Random Nix stuff I learned

getExe uses meta.mainProgram

The nixpkgs lib function getExe uses meta.mainProgram to get a path to the executable in a package. It’s final path will then be interpolated into $out/bin/{name}.

Source

Creating inline derivations in nix files

Sometimes one simply wants to write a quick script, or some other text file in a nix derivation.

The easiest way to achieve that is with the ‘trivial’ builders that nixpkgs has to offer.

Their descriptions in the manual

Virtual Machine tests

Nix has some cool infrastructure to test anything you can in a configuration of multiple virtual machines.

It has some warts, as the test script is written in python… But overall, it gets its job done.

Manual

Getting a reproducible build time in flakes

Flakes, like anything in nix are meant to be reproducible. The way this is usually achieved is to set some static time, like 1 in the unix epoch.

But this is 1970… So lots of stuff will display as being decades old.

But flakes are always (I think?) git repositories and as such you have a reproducible time, that also gets exposed.

This now needs to be transformed to something more standardized:

This will spit out a date in ISO8601 format, in UTC.

created =
  let
    sub = from: len: builtins.substring from len inputs.self.lastModifiedDate;
    year = sub 0 4;
    month = sub 4 2;
    day = sub 6 2;
    hour = sub 8 2;
    minute = sub 10 2;
    second = sub 12 2;
  in
  "${year}-${month}-${day}T${hour}:${minute}:${second}Z";