Revert from flakes to channels?

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1

I’ve been migrating my nixos config from channels to flakes and I think I’m ready to flip the switch.

However, one thing that concerned me when testing is that I couldn’t revert to using channels without rebooting into an older system build.

If I switch to flakes, nix-collect-garbage -d, and want to switch back to channels, is there any way of doing it? Or am I stuck with flakes at that point? Thanks!

2

nix-collect-garbage does not remove channels themselves so it should just work.

If you removed them, you can just add e.g. nixos-unstable back using appropriate nix-channel command (nix-channel --add as root, and do not forget nix-channel --update afterwards).

One complication might be if you also unset NIX_PATH (e.g. with nix.nixPath option). Then you will need to pass the default values of the option to nixos-rebuild using -I flags or NIX_PATH environment variable, or it will not be able to find Nixpkgs.

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3

I didn’t actually remove channels at all when testing, I just added flake.nix and rebuilt (in /etc/nixos, and I built a user-specific profile from a second one).

In both cases, removing the flake files and rebuilding (nixos-rebuild or nix-env -iA) without them errored out.

4

The profiles managed with nix profile are not backwards compatible. I forgot about that since I avoid imperative package management.

The only other reason why nixos-rebuild could fail that comes to mind is that you updated the Nixpkgs input and changed the NixOS config in a backwards incompatible way. Then it would not work with older version of Nixpkgs from a channel.

But it is really impossible to tell without a concrete error message.

5

Thanks, that link answers my question for nix-env, do you have any idea about nixos-rebuild?

6

Without a concrete error message, only the stuff mentioned above.

7

Error message when trying to revert to nixos-rebuild with channels:

❯❯❯ sudo nixos-rebuild switch
error:
       … while evaluating the attribute 'config'

         at /nix/store/qgbn0imyridkb9527v6gnv6z3jzzprb9-source/lib/modules.nix:322:9:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |         ^
          323|         _module = checked (config._module);

       … while calling the 'seq' builtin

         at /nix/store/qgbn0imyridkb9527v6gnv6z3jzzprb9-source/lib/modules.nix:322:18:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |                  ^
          323|         _module = checked (config._module);

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: file 'nixos-config' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
building Nix...
error:
       … while evaluating the attribute 'config'

         at /nix/store/qgbn0imyridkb9527v6gnv6z3jzzprb9-source/lib/modules.nix:322:9:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |         ^
          323|         _module = checked (config._module);

       … while calling the 'seq' builtin

         at /nix/store/qgbn0imyridkb9527v6gnv6z3jzzprb9-source/lib/modules.nix:322:18:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |                  ^
          323|         _module = checked (config._module);

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: file 'nixos-config' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
building the system configuration...
error:
       … while evaluating the attribute 'config.system.build.toplevel'

         at /nix/store/qgbn0imyridkb9527v6gnv6z3jzzprb9-source/lib/modules.nix:322:9:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |         ^
          323|         _module = checked (config._module);

       … while calling the 'seq' builtin

         at /nix/store/qgbn0imyridkb9527v6gnv6z3jzzprb9-source/lib/modules.nix:322:18:

          321|         options = checked options;
          322|         config = checked (removeAttrs config [ "_module" ]);
             |                  ^
          323|         _module = checked (config._module);

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: file 'nixos-config' was not found in the Nix search path (add it using $NIX_PATH or -I)

       at «none»:0: (source not available)
8

You either need to put your config’s entrypoint in /etc/nixos/configuration.nix, or specify what the entrypoint is (using nixos-config in NIX_PATH, as the error instructs).

9

Config is at /etc/nixos/configuration.nix (as before). I suspect it errors because it is expecting flake.nix not configuration.nix?

10

There is no default configuration path – NixOS relies on NIX_PATH if NIXOS_CONFIG environment variable is not set. But even if you set it, you might still not be using Nixpkgs from channels as mentioned above:

11

Thanks to everyone for their help, I’ve tested this out now:

  • Removing $XDG_STATE_HOME/nix/profiles/profile let me revert to nix-env with no obvious problems (as per the above link nix profile - Nix Reference Manual). :sparkles:

  • However, noting $NIX_PATH before sudo nixos-rebuild switching with flakes, then removing flake.nix and calling the command again but manually setting $NIX_PATH didn’t let me revert to channels. I tried

    • NIX_PATH=... sudo nixos-rebuild switch,
    • sudo NIX_PATH=... nixos-rebuild switch, and
    • sudo nixos-rebuild -I ... swicth,

    but none of these worked—I got the same file 'nixos-config' was not found error as above.

Weirdly I get warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix:/nix/var/nix/profiles/per-user/root/channels' does not exist, ignoring, even though all those files are clearly there when I check manually.

12

Ok, the incantation that works for nixos-rebuild is sudo nixos-rebuild switch -I nixos-config=/etc/nixos/configuration.nix. Thanks to everyone for their help in solving this!

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