diff --git a/src/bin/main.ml b/src/bin/main.ml
index 80e0a63..7405f16 100644
--- a/src/bin/main.ml
+++ b/src/bin/main.ml
@@ -27,6 +27,7 @@ let sh ~command ~dump ~file ~rest env =
         argv = Array.of_list (pos_zero :: rest);
         program = pos_zero;
         functions = [];
+        hash = Merry.Hash.empty;
       }
   in
   match (file, command) with
diff --git a/src/lib/built_ins.ml b/src/lib/built_ins.ml
index 28bda36..d2cb40a 100644
--- a/src/lib/built_ins.ml
+++ b/src/lib/built_ins.ml
@@ -34,6 +34,7 @@ module Options = struct
 end
 
 type set = { update : Options.option list; print_options : bool }
+type hash = Hash_remove | Hash_stats | Hash_add of string list
 
 (* Built-in Actions *)
 type t =
@@ -44,6 +45,7 @@ type t =
   | Wait of int
   | Dot of string (* a.k.a source *)
   | Unset of [ `Variables of string list | `Functions of string list ]
+  | Hash of hash
 
 (* Change Directory *)
 module Cd = struct
@@ -159,8 +161,32 @@ module Unset = struct
     in
     let term = Term.(const make_unset $ kind $ names) in
     let info =
-      let doc = "Wait for a particular PID (default is 0)" in
-      Cmd.info "wait" ~doc
+      let doc = "Unset names of variables or functions." in
+      Cmd.info "unset" ~doc
+    in
+    Cmd.v info term
+end
+
+module Hash = struct
+  open Cmdliner
+
+  let remove =
+    let doc = "Empty the location table." in
+    Arg.(value & flag & info [ "r" ] ~docv:"REMOVE" ~doc)
+
+  let utilities =
+    let doc = "Utilities to search for and add to the location table." in
+    Arg.(value & pos_all string [] & info [] ~docv:"UTILITIES" ~doc)
+
+  let t =
+    let make_hash remove pos_all =
+      if remove then Hash Hash_remove
+      else match pos_all with [] -> Hash Hash_stats | us -> Hash (Hash_add us)
+    in
+    let term = Term.(const make_hash $ remove $ utilities) in
+    let info =
+      let doc = "Remember or report utility locations." in
+      Cmd.info "hash" ~doc
     in
     Cmd.v info term
 end
@@ -212,4 +238,5 @@ let of_args (w : string list) =
   | "source" :: _ as cmd -> exec_cmd cmd Source.t
   | "." :: _ as cmd -> exec_cmd cmd Dot.t
   | "unset" :: _ as cmd -> exec_cmd cmd Unset.t
+  | "hash" :: _ as cmd -> exec_cmd cmd Hash.t
   | _ -> None
diff --git a/src/lib/built_ins.mli b/src/lib/built_ins.mli
index 49f5e7d..d8892ff 100644
--- a/src/lib/built_ins.mli
+++ b/src/lib/built_ins.mli
@@ -11,6 +11,7 @@ module Options : sig
 end
 
 type set = { update : Options.option list; print_options : bool }
+type hash = Hash_remove | Hash_stats | Hash_add of string list
 
 type t =
   | Cd of { path : string option }
@@ -21,6 +22,7 @@ type t =
   | Wait of int
   | Dot of string
   | Unset of [ `Variables of string list | `Functions of string list ]
+  | Hash of hash
 
 val of_args : string list -> (t, string) result option
 (** Parses a command-line to the built-ins, errors are returned if parsing. *)
diff --git a/src/lib/eval.ml b/src/lib/eval.ml
index 2bf3ad8..f192690 100644
--- a/src/lib/eval.ml
+++ b/src/lib/eval.ml
@@ -43,6 +43,7 @@ module Make (S : Types.State) (E : Types.Exec) = struct
     program : string;
     argv : string array;
     functions : (string * Ast.compound_command) list;
+    hash : Hash.t;
   }
 
   let clear_local_state ctx = { ctx with local_state = [] }
@@ -157,6 +158,25 @@ module Make (S : Types.State) (E : Types.Exec) = struct
   let apply_pair (a, b) f = f a b
   let ( ||> ) = apply_pair
 
