I was installing some dependencies for PaulStretch using Homebrew and was getting download errors. I thought that maybe Home Brew itself was out of date so I tried updating it and came across this error:
$ brew update
error: unable to unlink old '.gitignore' (Permission denied)
error: unable to unlink old 'README.md' (Permission denied)
error: unable to create file SUPPORTERS.md (Permission denied)
Checking out files: 100% (2135/2135), done.
Error: Failure while executing: git pull -q origin refs/heads/master:refs/remotes/origin/masterTurns out its easy enough to fix. I followed the instructions on Stack Overflow here
Note: There is an edit about setting permissions towards the end of the top answer which I needed to follow as well, the original answer on its own was not enough.