diff --git a/bin/main.ml b/bin/main.ml
index 35adfb5..d857031 100644
--- a/bin/main.ml
+++ b/bin/main.ml
@@ -3,7 +3,7 @@ open Ns.Cmd
 open Ns.Util
 
 let main
-      ({ installables; target_info; printcmd; force_experimental_features } :
+      ({ installables; target_info; printcmd; force_experimental_features; sh; _ } :
         Cli.strategy)
   =
   let ({ entrypoint; attribute; subshell_dir } : Cli.target_info) = target_info in
@@ -17,19 +17,20 @@ let main
       | Some entrypoint ->
         if Unix.flake_exists_at entrypoint
         then
-          ( Cmd.nix_develop entrypoint attribute force_experimental_features
+          ( Cmd.nix_develop entrypoint attribute force_experimental_features sh
           , if Unix.shell_exists_at entrypoint
-            then Some (Cmd.legacy_nix_shell_from_entrypoint entrypoint attribute)
+            then Some (Cmd.legacy_nix_shell_from_entrypoint entrypoint attribute sh)
             else None )
         else if Unix.shell_exists_at entrypoint
-        then Cmd.legacy_nix_shell_from_entrypoint entrypoint attribute, None
+        then Cmd.legacy_nix_shell_from_entrypoint entrypoint attribute sh, None
         else Error.handle_ns_error "no available devshell entrypoint: %s\n%!" entrypoint
       | None ->
         ( Cmd.nix_shell
             (List.map Uri.uri_to_string installables)
             force_experimental_features
+            sh
         , Option.map
-            (fun installables -> Cmd.legacy_nix_shell_from_installables installables)
+            (fun installables -> Cmd.legacy_nix_shell_from_installables installables sh)
             (Uri.combine_installables_tr installables) )
     in
     { workdir; primary; fallback }
diff --git a/default.nix b/default.nix
new file mode 100644
index 0000000..7e713e7
--- /dev/null
+++ b/default.nix
@@ -0,0 +1 @@
+(builtins.getFlake (toString ./.)).outputs.packages.${builtins.currentSystem}.default
diff --git a/lib/cli.ml b/lib/cli.ml
index 703b8c6..67a9685 100644
--- a/lib/cli.ml
+++ b/lib/cli.ml
@@ -19,6 +19,9 @@ module Cli = struct
     ; target_info : target_info
     ; printcmd : bool
     ; force_experimental_features : string list option
+    ; sh : string
+    ; sharg : string list
+    ; impure : bool
     }
 
   (** Positional URI arguments passed to the program via the command line. *)
@@ -39,9 +42,28 @@ module Cli = struct
     Arg.(value & flag & info [ "force"; "f" ] ~doc)
   ;;
 
+  let sh =
+    let doc =
+      "The shell to use in the subshell. Accepts a package name (e.g. zsh), a nix-store \
+       path, or a local path (e.g. /bin/sh). Defaults to the current user's login shell \
+       or bash if no user is logged in."
+    in
+    Arg.(value & opt filepath (Unix.eval_sh (Unix.pw_shell ())) & info [ "sh" ] ~doc)
+  ;;
+
+  let sharg =
+    let doc = "Supply arguments to the shell." in
+    Arg.(value & opt_all string [] & info [ "sharg" ] ~doc)
+  ;;
+
+  let impure =
+    let doc = "Allow the current shell environment to bleed into the subshell." in
+    Arg.(value & flag & info [ "impure" ] ~doc)
+  ;;
+
   (** Processes the args so that the main function knows what to do with them. *)
   let make_strategy =
-    let build original_args printcmd force_experimental_features =
+    let build original_args printcmd force_experimental_features sh sharg impure =
       let installables, target_info =
         let default_installables = []
         and default_target =
@@ -111,9 +133,13 @@ module Cli = struct
                ; "nix-command"
                ]
            else None)
+      ; sh = Unix.eval_sh sh
+      ; sharg
+      ; impure
       }
     in
-    Term.(const build $ uris $ printcmd $ force_experimental_features)
+    Term.(
+      const build $ uris $ printcmd $ force_experimental_features $ sh $ sharg $ impure)
   ;;
 
   let cmd entrypoint =
diff --git a/lib/cmd.ml b/lib/cmd.ml
index f103e5b..2e5612b 100644
--- a/lib/cmd.ml
+++ b/lib/cmd.ml
@@ -15,32 +15,34 @@ module Cmd = struct
     | args -> List.fold_left (fun cmd arg -> Bos.Cmd.add_arg cmd arg) cmd args
   ;;
 
