夏にjavaの授業を取る予定なので先取りしようとクラスのHP参考にbrew install gitを動かそうとしするもこんなエラーを吐く。。

/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- mach (LoadError)
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Library/Homebrew/extend/pathname.rb:2:in `<top (required)>'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Library/Homebrew/global.rb:3:in `<top (required)>'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Library/brew.rb:15:in `<main>'
airbears2-10-142-122-61:~ hironorisama$ brew install python3
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- mach (LoadError)
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Library/Homebrew/extend/pathname.rb:2:in `<top (required)>'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Library/Homebrew/global.rb:3:in `<top (required)>'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/2.0.0/rubygems/core_ext/kernel_require.rb:55:in `require'
	from /usr/local/Library/brew.rb:15:in `<main>'

stackoverflowでざっとスキミングしながらback to backしても全く改善の余地なし。以下、直接的な問題の解決という点ではあまり参考にならなかったリンク：

*とは言えhomebrew周辺の構造理解に少しでも役に立ったということは言うまでもないので興味のある方は自分もエラーを吐いた気に鳴って読んでみると面白いかもしれない。

そこで包括的にググってみることに。なんでこれを最初にしなかったのか。。

すると最初のトップページで以下のようなリンクを発見:

homebrew not working on OSXに書いてあるとおりにやってみる:

First, open terminal and cd /usr/local/, and git status to see if Homebrew is clean.
if dirty, git reset --hard && git clean -df
then brew doctor, brew update

