CodeNote

$ brew update
Error: /usr/local must be writable!

$ brew doctor
 
Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry and just ignore them. Thanks!
 
Warning: /usr/local is not writable.
 
You should probably change the ownership and permissions of /usr/local
back to your user account.
  sudo chown -R $(whoami) /usr/local
 
Warning: /usr/local is not writable.
Even if this directory was writable when you installed Homebrew, other
software may change permissions on this directory. For example, upgrading
to OS X El Capitan has been known to do this. Some versions of the
"InstantOn" component of Airfoil or running Cocktail cleanup/optimizations
are known to do this as well.
 
You should probably change the ownership and permissions of /usr/local
back to your user account.
  sudo chown -R $(whoami) /usr/local
 
Warning: Some installed formula are missing dependencies.
You should `brew install` the missing dependencies:
  brew install oniguruma
 
Run `brew missing` for more details.
 
Warning: Your Homebrew is outdated.
You haven't updated for at least 24 hours. This is a long time in brewland!
To update Homebrew, run `brew update`.

sudo chown -R $(whoami) /usr/local

brew update
 
/usr/local/Library/Homebrew/cmd/update.sh: line 13: /usr/local/Library/ENV/scm/git: No such file or directory
(略)
/usr/local/Library/Homebrew/cmd/update.sh: line 13: /usr/local/Library/ENV/scm/git: No such file or directory
Error: update-report should not be called directly!

brew prune

$ brew update
fatal: cannot create directory at '.github': Permission denied
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/master

$ ls -la /usr/local
total 112
drwxr-xr-x   24 root     wheel    816 Oct 28 08:47 .

$ sudo chown $(whoami):admin /usr/local

$ ls -la /usr/local
total 112
drwxr-xr-x   24 myname  admin    816 Oct 28 08:47 .

cd `brew --prefix`
git fetch origin
git reset --hard origin/master

$ brew update

$ brew -v
Homebrew 0.9.9 (git revision 145155; last commit 2016-04-23)
Homebrew/homebrew-core (git revision bd06; last commit 2016-04-23)

% brew -h
/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

git diff
 
diff --git a/Library/brew.rb b/Library/brew.rb
index 4fa22b4..ee05650 100755
--- a/Library/brew.rb
+++ b/Library/brew.rb
@@ -1,4 +1,4 @@
-#!/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby -W0
+#!/System/Library/Frameworks/Ruby.framework/Versions/current/usr/bin/ruby -W0
 # encoding: UTF-8
 
 std_trap = trap("INT") { exit! 130 } # no backtrace thanks

cd $(brew --prefix)
git add .
git commit -m 'Change the path for ruby'
brew update

% brew update
 
error: Your local changes to the following files would be overwritten by merge:
	Library/Formula/mongodb.rb
Please, commit your changes or stash them before you can merge.
error: Your local changes to the following files would be overwritten by merge:
	Library/brew.rb
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

git reset --hard HEAD
git merge -Xtheirs origin/master