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+# Garbage Collection
+
+`nix-env` operations such as upgrades (`-u`) and uninstall (`-e`) never
+actually delete packages from the system. All they do (as shown above)
+is to create a new user environment that no longer contains symlinks to
+the “deleted” packages.
+
+Of course, since disk space is not infinite, unused packages should be
+removed at some point. You can do this by running the Nix garbage
+collector. It will remove from the Nix store any package not used
+(directly or indirectly) by any generation of any profile.
+
+Note however that as long as old generations reference a package, it
+will not be deleted. After all, we wouldn’t be able to do a rollback
+otherwise. So in order for garbage collection to be effective, you
+should also delete (some) old generations. Of course, this should only
+be done if you are certain that you will not need to roll back.
+
+To delete all old (non-current) generations of your current profile:
+
+    $ nix-env --delete-generations old
+
+Instead of `old` you can also specify a list of generations, e.g.,
+
+    $ nix-env --delete-generations 10 11 14
+
+To delete all generations older than a specified number of days (except
+the current generation), use the `d` suffix. For example,
+
+    $ nix-env --delete-generations 14d
+
+deletes all generations older than two weeks.
+
+After removing appropriate old generations you can run the garbage
+collector as follows:
+
+    $ nix-store --gc
+
+The behaviour of the gargage collector is affected by the
+`keep-derivations` (default: true) and `keep-outputs` (default: false)
+options in the Nix configuration file. The defaults will ensure that all
+derivations that are build-time dependencies of garbage collector roots
+will be kept and that all output paths that are runtime dependencies
+will be kept as well. All other derivations or paths will be collected.
+(This is usually what you want, but while you are developing it may make
+sense to keep outputs to ensure that rebuild times are quick.) If you
+are feeling uncertain, you can also first view what files would be
+deleted:
+
+    $ nix-store --gc --print-dead
+
+Likewise, the option `--print-live` will show the paths that *won’t* be
+deleted.
+
+There is also a convenient little utility `nix-collect-garbage`, which
+when invoked with the `-d` (`--delete-old`) switch deletes all old
+generations of all profiles in `/nix/var/nix/profiles`. So
+
+    $ nix-collect-garbage -d
+
+is a quick and easy way to clean up your system.