+  let resolve_program ?(update = true) ctx name =
+    let v =
+      if not (String.contains name '/') then
+        Sys.getenv_opt "PATH"
+        |> Option.value ~default:"/bin:/usr/bin"
+        |> String.split_on_char ':'
+        |> List.find_map (fun dir ->
+            let p = Filename.concat dir name in
+            if Sys.file_exists p then Some p else None)
+      else if Sys.file_exists name then Some name
+      else None
+    in
+    match (update, v) with
+    | true, Some loc ->
+        let hash = Hash.add ~utility:name ~loc ctx.hash in
+        ({ ctx with hash }, Some loc)
+    | false, Some loc -> (ctx, Some loc)
+    | _, None -> (ctx, None)
+
   let get_env ?(extra = []) ctx =
     let extra =
       extra
@@ -193,12 +213,18 @@ module Make (S : Types.State) (E : Types.Exec) = struct
         Eio.Flow.close some_write
       end
     in
-    let exec_process ctx job ?fds ?stdin ~stdout ~pgid args =
-      let process =
-        E.exec ctx.executor ?fds ?stdin ~stdout ~pgid ~mode
-          ~cwd:(cwd_of_ctx ctx)
-          ~env:(get_env ~extra:ctx.local_state ctx)
-          args
+    let exec_process ctx job ?fds ?stdin ~stdout ~pgid executable args =
+      let ctx, process =
+        match resolve_program ctx executable with
+        | ctx, None ->
+            Fmt.epr "msh: command not found: %s\n%!" executable;
+            (ctx, Error (127, `Not_found))
+        | ctx, Some full_path ->
+            ( ctx,
+              E.exec ctx.executor ?fds ?stdin ~stdout ~pgid ~mode
+                ~cwd:(cwd_of_ctx ctx)
+                ~env:(get_env ~extra:ctx.local_state ctx)
+                ~executable:full_path (executable :: args) )
       in
       match process with
       | Error (n, _) ->
@@ -243,10 +269,11 @@ module Make (S : Types.State) (E : Types.Exec) = struct
           in
           match Built_ins.of_args (executable :: args_as_strings) with
           | Some (Ok bi) ->
-              let ctx = handle_built_in ctx bi in
+              let ctx = handle_built_in ~stdout:some_write ctx bi in
+              close_stdout ~is_global some_write;
               let built_in = ctx >|= fun _ -> () in
               let job = handle_job ~pgid job (`Built_in built_in) in
-              loop (Exit.value ctx) job (pgid, stdout_of_previous) rest
+              loop (Exit.value ctx) job (pgid, some_read) rest
           | Some (Error _) ->
               (ctx, handle_job ~pgid job (`Built_in (Exit.nonzero () 1)))
           | None -> (
@@ -289,16 +316,16 @@ module Make (S : Types.State) (E : Types.Exec) = struct
                       | None ->
                           let ctx, job =
                             exec_process ctx job ~fds:redirect
-                              ~stdout:some_write ~pgid
-                              (executable :: args_as_strings)
+                              ~stdout:some_write ~pgid executable
+                              args_as_strings
                           in
                           close_stdout ~is_global some_write;
                           loop ctx job (pgid, some_read) rest
                       | Some stdout ->
                           let ctx, job =
                             exec_process ctx job ~fds:redirect ~stdin:stdout
-                              ~stdout:some_write ~pgid
-                              (executable :: args_as_strings)
+                              ~stdout:some_write ~pgid executable
+                              args_as_strings
                           in
                           close_stdout ~is_global some_write;
                           loop ctx job (pgid, some_read) rest))))
@@ -320,14 +347,17 @@ module Make (S : Types.State) (E : Types.Exec) = struct
        process that last just until all of the processes are setup. *)
     let ctx, job =
       let ghost_process =
-        E.exec ~mode:(Types.Switched pipeline_switch) ~pgid:0
-          ~cwd:(cwd_of_ctx initial_ctx) initial_ctx.executor
-          [ "sleep"; "99999999" ]
-        |> function
-        | Ok p -> p
-        | Error (n, `Not_found) ->
-            Fmt.epr "Interal error ghost process: not found";
-            exit n
+        match resolve_program ~update:false initial_ctx "sleep" with
+        | _, None -> Fmt.failwith "Sleep not found\n%!"
+        | ctx, Some sleep -> (
+            E.exec ~mode:(Types.Switched pipeline_switch) ~pgid:0
+              ~cwd:(cwd_of_ctx ctx) ctx.executor ~executable:sleep
+              [ "sleep"; "99999999" ]
+            |> function
+            | Ok p -> p
+            | Error (n, `Not_found) ->
+                Fmt.epr "Interal error ghost process: not found";
+                exit n)
       in
       loop initial_ctx None (E.pid ghost_process, None) p
     in
@@ -672,6 +702,7 @@ module Make (S : Types.State) (E : Types.Exec) = struct
         | Ast.Prefix_assignment (Name param, v) ->
             (* Expand the values *)
             let ctx, v = expand_cst ctx v in
+            let v = handle_subshell ctx v in
             let state =
               if update then S.update ctx.state ~param v else ctx.state
             in
@@ -693,7 +724,8 @@ module Make (S : Types.State) (E : Types.Exec) = struct
             (ctx, acc @ word_glob_expand ctx cst))
       (ctx, []) swc
 