On branch master
Changes not staged for commit:
  (use "git add/rm <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	deleted:    Library/Aliases/caf
	deleted:    Library/Aliases/docker-compose
	deleted:    Library/Aliases/gperftools
	deleted:    Library/Aliases/htop
	deleted:    Library/Aliases/kotlin
	deleted:    Library/Aliases/mbedtls
	deleted:    Library/Aliases/mpich
	deleted:    Library/Aliases/nim
	deleted:    Library/Aliases/ocaml
	deleted:    Library/Aliases/offlineimap
	deleted:    Library/Aliases/shell-fm
	deleted:    Library/Aliases/wolfssl
	deleted:    Library/Contributions/example-formula.rb
	deleted:    Library/ENV/include/6.3/__debug
	deleted:    Library/Formula/app-engine-java-sdk.rb
	deleted:    Library/Formula/cdimgtools.rb
	deleted:    Library/Formula/clay.rb
	deleted:    Library/Formula/cppdom.rb
	deleted:    Library/Formula/cracklib-words.rb
	deleted:    Library/Formula/cyassl.rb
	deleted:    Library/Formula/echoping.rb
	deleted:    Library/Formula/ffts.rb
	deleted:    Library/Formula/fig.rb
	deleted:    Library/Formula/fsharp.rb
	deleted:    Library/Formula/gant.rb
	deleted:    Library/Formula/gdrive.rb
	deleted:    Library/Formula/git-encrypt.rb
	deleted:    Library/Formula/go-app-engine-32.rb
	deleted:    Library/Formula/go-app-engine-64.rb
	deleted:    Library/Formula/google-app-engine.rb
	deleted:    Library/Formula/google-perftools.rb
	deleted:    Library/Formula/googlecl.rb
	deleted:    Library/Formula/gptfdisk.rb
	deleted:    Library/Formula/groonga-normalizer-mysql.rb
	deleted:    Library/Formula/gtkglarea.rb
	deleted:    Library/Formula/hamsterdb.rb
	deleted:    Library/Formula/honeyd.rb
	deleted:    Library/Formula/iojs.rb
	deleted:    Library/Formula/ipe.rb
	deleted:    Library/Formula/kbtin.rb
	deleted:    Library/Formula/kotlin-compiler.rb
	deleted:    Library/Formula/kumofs.rb
	deleted:    Library/Formula/libcppa.rb
	deleted:    Library/Formula/magit.rb
	deleted:    Library/Formula/mpich2.rb
	deleted:    Library/Formula/mysql-connector-odbc.rb
	deleted:    Library/Formula/mysql-proxy.rb
	deleted:    Library/Formula/net6.rb
	deleted:    Library/Formula/nimrod.rb
	deleted:    Library/Formula/objective-caml.rb
	deleted:    Library/Formula/offline-imap.rb
	deleted:    Library/Formula/p11-kit.rb
	deleted:    Library/Formula/paq8px.rb
	deleted:    Library/Formula/par2tbb.rb
	deleted:    Library/Formula/pdf-tools.rb
	deleted:    Library/Formula/perlmagick.rb
	deleted:    Library/Formula/phash.rb
	deleted:    Library/Formula/plt-racket.rb
	deleted:    Library/Formula/polarssl.rb
	deleted:    Library/Formula/qi.rb
	deleted:    Library/Formula/qiv.rb
	deleted:    Library/Formula/rbenv-gem-rehash.rb
	deleted:    Library/Formula/rocket.rb
	deleted:    Library/Formula/rubber.rb
	deleted:    Library/Formula/sedna.rb
	deleted:    Library/Formula/synfigstudio.rb
	deleted:    Library/Formula/telepathy-gabble.rb
	deleted:    Library/Formula/telepathy-glib.rb
	deleted:    Library/Formula/telepathy-idle.rb
	deleted:    Library/Formula/telepathy-mission-control.rb
	deleted:    Library/Formula/uim.rb
	deleted:    Library/Formula/vobcopy.rb
	deleted:    Library/Formula/whereami.rb
	deleted:    Library/Formula/why3.rb
	deleted:    Library/Formula/wps2odt.rb
	deleted:    Library/Formula/xastir.rb
	deleted:    Library/Formula/ydict.rb
	deleted:    Library/Homebrew/cmd/aspell-dictionaries.rb
	deleted:    Library/Homebrew/cmd/update.rb
	deleted:    Library/Homebrew/extend/symbol.rb
	deleted:    Library/Homebrew/keg_fix_install_names.rb
	deleted:    Library/Homebrew/mach.rb
	deleted:    Library/Homebrew/requirements/apr_dependency.rb
	deleted:    Library/Homebrew/requirements/fortran_dependency.rb
	deleted:    Library/Homebrew/requirements/java_dependency.rb
	deleted:    Library/Homebrew/requirements/language_module_dependency.rb
	deleted:    Library/Homebrew/requirements/ld64_dependency.rb
	deleted:    Library/Homebrew/requirements/mpi_dependency.rb
	deleted:    Library/Homebrew/requirements/osxfuse_dependency.rb
	deleted:    Library/Homebrew/requirements/python_dependency.rb
	deleted:    Library/Homebrew/requirements/tuntap_dependency.rb
	deleted:    Library/Homebrew/requirements/x11_dependency.rb
	deleted:    Library/Homebrew/test/.gitignore
	deleted:    Library/Homebrew/test/test_cmd_cleanup.rb
	deleted:    Library/Homebrew/test/test_language_module_dependency.rb
	deleted:    Library/Homebrew/test/test_mpi_dependency.rb
	deleted:    Library/Homebrew/test/test_updater.rb
	deleted:    Library/Homebrew/test/test_x11_dependency.rb
	deleted:    share/doc/homebrew/Homebrew-0.8.md
	deleted:    share/doc/homebrew/Homebrew-0.9.3.md
	deleted:    share/doc/homebrew/Homebrew-0.9.md
	deleted:    share/doc/homebrew/Homebrew-2.md
	deleted:    share/doc/homebrew/Press.md
	deleted:    share/doc/homebrew/The-brew-command.md
	deleted:    share/doc/homebrew/brew-test-bot-failed-jenkins.png
	deleted:    share/doc/homebrew/brew-test-bot-failed-pr.png
	deleted:    share/doc/homebrew/brew-test-bot-failed-test.png
	deleted:    share/doc/homebrew/brew-test-bot-for-core-contributors-block.png
	deleted:    share/doc/homebrew/brew-test-bot-for-core-contributors-console.png
	deleted:    share/doc/homebrew/brew-test-bot-for-core-contributors-tabs.png
	deleted:    share/doc/homebrew/brew-test-bot-for-core-contributors-trigger.png
	deleted:    share/doc/homebrew/brew-test-bot-passed-jenkins.png
	deleted:    share/doc/homebrew/brew-test-bot-passed-pr.png
	deleted:    share/doc/homebrew/brew-test-bot-triggered-pr.png

no changes added to commit (use "git add" and/or "git commit -a")

