small section on nix garbage collection

COMMANDS.md

@@ -6,4 +6,32 @@ nix repl
 
 # fenix src FUNC (maybe a different name then src?)
 fenix repl <<< ":e $FUNC"
+
+# fenix clean
+nix-store --gc # aka nixos-collect-garbage
 ```
+
+
+
+### Notes
+#### Useful Tricks
+The path `/nix/var/nix/profiles/per-user/root/channels/` seems to be where
+Nix clones the git repos for chosen channels. Could be useful instead
+of querying `search.nixos.org`.
+
+#### "Alias" Commands
+The tool `nix-collect-garbage` (as per `man nix-collect garbage`):
+> ...is mostly  an alias of nix-store --gc ⟨../command-ref/nix-store/gc.md⟩.
+> That is, it deletes all unreachable store objects ⟨../store/store-object.md⟩
+> in the Nix store to clean up your system.
+>
+> However, it provides two additional options, --delete-old ⟨#opt-delete-old⟩
+> and --delete-older-than ⟨#opt-delete-older-than⟩,  which  also  delete
+> old  profiles  ⟨../command-ref/files/profiles.md⟩, allowing potentially more
+> store objects ⟨../store/store-object.md⟩ to be deleted because profiles are
+> also garbage collection roots.
+
+> These options are the equivalent of running nix-env --delete-generations
+> ⟨../command-ref/nix-env/delete-generations.md⟩ with various augments on
+> multiple profiles, prior to running nix-collect-garbage (or just nix-store --gc)
+> without any flags.