How to override package version in xfce application?

Super new to Nix and NixOS in general so bear with me… I’m trying to set up an XFCE desktop with NixOS and make it as close as I can to my current Xubuntu system. I use a built-from-source version of xfce4-terminal that I’d like to have in NixOS as well. I found the source of the xfce4-terminal package. But I have no idea how to go about overriding the version there. I tried the generic instructions on overriding packages to no avail, maybe because it uses a custom mkXfceDerivation? Any advice on how to go about this?

Hi. I think normally, the best way to do this is using <pkg>.override which is the highest level (closer to the input as opposed to the output derivation) one out of the three functions used to override a derivation (the other two being <pkg>.overrideAttrs and <pkg>.overrideDerivation). When a value is overridden that change won’t affect the higher levels, which might include logic that is necessary to correctly package the project.

So in this case, I think the levels from the highest to the lowest are like this:

I checked which one of the functions override actually affects in nix repl:

nix-repl> xfce.xfce4-terminal.override.__functionArgs
{ docbook_xml_dtd_45 = false; docbook_xsl = false; gtk3 = false; libxfce4ui = false; libxslt = false; mkXfceDerivation = false; pcre2 = false; vte = false; xfconf = false; }

Which looks like “xfce4-terminal/default.nix”. But since that function doesn’t have any parameters for the version (and the hash), I decided to override the value of mkXfceDerivation key with another function that takes the same parameters as mkXfceDerivation. This function acts as a shim/wrapper by merging the arguments passed to it with the set { version = <new-version>; sha256 = <new-hash>; } before passing it to the actual mkXfceDerivation:

  createMkXfceDerivationShim = overrideArgs: args: xfce.mkXfceDerivation (args // overrideArgs);
  mkXfceDerivationShim = createMkXfceDerivationShim {
    version = "0.8.9";
    sha256 = "sha256-q/HLoILt5f9F2C0f2/OPtukuRCvUF10LV9VWo4OkCyQ=";
  customTerminal = xfce.xfce4-terminal.override {
    mkXfceDerivation = mkXfceDerivationShim;

To get the correct hash, I first used lib.fakeSha256.

Wow, thank you for the detailed response! This is great for learning how to do this kind of thing in general. I think I need to do some more research into how exactly overriding works, but I’ll try out your suggestion. Thanks!