How do I determine which commit a channel references with Nixos?

Question

I'd like to figure out the commit and date that a channel references.

For example how can determine that for the nixos2009 channel below?

sudo nix-channel --list
nixos https://nixos.org/channels/nixos-20.09
nixos-2003 https://nixos.org/channels/nixos-20.03
nixos2003 https://nixos.org/channels/nixos-20.03
nixos2009 https://nixos.org/channels/nixos-20.09
unstable https://nixos.org/channels/nixpkgs-unstable

---

I can see ls /nix/var/nix/profiles/per-user/root/channels-*/manifest.nix shows us the channel generations.. Which the latest channel generation has the following content:

cat /nix/var/nix/profiles/per-user/root/channels-61-link/manifest.nix | nixfmt
[
  {
    meta = { };
    name = "nixos-20.09.2538.0cfd08f4881";
    out = {
      outPath =
        "/nix/store/7s917s7ipvq3zmbx5g3kssldwc029r8r-nixos-20.09.2538.0cfd08f4881";
    };
    outPath =
      "/nix/store/7s917s7ipvq3zmbx5g3kssldwc029r8r-nixos-20.09.2538.0cfd08f4881";
    outputs = [ "out" ];
    system = "x86_64-linux";
    type = "derivation";
  }
  {
    meta = { };
    name = "nixos-2003-20.03.3324.929768261a3";
    out = {
      outPath =
        "/nix/store/zajz4gpq506g68w47pnl11k8mhz06jz0-nixos-2003-20.03.3324.929768261a3";
    };

Potentially I can get it from the store path above like /nix/store/zajz4gpq506g68w47pnl11k8mhz06jz0-nixos-2003-20.03.3324.929768261a3 - but I'm not sure (and not sure what that "path" is called either).

Seems like an answer for this is documented here: https://discourse.nixos.org/t/how-to-see-what-commit-is-my-channel-on/4818/6?u=chrissound though that is not a programmatic way (which is what I'm looking for).

Answer 1

You can use nixos-version. Mine gives 20.03pre194293.2436c27541b (Markhor) where the last part is the commit hash. Not sure if it also works on stable, but it should.

Answer 2

For an arbitrary channel I do it this way (replace nixpkgs with your favorite channel):

$ nix-instantiate --eval -E '(import <nixpkgs> {}).lib.version'
"22.11pre405560.2da64a81275"

Note that the git commit is the last number 2da….

Answer 3

1. That manifest might be a nice way.

2. Another (also in the linked forum) is

   readlink /nix/var/nix/profiles/per-user/root/channels-1-link
   

3. And also, perhaps the easiest one:

   cat /nix/var/nix/profiles/per-user/root/channels/*/svn-revision
   

   On newer versions of nix, the full git commit sha will also be in:

   /nix/var/nix/profiles/per-user/root/channels/*/.git-revision
   

Answer 4

For your 'user' nix channels:

channelName="nixos2311"
cat /nix/var/nix/profiles/per-user/$USER/channels/$channelName/.git-revision