ローカル環境にて、nokogiriが原因(?)でbundle installできないときの解決策

ローカル環境にrailsの開発環境を作ろうと以下の記事[Ruby初学者のRuby On Rails 環境構築【Mac】](https://qiita.com/TAByasu/items/47c6cfbeeafad39eda07)
を参考に進めていると、以下のエラーに遭遇した。

```ターミナル
$bundle install

#以下が実行結果

Fetching gem metadata from https://rubygems.org/.............
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Using rake 13.0.1
Using concurrent-ruby 1.1.6
Using i18n 1.8.2
Using minitest 5.14.0
Using thread_safe 0.3.6
Using tzinfo 1.2.6
Using zeitwerk 2.2.2
Using activesupport 6.0.2.1
Using builder 3.2.4
Using erubi 1.9.0
Using mini_portile2 2.4.0
Fetching nokogiri 1.10.8
Installing nokogiri 1.10.8 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/(user)/Desktop/Rails/vendor/bundle/ruby/2.5.0/gems/nokogiri-1.10.8/ext/nokogiri
/Users/(user)/.rbenv/versions/2.5.0/bin/ruby -I /Users/(user)/.rbenv/versions/2.5.0/lib/ruby/site_ruby/2.5.0 -r
./siteconf20200212-25664-dl2tcu.rb extconf.rb --use-system-libraries
checking if the C compiler accepts  -I
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/libxml2... *** 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
        --without-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=/Users/(user)/.rbenv/versions/2.5.0/bin/$(RUBY_BASE_NAME)
        --help
        --clean
/Users/(user)/.rbenv/versions/2.5.0/lib/ruby/2.5.0/mkmf.rb:456:in `try_do': The compiler failed to generate an
executable file. (RuntimeError)
You have to install development tools first.
        from /Users/(user)/.rbenv/versions/2.5.0/lib/ruby/2.5.0/mkmf.rb:574:in `block in try_compile'
        from /Users/(user)/.rbenv/versions/2.5.0/lib/ruby/2.5.0/mkmf.rb:521:in `with_werror'
        from /Users/(user)/.rbenv/versions/2.5.0/lib/ruby/2.5.0/mkmf.rb:574:in `try_compile'
        from extconf.rb:138:in `nokogiri_try_compile'
        from extconf.rb:162:in `block in add_cflags'
        from /Users/(user)/.rbenv/versions/2.5.0/lib/ruby/2.5.0/mkmf.rb:632:in `with_cflags'
        from extconf.rb:161:in `add_cflags'
        from extconf.rb:416:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

/Users/(user)/Desktop/Rails/vendor/bundle/ruby/2.5.0/extensions/x86_64-darwin-18/2.5.0/nokogiri-1.10.8/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/(user)/Desktop/Rails/vendor/bundle/ruby/2.5.0/gems/nokogiri-1.10.8
for inspection.
Results logged to
/Users/(user)/Desktop/Rails/vendor/bundle/ruby/2.5.0/extensions/x86_64-darwin-18/2.5.0/nokogiri-1.10.8/gem_make.out

An error occurred while installing nokogiri (1.10.8), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.10.8' --source 'https://rubygems.org/'` succeeds before
bundling.

In Gemfile:
  rails was resolved to 6.0.2.1, which depends on
    actioncable was resolved to 6.0.2.1, which depends on
      actionpack was resolved to 6.0.2.1, which depends on
        actionview was resolved to 6.0.2.1, which depends on
          rails-dom-testing was resolved to 2.0.3, which depends on
            nokogiri

```

**どうやらnokogiriがインストールできてないみたい..**

## まず、解決策

以下の記事[macOSアップデート後の『xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools)...』の対処法](https://qiita.com/nishina555/items/e23d73067a5cac182a63)
を参考にした。

以下を実行し、xcodeのツール（？）のインストールをした後、再度bundle installをしたら解決した。

```ターミナル
$ xcode-select --install
```
##どのように解決に至ったか

エラーメッセージに

```terminal

Check the mkmf.log file for more details.  You may
need configuration options.
```

と出ていたので、mkmk.logを開いた。
そこに

```mkmk.log
xcrun: error: invalid active developer path
```
というエラーが出ていたので、ググったところ上記の解決につながった記事がヒットした。