-  let nix_develop entrypoint attribute force_experimental_features =
+  (* TODO: nix develop --ignore-env behaves differently than nix-shell --pure
+           which is causing none of the packages to show up. *)
+  let nix_develop entrypoint attribute force_experimental_features sh =
     Bos.Cmd.v "nix"
     |>+ [ "develop" ] @ [ Uri.sprintf_uri_attr_opt entrypoint attribute ]
     |>+ Option.value ~default:[] force_experimental_features
-    |>+ [ "--command"; Unix.shell () ]
+    (* |>+ [ "--ignore-env"; "-k"; "TERM"; "-k"; "TERMINFO"; "-k"; "HOME"; "-k"; "USER"; "-k"; "DISPLAY" ] *)
+    |>+ [ "--command"; sh ]
   ;;
 
-  let legacy_nix_shell_from_entrypoint entrypoint attribute =
+  let legacy_nix_shell_from_entrypoint entrypoint attribute sh =
     Bos.Cmd.v "nix-shell"
     |>+ Option.value ~default:[] (Option.map (fun attr -> [ "--attr"; attr ]) attribute)
     |>+ [ entrypoint ]
-    |>+ [ "--command"; Unix.shell () ]
+    |>+ [ "--pure" ]
+    |>+ [ "--command"; sh ]
   ;;
 
-  let nix_shell installables force_experimental_features =
+  let nix_shell installables force_experimental_features sh =
     Bos.Cmd.v "nix"
     |>+ [ "shell" ]
     |>+ installables
     |>+ Option.value ~default:[] force_experimental_features
+    |>+ [ "--command"; sh ]
   ;;
 
-  let legacy_nix_shell_from_installables installables =
-    (* Need to combine/validate that the installables given are all Nixpkgs *)
-    Bos.Cmd.v "nix-shell"
-    |>+ [ "--packages" ] @ installables
-    |>+ [ "--command"; Unix.shell () ]
+  let legacy_nix_shell_from_installables installables sh =
+    Bos.Cmd.v "nix-shell" |>+ [ "--packages" ] @ installables |>+ [ "--command"; sh ]
   ;;
 
   let print_strategy ({ workdir; primary; fallback } : strategy) =
diff --git a/lib/util.ml b/lib/util.ml
index 5b420f9..6987956 100644
--- a/lib/util.ml
+++ b/lib/util.ml
@@ -23,7 +23,7 @@ module Util = struct
     ;;
 
     (** Get the user's login shell *)
-    let shell () = (Unix.getpwuid (Unix.getuid ())).Unix.pw_shell
+    let pw_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")
@@ -33,6 +33,39 @@ module Util = struct
       Sys.file_exists (Filename.concat dir "shell.nix")
       || Sys.file_exists (Filename.concat dir "default.nix")
     ;;
+
+    (** Determines which shell to use. If the user gives a package
+    we build it and evaluate the main program to get the store path.
+    If there is no login shell, default to nixpkgs#bash. *)
+    let eval_sh sh =
+      let default = "bash"
+      and get_bin pkg =
+        let run_cmd cmd =
+          let ic = Unix.open_process_in cmd in
+          let result = Option.get (In_channel.input_line ic) in
+          (* TODO: Handle this potential error *)
+          let _ = Unix.close_process_in ic in
+          result
+        in
+        let store_path =
+          run_cmd (Printf.sprintf "nix-build '<nixpkgs>' -A %s --no-out-link --quiet" pkg)
+        and program =
+          run_cmd
+            (Printf.sprintf
+               "nix-instantiate '<nixpkgs>' --eval --raw -A %s.meta.mainProgram"
+               pkg)
+        in
+        let bin_path = Printf.sprintf "%s/bin/%s" store_path program in
+        bin_path
+      in
+      if sh = "/noshell" || sh = "/sbin/nologin"
+      then get_bin default
+      else (
+        match String.split_on_char '/' sh with
+        | [ "" ] -> Error.handle_ns_error "invalid argument: shell cannot be empty\n%!"
+        | [ pkg ] -> get_bin pkg
+        | _ -> sh)
+    ;;
   end
 
   (** Types and functions for parsing, interacting and formatting URIs. *)
diff --git a/shell.nix b/shell.nix
new file mode 100644
index 0000000..a605591
--- /dev/null
+++ b/shell.nix
@@ -0,0 +1 @@
+(builtins.getFlake (toString ./.)).outputs.devShells.${builtins.currentSystem}.default