fix: Inherit the login shell variable from the user session (#16)

Closes #15 

This fixes the issue where the subshell's `SHELL` environment variable
changes once already in a nix-shell by always getting the user's
preferred shell from their UID.

Author: eureka-cpu

README.md

@@ -36,7 +36,7 @@ nix run github:eureka-cpu/ns -- <URI>#<ATTR>
 Add or remove it from your user profile:
 
 ```sh
-nix profile install github:eureka-cpu/ns
+nix profile add github:eureka-cpu/ns
 nix profile remove ns
 ```
 

dune-project

@@ -2,7 +2,7 @@
 
 (name ns)
 
-(version 0.4.2)
+(version 0.4.3)
 
 (generate_opam_files true)
 

lib/cmd.ml

@@ -19,14 +19,14 @@ module Cmd = struct
     Bos.Cmd.v "nix"
     |>+ [ "develop" ] @ [ Uri.sprintf_uri_attr_opt entrypoint attribute ]
     |>+ Option.value ~default:[] force_experimental_features
-    |>+ [ "--command"; Unix.shell ]
+    |>+ [ "--command"; Unix.shell () ]
   ;;
 
   let legacy_nix_shell_from_entrypoint entrypoint attribute =
     Bos.Cmd.v "nix-shell"
     |>+ Option.value ~default:[] (Option.map (fun attr -> [ "--attr"; attr ]) attribute)
     |>+ [ entrypoint ]
-    |>+ [ "--command"; Unix.shell ]
+    |>+ [ "--command"; Unix.shell () ]
   ;;
 
   let nix_shell installables force_experimental_features =
@@ -40,7 +40,7 @@ module Cmd = struct
     (* Need to combine/validate that the installables given are all Nixpkgs *)
     Bos.Cmd.v "nix-shell"
     |>+ [ "--packages" ] @ installables
-    |>+ [ "--command"; Unix.shell ]
+    |>+ [ "--command"; Unix.shell () ]
   ;;
 
   let print_strategy ({ workdir; primary; fallback } : strategy) =

lib/util.ml

@@ -22,12 +22,8 @@ module Util = struct
         Error.handle_ns_error "%s" (Error.sprintf_unix_error e path)
     ;;
 
-    (** Get the user's shell from the environment *)
-    let shell =
-      match Sys.getenv_opt "SHELL" with
-      | Some s -> s
-      | None -> "/bin/sh"
-    ;;
+    (** Get the user's login shell *)
+    let shell () = (Unix.getpwuid (Unix.getuid ())).Unix.pw_shell
 
     (** Whether a flake.nix file exists at the given directory *)
     let flake_exists_at dir = Sys.file_exists (Filename.concat dir "flake.nix")

ns.opam

@@ -1,6 +1,6 @@
 # This file is generated by dune, edit dune-project instead
 opam-version: "2.0"
-version: "0.4.2"
+version: "0.4.3"
 synopsis: "A unified interface for nix shell."
 description:
   "An intuitive nix shell interface that unifies the `nix shell`, `nix develop` and `nix-shell` commands."

test/dune

@@ -16,4 +16,4 @@
  (targets test_ns) ; the Dune target name
  (deps test_ns.rs) ; path relative to this dune file
  (action
-  (run rustc --test %{deps} -o %{targets} --allow warnings)))
+  (run rustc --test %{deps} -o %{targets} -A warnings -l c)))

test/test_ns.rs

@@ -1,13 +1,46 @@
-use std::{env, path, process, sync};
+use std::{env, ffi, path, process, sync};
 
 static DUNE_TEST_DIR: sync::OnceLock<path::PathBuf> = sync::OnceLock::new();
 static ENV_SHELL: sync::OnceLock<String> = sync::OnceLock::new();
 
 fn dune_test_dir() -> &'static path::PathBuf {
     DUNE_TEST_DIR.get_or_init(|| env::current_dir().expect("failed to get test directory"))
 }
-fn env_shell() -> &'static String {
-    ENV_SHELL.get_or_init(|| env::var("SHELL").expect("failed to get shell from environment"))
+
+/// Minimal bindings to dynamically linked libc, used for getting the shell of the logged in user.
+mod libc {
+    unsafe extern "C" {
+        pub fn getuid() -> u32;
+        pub fn getpwuid(uid: u32) -> *mut Passwd;
+    }
+
+    #[repr(C)]
+    pub struct Passwd {
+        pub pw_name: *mut i8,
+        pub pw_passwd: *mut i8,
+        pub pw_uid: u32,
+        pub pw_gid: u32,
+        #[cfg(target_os = "macos")]
+        pub pw_change: i64,
+        #[cfg(target_os = "macos")]
+        pub pw_class: *mut i8,
+        pub pw_gecos: *mut i8,
+        pub pw_dir: *mut i8,
+        pub pw_shell: *mut i8,
+        #[cfg(target_os = "macos")]
+        pub pw_expire: i64,
+    }
+}
+
+fn env_shell() -> &'static str {
+    ENV_SHELL.get_or_init(|| unsafe {
+        let uid = libc::getuid();
+        let pw = libc::getpwuid(uid);
+        assert!(!pw.is_null());
+        ffi::CStr::from_ptr((*pw).pw_shell)
+            .to_string_lossy()
+            .into_owned()
+    })
 }
 
 fn ns() -> path::PathBuf {