-  and handle_built_in (ctx : ctx) = function
+  and handle_built_in ~(stdout : Eio_unix.sink_ty Eio.Flow.sink) (ctx : ctx) =
+    function
     | Built_ins.Cd { path } ->
         let cwd = S.cwd ctx.state in
         let+ state =
@@ -708,7 +740,9 @@ module Make (S : Types.State) (E : Types.Exec) = struct
         in
         { ctx with state }
     | Pwd ->
-        Fmt.pr "%a\n%!" Fpath.pp (S.cwd ctx.state);
+        Eio.Flow.copy_string
+          (Fmt.str "%a\n%!" Fpath.pp (S.cwd ctx.state))
+          stdout;
         Exit.zero ctx
     | Exit n ->
         let should_exit =
@@ -720,27 +754,19 @@ module Make (S : Types.State) (E : Types.Exec) = struct
           Exit.zero
             { ctx with options = Built_ins.Options.update ctx.options update }
         in
-        if print_options then Fmt.pr "%a%!" Built_ins.Options.pp ctx.options;
+        if print_options then
+          Eio.Flow.copy_string
+            (Fmt.str "%a" Built_ins.Options.pp ctx.options)
+            stdout;
         v
     | Wait i -> (
         match Unix.waitpid [] i with
         | _, WEXITED 0 -> Exit.zero ctx
         | _, (WEXITED n | WSIGNALED n | WSTOPPED n) -> Exit.nonzero ctx n)
     | Dot file -> (
-        let resolve_program name =
-          if not (String.contains name '/') then
-            Sys.getenv_opt "PATH"
-            |> Option.value ~default:"/bin:/usr/bin"
-            |> String.split_on_char ':'
-            |> List.find_map (fun dir ->
-                let p = Filename.concat dir name in
-                if Sys.file_exists p then Some p else None)
-          else if Sys.file_exists name then Some name
-          else None
-        in
-        match resolve_program file with
-        | None -> Exit.nonzero ctx 127
-        | Some f ->
+        match resolve_program ctx file with
+        | ctx, None -> Exit.nonzero ctx 127
+        | ctx, Some f ->
             let program = Ast.of_file (ctx.fs / f) in
             let ctx, _ = run (Exit.zero ctx) program in
             ctx)
@@ -760,6 +786,13 @@ module Make (S : Types.State) (E : Types.Exec) = struct
                 ctx.functions names
             in
             Exit.zero { ctx with functions })
+    | Hash v -> (
+        match v with
+        | Built_ins.Hash_remove -> Exit.zero { ctx with hash = Hash.empty }
+        | Built_ins.Hash_stats ->
+            Eio.Flow.copy_string (Fmt.str "%a" Hash.pp ctx.hash) stdout;
+            Exit.zero ctx
+        | _ -> assert false)
 
