How to build software outside Homebrew with Homebrew keg_only dependencies

What does “keg-only” mean?

The FAQ briefly explains this.

As an example:

OpenSSL isn’t symlinked into my PATH and non-Homebrew builds can’t find it!

This is because Homebrew keeps it locked inside its individual prefix, rather than symlinking to the publicly-available location, usually /usr/local.

Advice on potential workarounds

A number of people in this situation are either forcefully linking keg_only tools with brew link --force or moving default system utilities out of the PATH and replacing them with manually-created symlinks to the Homebrew-provided tool.

Please do not remove macOS native tools and forcefully replace them with symlinks back to the Homebrew-provided tool. Doing so can and likely will cause significant breakage when attempting to build software.

brew link --force creates a warning in brew doctor to let both you and maintainers know that a link exists that could be causing issues. If you’ve linked something and there’s no problems at all? Feel free to ignore the brew doctor error.

How do I use those tools outside of Homebrew?

Useful, reliable alternatives exist should you wish to use keg_only tools outside of Homebrew.

Build flags

You can set flags to give configure scripts or Makefiles a nudge in the right direction. An example of flag setting:

./configure --prefix=/Users/Dave/Downloads CFLAGS=-I$(brew --prefix)/opt/openssl/include LDFLAGS=-L$(brew --prefix)/opt/openssl/lib

An example using pip:

CFLAGS=-I$(brew --prefix)/opt/icu4c/include LDFLAGS=-L$(brew --prefix)/opt/icu4c/lib pip install pyicu

PATH modification

You can temporarily prepend your PATH with the tool’s bin directory, such as:

export PATH=$(brew --prefix)/opt/openssl/bin:$PATH

This will prepend that folder to your PATH, ensuring any build script that searches the PATH will find it first.

Changing your PATH using that command ensures the change only exists for the duration of that shell session. Once you are no longer in that session, the PATH reverts to the prior state.

pkg-config detection

If the tool you are attempting to build is pkg-config aware, you can amend your PKG_CONFIG_PATH to find that keg_only utility’s .pc file, if it has them. Not all formulae ship with those files.

An example of this is:

export PKG_CONFIG_PATH=$(brew --prefix)/opt/openssl/lib/pkgconfig

If you’re curious about the PKG_CONFIG_PATH variable man pkg-config goes into more detail.

You can get pkg-config to detail the default search path with:

pkg-config --variable pc_path pkg-config