I have uninstalled and installed Homebrew 3 times now because it seems to never allow me to install anything as it denies me permissions at the end of most installations.
As an example I will post this libjpeg download scenario that I'm currently facing.
I try to install libjpeg and get:
$ brew install libjpeg
==> Downloading https://downloads.sf.net/project/machomebrew/Bottles/jpeg-8d.mountain_lion.bottle.1.tar.gz
Already downloaded: /Library/Caches/Homebrew/jpeg-8d.mountain_lion.bottle.1.tar.gz
==> Pouring jpeg-8d.mountain_lion.bottle.1.tar.gz
Warning: Could not link jpeg. Unlinking...
Error: The brew link step did not complete successfully
The formula built, but is not symlinked into /usr/local
You can try again using `brew link jpeg'
Error: Permission denied - /usr/local/opt/jpeg
'brew link jpeg' results in
Error: Permission denied - /usr/local/opt/jpeg
Here is what my brew doctor reads
$ brew doctor
Warning: "config" scripts exist outside your system or Homebrew directories.
./configure scripts often look for *-config scripts to determine if
software packages are installed, and what additional flags to use when
compiling and linking.
Having additional scripts in your path can confuse software installed via
Homebrew if the config script overrides a system or Homebrew provided
script of the same name. We found the following "config" scripts:
/Library/Frameworks/Python.framework/Versions/2.7/bin/python-config
/Library/Frameworks/Python.framework/Versions/2.7/bin/python2-config
/Library/Frameworks/Python.framework/Versions/2.7/bin/python2.7-config
Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run brew link on these:
jpeg
This permission issue has been making it impossible to use brew on anything and I would really appreciate any suggestions.
I was able to solve the problem by using chown on the folder:
sudo chown -R "$USER":admin /usr/local
Also you'll (most probably) have to do the same on /Library/Caches/Homebrew:
sudo chown -R "$USER":admin /Library/Caches/Homebrew
Apparently I had used sudo before in a way that altered my folder permission on /usr/local,
from here on forward all installations with brew have proven to be successful.
This answer comes courtesy of gitHub's homebrew issue tracker