundle
$ gem uninstall psych
$ gem install psych