How to resolve a permission error when installing gnuplot from homebrew

EDIT: The problem is now fixed in Homebrew:

• https://github.com/mxcl/homebrew/commit/8aa4f9dd3bbd885350776171d3d9403d381e1a59

If you still experience the problem, update Homebrew like this:

brew update

If you want to know what the problem was, I have kept my original answer below.

Ignore the permisson issue for now

I am experiencing the exact same problem and in my opinion the problem is in brew doctor rather than in your and my installation.

I think you should ignore the issue rather than changing the ownership of /usr/local. Alternatively, you could fix your local brew doctor script until a fix is released. See below.

I don't consider it correct to make /usr/local owned by a specific user. I have more than one admin user on this machine. You should leave /usr/local owned by root:admin as owner and group.

My investigation

Like for you I have a /usr/local that is perfectly writable by my user which is also a member of the admin group:

$ ls -ld /usr/local/
drwxrwxr-x  14 root  admin  476 22 Jun 23:33 /usr/local/
$ whoami
mgd
$ dscl . -read /Groups/admin GroupMembership
GroupMembership: root mgd rgd

Let's test that the dir is really writable:

$ ls -l /usr/local/newfile
ls: /usr/local/newfile: No such file or directory
$ touch /usr/local/newfile
$ ls -l /usr/local/newfile
-rw-r--r--  1 mgd  admin  0 23 Jun 14:52 /usr/local/newfile

Further investigation into the brew doctorcode led my to the conclusion that the use of the ruby function Pathname.writable? is causing the problem. Consider this interactive Ruby session:

$ irb
>> require 'pathname'
=> true
>> Pathname('/usr/local').writable?
=> false

Function Pathname.writable? says /usr/local is not writable even though we know it is.

Using Pathname.writable_real? instead gives the correct result – it says that the dir is writable:

>> Pathname('/usr/local').writable_real?
=> true

This should be fixed in /usr/local/Library/Homebrew/cmd/doctor.rb. You can fix it in your own installation while waiting for a fix.

The difference between the two functions are (according to the Ruby docs here and here):

writable?(file_name) → true or false: Returns true if the named file is writable by the effective user id of this process.

writable_real?(file_name) → true or false: Returns true if the named file is writable by the real user id of this process.

Do you have several versions of Node installed via homebrew? Every time it updates node homebrew puts the newest version alongside the old one and then updates the symlinks to the newest version. Running the brew cleanup should reduce that up a bit for you.