   and exec initial_ctx ((command, sep) : Ast.complete_command) =
     let rec loop : Eio.Switch.t -> ctx -> Ast.clist -> ctx Exit.t =
diff --git a/src/lib/hash.ml b/src/lib/hash.ml
new file mode 100644
index 0000000..073c2f3
--- /dev/null
+++ b/src/lib/hash.ml
@@ -0,0 +1,25 @@
+(* The hash table for utility locations *)
+
+module M = Map.Make (String)
+
+type entry = { hits : int; loc : string }
+type t = entry M.t
+
+let empty = M.empty
+
+let add ~utility ~loc t =
+  match M.find_opt utility t with
+  | Some { hits; loc = loc' } when String.equal loc loc' ->
+      M.add utility { hits = hits + 1; loc } t
+  | None | Some _ -> M.add utility { hits = 1; loc } t
+
+let lookup ~utility t = M.find_opt utility t |> Option.map (fun v -> v.loc)
+
+let pp ppf t =
+  let entries = M.to_list t in
+  match entries with
+  | [] -> ()
+  | _ ->
+      let pp_entry ppf (_, { hits; loc }) = Fmt.pf ppf "%-7i %s@." hits loc in
+      let pp_header ppf () = Fmt.pf ppf "%-7s %s@." "hits" "command" in
+      Fmt.pf ppf "@[<v>%a%a@]" pp_header () Fmt.(list pp_entry) entries
diff --git a/src/lib/hash.mli b/src/lib/hash.mli
new file mode 100644
index 0000000..f6ebff6
--- /dev/null
+++ b/src/lib/hash.mli
@@ -0,0 +1,14 @@
+type t
+(** A lookup table for utilities *)
+
+val empty : t
+(** The empty table *)
+
+val add : utility:string -> loc:string -> t -> t
+(** [add ~utility ~loc t] adds the [utility] with [loc] location. *)
+
+val lookup : utility:string -> t -> string option
+(** [lookup ~utility t] will try to find [utility] in [t]. *)
+
+val pp : t Fmt.t
+(** A pretty printer *)
diff --git a/src/lib/merry.ml b/src/lib/merry.ml
index d98fd00..175ae93 100644
--- a/src/lib/merry.ml
+++ b/src/lib/merry.ml
@@ -1,4 +1,5 @@
 module Import = Import
+module Hash = Hash
 module Exit = Exit
 module Eunix = Eunix
 module Ast = Ast
diff --git a/src/lib/merry.mli b/src/lib/merry.mli
index 9a7e5f8..b5df63f 100644
--- a/src/lib/merry.mli
+++ b/src/lib/merry.mli
@@ -1,4 +1,5 @@
 module Ast = Ast
+module Hash = Hash
 module Exit = Exit
 module Eunix = Eunix
 module Types = Types
diff --git a/src/lib/posix/exec.ml b/src/lib/posix/exec.ml
index 2cd616c..24a0883 100644
--- a/src/lib/posix/exec.ml
+++ b/src/lib/posix/exec.ml
@@ -128,17 +128,6 @@ let process =
   let handler = Eio.Process.Pi.process (module Process_impl) in
   fun proc -> Eio.Resource.T (proc, handler)
 
