What does “brew link” do?

Question

When I run brew doctor I get the common warning:

Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble          


        

Answer 1

Homebrew can allow multiple versions of a formula to be installed. For example, there is formulae called node and node@10 and similar.

$ brew info node@10
...
==> Caveats
node@10 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.

If I have both node and node@10 installed, where node is at v11. I can decide later to active the earlier version with brew link:

$ brew unlink node

$ brew link node@10

$ cd /urs/local/bin
$ ls -l node
lrwxr-xr-x  1 user  admin  34 12 Dec 20:07 node -> ../Cellar/node@10/10.14.1/bin/node

Here the symlink node is pointing to an earlier version (keg-only) installed in Cellar.