Lookup bash in the PATH

Closes #36

src/action.ml

@@ -307,18 +307,14 @@ module Mini_shexp = struct
           Printf.fprintf oc "# 1 %S\n" (Path.to_string fn);
           copy_channels ic oc));
       return ()
-    | System cmd -> begin
-      let path, arg, err =
-        Utils.system_shell ~needed_to:"interpret (system ...) actions"
+    | System cmd ->
+      let path, arg =
+        Utils.system_shell_exn ~needed_to:"interpret (system ...) actions"
       in
-      match err with
-      | Some err -> err.fail ()
-      | None ->
-        run ~dir ~env ~env_extra ~stdout_to ~stderr_to path [arg; cmd]
-      end
+      run ~dir ~env ~env_extra ~stdout_to ~stderr_to path [arg; cmd]
     | Bash cmd ->
       run ~dir ~env ~env_extra ~stdout_to ~stderr_to
-        (Path.absolute "/bin/bash")
+        (Utils.bash_exn ~needed_to:"interpret (bash ...) actions")
         ["-e"; "-u"; "-o"; "pipefail"; "-c"; cmd]
     | Update_file (fn, s) ->
       let fn = Path.to_string fn in

src/utils.ml

@@ -1,6 +1,6 @@
 open Import
 
-let system_shell =
+let system_shell_exn =
   let cmd, arg, os =
     if Sys.win32 then
       ("cmd", "/c", "on Windows")
@@ -10,14 +10,20 @@ let system_shell =
   let bin = lazy (Bin.which cmd) in
   fun ~needed_to ->
     match Lazy.force bin with
-    | Some path -> (path, arg, None)
+    | Some path -> (path, arg)
     | None ->
-      (Path.absolute ("/" ^ cmd),
-       arg,
-       Some { fail = fun () ->
-         die "I need %s to %s but I couldn't find it :(\n\
-              Who doesn't have %s%s?!"
-           cmd needed_to cmd os })
+      die "I need %s to %s but I couldn't find it :(\n\
+           Who doesn't have %s%s?!"
+        cmd needed_to cmd os
+
+let bash_exn =
+  let bin = lazy (Bin.which "bash") in
+  fun ~needed_to ->
+    match Lazy.force bin with
+    | Some path -> path
+    | None ->
+      die "I need bash to %s but I couldn't find it :("
+        needed_to
 
 let signal_name =
   let table =

src/utils.mli

@@ -1,10 +1,13 @@
 (** Utilities that can't go in [Import] *)
 
-open Import
+open! Import
 
-(** Return the absolute path to the shell, the argument to pass it (-c or /c) and a
-    failure in case the shell can't be found. *)
-val system_shell : needed_to:string -> Path.t * string * fail option
+(** Return the absolute path to the shell and the argument to pass it (-c or /c). Raise in
+    case in cannot be found. *)
+val system_shell_exn : needed_to:string -> Path.t * string
+
+(** Same as [system_shell_exn] but for bash *)
+val bash_exn : needed_to:string -> Path.t
 
 (** Convert a signal number to a name: INT, TERM, ... *)
 val signal_name : int -> string