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
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.
Useful, reliable alternatives exist should you wish to use
keg_only tools outside of Homebrew.
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
CFLAGS=-I$(brew --prefix)/opt/icu4c/include LDFLAGS=-L$(brew --prefix)/opt/icu4c/lib pip install pyicu
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.
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.
If the tool you are attempting to build is pkg-config aware, you can amend your
PKG_CONFIG_PATH to find that
.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
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