-let resolve_program name =
-  if not (String.contains name '/') then
-    Sys.getenv_opt "PATH"
-    |> Option.value ~default:"/bin:/usr/bin"
-    |> String.split_on_char ':'
-    |> List.find_map (fun dir ->
-        let p = Filename.concat dir name in
-        if Sys.file_exists p then Some p else None)
-  else if Sys.file_exists name then Some name
-  else None
-
 let read_of_fd ~mode ~default ~to_close v =
   match (mode, v) with
   | Merry.Types.Async, _ | _, None -> default
@@ -186,10 +175,7 @@ let get_executable ~args = function
   | None -> (
       match args with
       | [] -> invalid_arg "Arguments list is empty and no executable given!"
-      | x :: _ -> (
-          match resolve_program x with
-          | Some x -> x
-          | None -> raise (Eio.Process.err (Executable_not_found x))))
+      | x :: _ -> x)
 
 let get_env = function Some e -> e | None -> Unix.environment ()
 
diff --git a/src/lib/posix/merry_posix.ml b/src/lib/posix/merry_posix.ml
index a1ae698..2f3cacd 100644
--- a/src/lib/posix/merry_posix.ml
+++ b/src/lib/posix/merry_posix.ml
@@ -16,7 +16,7 @@ module Exec = struct
     | `Signaled n -> Merry.Exit.nonzero () n
 
   let exec ?(fork_actions = []) ?(fds = []) ?stdin ?stdout ?stderr ?env ~mode
-      ~pgid ~cwd t args =
+      ~pgid ~cwd ~executable t args =
     let env =
       Option.map
         (fun lst -> List.map (fun (a, b) -> a ^ "=" ^ b) lst |> Array.of_list)
@@ -25,7 +25,7 @@ module Exec = struct
     try
       Ok
         (Exec.run ~fork_actions ~mode ~fds ~pgid ~cwd ?stdin ?stdout ?stderr
-           ?env t args)
+           ?env t ~executable args)
     with Eio.Io (Eio.Process.E (Eio.Process.Executable_not_found m), _ctx) ->
       Fmt.epr "msh: command not found: %s\n%!" m;
       Error (127, `Not_found)
diff --git a/src/lib/types.ml b/src/lib/types.ml
index bcdab91..3dac835 100644
--- a/src/lib/types.ml
+++ b/src/lib/types.ml
@@ -70,6 +70,7 @@ module type Exec = sig
     mode:exec_mode ->
     pgid:int ->
     cwd:Eio.Fs.dir_ty Eio.Path.t ->
+    executable:string ->
     t ->
     string list ->
     (process, int * [ `Not_found ]) result
diff --git a/test/built_ins.t b/test/built_ins.t
index abe31c4..29079e3 100644
--- a/test/built_ins.t
+++ b/test/built_ins.t
@@ -93,3 +93,53 @@ Testing some shell built-ins
   HEY
   msh: command not found: shout
   [127]
+
+5. Hash
+
+  $ cat > test.sh << EOF
+  > reproducible_hash () {
+  >   hash | sed 's/|/ /' | awk '{print \$1, \$8}'
+  > }
+  > ls
+  > ls
+  > reproducible_hash
+  > hash -r
+  > reproducible_hash
+  > EOF
+
+  $ sh test.sh 
+  hello.txt
+  run.sh
+  test.sh
+  test_bad.sh
+  test_good.sh
+  testing
+  hello.txt
+  run.sh
+  test.sh
+  test_bad.sh
+  test_good.sh
+  testing
+  hits 
+  2 
+  $ msh test.sh 
+  hello.txt
+  run.sh
+  test.sh
+  test_bad.sh
+  test_good.sh
+  testing
+  hello.txt
+  run.sh
+  test.sh
+  test_bad.sh
+  test_good.sh
+  testing
+  hits 
+  2 
+6. Built-in redirection and pipelining
+
+  $ sh -c "FOO=\$(pwd | rev | rev); echo \$(basename \$FOO)" 
+  test
+  $ msh -c "FOO=\$(pwd | rev | rev); echo \$(basename \$FOO)" 
+  test