feat(wpcarro/blog): nix-shell (note to self) r/5052

Publishing another "note to self"

Change-Id: If5d052f0360f3e1f371b0c1fdd3781e5bb846ea4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6861
Tested-by: BuildkiteCI
Reviewed-by: wpcarro <wpcarro@gmail.com>

1 files changed, 50 insertions, 0 deletions

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+## Background
+
+I rarely use `nix-shell` for its originally intended purpose of "reproducing the
+environment of a derivation for development". Instead, I often use it to put
+some executable on my `PATH` for some ad hoc task.
+
+What's `nix-shell`'s "intended purpose"? Let's ask The Man (`man nix-shell`):
+
+> The command nix-shell will build the dependencies of the specified derivation,
+> but not the derivation itself. It will then start an interactive shell in
+> which all environment variables defined by the derivation path have been set
+> to their corresponding values, and the script $stdenv/setup has been
+> sourced. This is useful for reproducing the environment of a derivation for
+> development.
+
+Because I'm abusing `nix-shell` in this way, I'm liable to forget that
+`nix-shell` puts `buildInputs` on `PATH` and *not* the derivation itself. But I
+often only want the derivation!
+
+## Solution
+
+Pass the Nix expression to `nix-shell -p`:
+
+```shell
+λ nix-shell -p '(import /depot {}).tvix.eval'
+```
+
+## Explanation
+
+This works because Nix forwards the arguments passed to `-p` (i.e. `--packages`)
+and interpolates them into this expression here: [source](nix-src)
+
+```nix
+{ ... }@args:
+
+with import <nixpkgs> args;
+
+(pkgs.runCommandCC or pkgs.runCommand) "shell" {
+  buildInputs = [
+    # --packages go here
+  ];
+}
+```
+
+So really you can pass-in *any* valid Nix expression that produces a derivation
+and `nix-shell` will put their outputs on your `PATH`.
+
+Enjoy!
+
+[nix-src]: https://sourcegraph.com/github.com/NixOS/nix@3ae9467d57188f9db41f85b0e5c41c0c9d141955/-/blob/src/nix-build/nix-build.cc?L266