A number of persistent settings of Nix are stored in the file prefix/etc/nix/nix.conf. This file is a list of name = value pairs, one per line. Comments start with a # character. An example configuration file is shown in . gc-keep-outputs = true # Nice for developers gc-keep-derivations = true # Idem env-keep-derivations = false The following variables are currently available: gc-keep-outputs If true, the garbage collector will keep the outputs of non-garbage derivations. If false (default), outputs will be deleted unless they are GC roots themselves (or reachable from other roots). In general, outputs must be registered as roots separately. However, even if the output of a derivation is registered as a root, the collector will still delete store paths that are used only at build time (e.g., the C compiler, or source tarballs downloaded from the network). To prevent it from doing so, set this option to true. gc-keep-derivations If true (default), the garbage collector will keep the derivations from which non-garbage store paths were built. If false, they will be deleted unless explicitly registered as a root (or reachable from other roots). Keeping derivation around is useful for querying and traceability (e.g., it allows you to ask with what dependencies or options a store path was built), so by default this option is on. Turn it off to safe a bit of disk space (or a lot if gc-keep-outputs is also turned on). env-keep-derivations If false (default), derivations are not stored in Nix user environments. That is, the derivation any build-time-only dependencies may be garbage-collected. If true, when you add a Nix derivation to a user environment, the path of the derivation is stored in the user environment. Thus, the derivation will not be garbage-collected until the user environment generation is deleted (nix-env --delete-generations). To prevent build-time-only dependencies from being collected, you should also turn on gc-keep-outputs. The difference between this option and gc-keep-derivations is that this one is “sticky”: it applies to any user environment created while this option was enabled, while gc-keep-derivations only applies at the moment the garbage collector is run.