After upgrading to Yosemite, your brew could run into an error:

/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 23: /usr/local/Library/brew.rb: Undefined error: 0

This is because the path to ruby has changed. It can be solved easily be editing /usr/local/Library/brew.rb.

Replace the first line “1.8” to “current” so it looks like this:

#!/System/Library/Frameworks/Ruby.framework/Versions/Current/usr/bin/ruby -W0

As a side effect, you could run into errors with “files would be overwritten by merge” when you brew update.

To solve, you need to update the brew repos.