Yosemiteアップグレード後に発生したHomebrewのエラー対応（2015年9月版） ref: http://qiita.com/youthkee/items/d7fcb8132b97a83e4171

brew.rb(変更前)

if MACOS and MACOS_VERSION < 10.5
  abort <<-EOABORT.undent
    Homebrew requires Leopard or higher. For Tiger support, see:
    http://github.com/sceaga/homebrew/tree/tiger
  EOABORT
end

brew.rb(変更後)

if MACOS and MACOS_VERSION < 10.5 and MACOS_VERSION != 10.1 and MACOS_VERSION != 10.11
  abort <<-EOABORT.undent
    Homebrew requires Leopard or higher. For Tiger support, see:
    http://github.com/sceaga/homebrew/tree/tiger
  EOABORT
end

brew.rb（リモート最新版）

#!/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby -W0
（中略）
if OS.mac? and MacOS.version < "10.6"
  abort <<-EOABORT.undent
    Homebrew requires Snow Leopard or higher. For Tiger and Leopard support, see:
    https://github.com/mistydemeo/tigerbrew
  EOABORT
end

file0.txt

/usr/local/bin/brew: /usr/local/Library/brew.rb: /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby: bad interpreter: No such file or directory
/usr/local/bin/brew: line 21: /usr/local/Library/brew.rb: Undefined error: 0

file10.txt

$ git status
（中略）
# Unmerged paths:
#   (use "git add <file>..." to mark resolution)
#
#   both modified:      Library/brew.rb

file12.txt

$ git reset --hard origin/master

file13.txt

$ brew update

file14.txt

$ git reset --hard origin/master

file3.txt

Homebrew requires Leopard or higher. For Tiger support, see:
https://github.com/mistydemeo/tigerbrew

file6.txt

Mac OS X `10.1' is invalid

file7.txt

cd /usr/local/
git add Library/brew.rb
git commit -m "Fix brew.rb for yosemite update"

[master 7d35673] Fix brew.rb for yosemite update
 1 file changed, 2 insertions(+), 2 deletions(-)

file8.txt

$ brew update

error: Your local changes to the following files would be overwritten by merge:
    README.md
Please, commit your changes or stash them before you can merge.
Aborting
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

file9.txt

$ git checkout README.md
$ brew update

Automatic merge failed; fix conflicts and then commit the result.
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master