Nix Feature: Packages Installing but not in PATH

[natescherer]

Hi All,
Wanted to make an issue to note that using the packages option on the nix feature seems to install the packages but they do not end up on the PATH, and I'm not sure if that was intentional or not based on the documentation? Here's a minimal example:

devcontainer.json

{
    "image": "mcr.microsoft.com/devcontainers/base:debian",
    "features": {
        "ghcr.io/devcontainers/features/nix:1": {
            "packages": "alejandra"
        }
    }
}

After the container builds, I can verify that it did install alejandra, but it's not on the PATH.
Zsh

vscode ➜ /workspaces/h2ek8s (main) $ alejandra
zsh: command not found: alejandra
vscode ➜ /workspaces/h2ek8s (main) $ find / -name alejandra
EXTRANEOUS PERMISSION DENIED LINES REMOVED
/nix/store/abdks1z1vr080rip7m1svddsl3hy500d-nixpkgs/nixpkgs/pkgs/by-name/al/alejandra
/nix/store/8l4jxp6j7qjw2jvhw8zzcnd52rrkg8wr-user-environment/bin/alejandra
/nix/store/9smc80g61acjf8kk8dr9lb8phcz0fxfv-user-environment/bin/alejandra
/nix/store/fqc2w0lqhnfilywlf8q2r6dc98q9ppby-alejandra-4.0.0/bin/alejandra

Interestingly, if I change the package installation to happen with an onCreateCommand, it works fine, even though that seems to be more or less what the feature is doing?

devcontainer.json

{
    "image": "mcr.microsoft.com/devcontainers/base:debian",
    "features": {
        "ghcr.io/devcontainers/features/nix:1": {}
    },
    "onCreateCommand": "nix-env -iA nixpkgs.alejandra"
}

After the container builds, alejandra is on the PATH.
Zsh

vscode ➜ /workspaces/h2ek8s (main) $ alejandra
Formatting stdin.
Use --help to see all command line options.
use --quiet to suppress this and other messages.

I played around with various permutations of adding nixpkgs. before the package name, setting useAttributePath to true, and setting multiUser to false, and none seemed to result in what I put in packages ending up on the PATH.

Here's my local environment:

OS: Win 11 25H2
VS Code: 1.109.3
Docker Desktop: 4.60.1

Please let me know if you have any questions or need any more information!

[V-Subhankar-infy]

Hi @natescherer , thanks for the detailed report!

It was ineed a bug and the reason was:
with multiUser: true (the default), the installer ran nix-env inside a su subshell, which silently installed packages into ~/.nix-profile instead of /nix/var/nix/profiles/default. Since only the latter is on PATH at runtime, the packages were invisible despite showing as installed.

The onCreateCommand workaround worked coz It ran after container startup, in a live shell where ~/.nix-profile was already symlinked onto the active PATH — so the install landed somewhere reachable.

Fix: The su subshell is removed. post-install-steps.sh now receives NIX_FEATURE_INSTALL_PROFILE=/nix/var/nix/profiles/default and passes -p to every nix-env / nix profile install call, ensuring packages go directly into the profile PATH points to. Single-user installs are unaffected.

Fix is in PR #1691 — let us know if anything still looks off!