これは汚いと考えてよろしいのでしょうか。。とりあえずgit reset --hard && git clean -dfを実行してみることに。

HEAD is now at ce8f101 Update minitest

と返ってきたのでbrew doctorとbrew updateをもう一度実行。まだ同じエラーを吐く。

warning: unable to unlink CONTRIBUTING.md: Permission denied
warning: unable to unlink SUPPORTERS.md: Permission denied
fatal: cannot create directory at '.github': Permission denied

どうやら/usr/local内の権限に問題があるのでは？と判断したので上記のリンク周辺を参考にsudo chown -R $(whoami):admin /usr/localを実行し、もう一度brew doctorとbrew updateをするも状況は変わらず。そこでpermissionを変更した後にgit reset --hard && git clean -dfをやったら何か変わるのでは？と考え試しに行ってみる。その後brew doctorとbrew updateを実行するとさっきとは少しだけ違うメッセージを受け取る:

Error: undefined method `new' for Patch:Module
Please report this bug:
    https://git.io/brew-troubleshooting
/usr/local/Library/Homebrew/patch.rb:115:in `initialize'
/usr/local/Library/Homebrew/patch.rb:18:in `new'
/usr/local/Library/Homebrew/patch.rb:18:in `create'
/usr/local/Library/Homebrew/software_spec.rb:168:in `patch'
/usr/local/Library/Homebrew/formula.rb:1920:in `block in patch'
/usr/local/Library/Homebrew/formula.rb:1920:in `each'
/usr/local/Library/Homebrew/formula.rb:1920:in `patch'
/usr/local/Library/Formula/a2ps.rb:19:in `<class:A2ps>'
/usr/local/Library/Formula/a2ps.rb:1:in `load_formula'
/usr/local/Library/Homebrew/formulary.rb:22:in `module_eval'
/usr/local/Library/Homebrew/formulary.rb:22:in `load_formula'
/usr/local/Library/Homebrew/formulary.rb:35:in `load_formula_from_path'
/usr/local/Library/Homebrew/formulary.rb:84:in `load_file'
/usr/local/Library/Homebrew/formulary.rb:75:in `klass'
/usr/local/Library/Homebrew/formulary.rb:71:in `get_formula'
/usr/local/Library/Homebrew/formulary.rb:208:in `factory'
/usr/local/Library/Homebrew/cmd/update.rb:173:in `block in report'
/usr/local/Library/Homebrew/cmd/update.rb:159:in `each_line'
/usr/local/Library/Homebrew/cmd/update.rb:159:in `report'
/usr/local/Library/Homebrew/cmd/update.rb:24:in `update'
/usr/local/Library/brew.rb:140:in `<main>'

いちいち報告している暇がなかったので警告をシカトしてもう一度brew update:

Already up-to-date.

この一言までたどり着くのに長かった。。念のためもう一度brew doctorを行い最後にもう一度brew install gitを実行。

==> Downloading https://homebrew.bintray.com/bottles/git-2.7.2.el_capitan.bottle.tar.gz
######################################################################## 100.0%
==> Pouring git-2.7.2.el_capitan.bottle.tar.gz
==> Caveats
The OS X keychain credential helper has been installed to:
  /usr/local/bin/git-credential-osxkeychain

The "contrib" directory has been installed to:
  /usr/local/share/git-core/contrib

Bash completion has been installed to:
  /usr/local/etc/bash_completion.d

zsh completion has been installed to:
  /usr/local/share/zsh/site-functions

Emacs Lisp files have been installed to:
  /usr/local/share/emacs/site-lisp/git
==> Summary
🍺  /usr/local/Cellar/git/2.7.2: 1,409 files, 236.4M

上のリンクにも書いてありますが、振り返ってみるとどうやらパソコン自体のバージョンアップデートした際の互換性にも原因があったみたい。