nix/doc/manual/source/package-management/garbage-collector-roots.md

Garbage Collector Roots

The roots of the garbage collector are all store paths to which there are symlinks in the directory prefix/nix/var/nix/gcroots. For instance, the following command makes the path /nix/store/d718ef...-foo a root of the collector:

$ ln -s /nix/store/d718ef...-foo /nix/var/nix/gcroots/bar

That is, after this command, the garbage collector will not remove /nix/store/d718ef...-foo or any of its dependencies.

Subdirectories of prefix/nix/var/nix/gcroots are searched recursively. Symlinks to store paths count as roots. Symlinks to non-store paths are ignored, unless the non-store path is itself a symlink to a store path.