nixos

You can override derivation inputs and derivation attributes. Let's take for example the nano editor.

You can enable tidy support by overriding the enableTidy input like so:

(final: prev: { nano = prev.nano.override { enableTidy = true; }; })

You can also override build steps, eg. if you want to apply an upstream patch because you don't want to wait for the next release version, you'd do it like this.

(final: prev: { nano = prev.nano.overrideAttrs ( old: { patches = (old.patches or [] ) ++ [ ./filename.patch ]; }); })

Now the thing about overlays is this: if you change a package in an overlay, all packages depending on that package, either directly or indirectly, will pull in that modified version, which is an extremely powerful tool as you can probably imagine.