brew install libusb linking failed - homebrew

Question

I am installing libusb with brew in my Mac
brew install libusb
The linking step failed as below
Error: The `brew link` step did not complete successfully The formula built, but is not symlinked into /usr/local
Could not symlink lib/libusb-1.0.0.dylib
Target /usr/local/lib/libusb-1.0.0.dylib already exists.
You may want to remove it: rm '/usr/local/lib/libusb-1.0.0.dylib'
To force the link and overwrite all conflicting files: brew link
--overwrite libusb
So I removed the existing libusb with
sudo rm '/usr/local/lib/libusb-1.0.0.dylib'
and then did a link
brew link --overwrite libusb
The linking doesn't work, shows error below
Error: Could not symlink lib/libusb-1.0.0.dylib
/usr/local/lib is not writable.
If I try
sudo brew link --overwrite libusb
instead, that doesn't work either. What am I missing?
I am using OSX El Capitan version 10.11.4 (15E65)

Answer 1

If things seem not to work with homebrew, my general strategy is first to try:
brew doctor
and do whatever the good doctor recommends.
If that fails, I tend to uninstall things, normally using --force which really does a good clean-up and removes old versions. So, in your case:
brew rm libusb --force
Then re-install the "unhappy" package. So, in your case:
brew install libusb
In answer to your new question in the comments. Your installation looks correct because libusb isn't an executable program - it is just a library without any associated command-line tools - so it won't show up when you run which libusb.
You can see the constituent parts of the package with this command:
brew ls libusb
/usr/local/Cellar/libusb/1.0.20/include/libusb-1.0/libusb.h
/usr/local/Cellar/libusb/1.0.20/lib/libusb-1.0.0.dylib
/usr/local/Cellar/libusb/1.0.20/lib/pkgconfig/libusb-1.0.pc
/usr/local/Cellar/libusb/1.0.20/lib/ (2 other files)
And, as you can see, there is no stand-alone executable program in /usr/local/bin called libusb, there are just
libusb.h - a C header file you would compile against
libusb...dylib - a dynamic library you would link against
libusb...pc - which supplies the info for the pkgconfig tool
So, if you wanted to compile and link an application against libusb, you would run pkg-config like this to find out the "Include path" and linker details
pkg-config --cflags --libs libusb
-I/usr/local/Cellar/libusb-compat/0.1.5/include \
-I/usr/local/Cellar/libusb/1.0.20/include/libusb-1.0 \
-L/usr/local/Cellar/libusb
which means your compilation command would look like this:
gcc yourApp.c $(pkg-config --cflags --libs libusb) -o yourApp