diff --git a/src/action.ml b/src/action.ml
index 497a4758..fe5ce286 100644
--- a/src/action.ml
+++ b/src/action.ml
@@ -1,62 +1,93 @@
 open Import
 open Sexp.Of_sexp
 
+type var_expansion =
+  | Not_found
+  | Path  of Path.t
+  | Paths of Path.t list
+  | Str   of string
+
+let expand_str ~dir ~f template =
+  String_with_vars.expand template ~f:(fun var ->
+    match f var with
+    | Not_found -> None
+    | Path path -> Some (Path.reach ~from:dir path)
+    | Paths l -> Some (List.map l ~f:(Path.reach ~from:dir) |> String.concat ~sep:" ")
+    | Str s -> Some s)
+
+let expand_path ~dir ~f template =
+  match String_with_vars.just_a_var template with
+  | None -> expand_str ~dir ~f template |> Path.relative dir
+  | Some v ->
+    match f v with
+    | Not_found -> expand_str ~dir ~f template |> Path.relative dir
+    | Path p -> p
+    | Str s -> Path.relative dir s
+    | Paths l ->
+      List.map l ~f:(Path.reach ~from:dir)
+      |> String.concat ~sep:" "
+      |> Path.of_string
+
 module Mini_shexp = struct
-  type 'a t =
-    | Run            of 'a * 'a list
-    | Chdir          of 'a * 'a t
-    | Setenv         of 'a * 'a * 'a t
-    | With_stdout_to of 'a * 'a t
-    | Progn          of 'a t list
+  type ('a, 'path) t =
+    | Run            of 'path * 'a list
+    | Chdir          of 'path * ('a, 'path) t
+    | Setenv         of 'a * 'a * ('a, 'path) t
+    | With_stdout_to of 'path * ('a, 'path) t
+    | Progn          of ('a, 'path) t list
     | Echo           of 'a
-    | Cat            of 'a
-    | Copy           of 'a * 'a
-    | Symlink        of 'a * 'a
-    | Copy_and_add_line_directive of 'a * 'a
+    | Create_file    of 'path
+    | Cat            of 'path
+    | Copy           of 'path * 'path
+    | Symlink        of 'path * 'path
+    | Copy_and_add_line_directive of 'path * 'path
     | System         of 'a
 
-  let rec t a sexp =
+  let rec t a p sexp =
     sum
-      [ cstr_rest "run" (a @> nil) a             (fun prog args -> Run (prog, args))
-      ; cstr "chdir"    (a @> t a @> nil)        (fun dn t -> Chdir (dn, t))
-      ; cstr "setenv"   (a @> a @> t a @> nil)   (fun k v t -> Setenv (k, v, t))
-      ; cstr "with-stdout-to" (a @> t a @> nil)  (fun fn t -> With_stdout_to (fn, t))
-      ; cstr_rest "progn"      nil (t a)         (fun l -> Progn l)
+      [ cstr_rest "run" (p @> nil) a             (fun prog args -> Run (prog, args))
+      ; cstr "chdir"    (p @> t a p @> nil)        (fun dn t -> Chdir (dn, t))
+      ; cstr "setenv"   (a @> a @> t a p @> nil)   (fun k v t -> Setenv (k, v, t))
+      ; cstr "with-stdout-to" (p @> t a p @> nil)  (fun fn t -> With_stdout_to (fn, t))
+      ; cstr_rest "progn"      nil (t a p)         (fun l -> Progn l)
       ; cstr "echo"           (a @> nil)         (fun x -> Echo x)
-      ; cstr "cat"            (a @> nil)         (fun x -> Cat x)
-      ; cstr "copy" (a @> a @> nil)              (fun src dst -> Copy (src, dst))
+      ; cstr "cat"            (p @> nil)         (fun x -> Cat x)
+      ; cstr "create-file"    (p @> nil)         (fun x -> Create_file x)
+      ; cstr "copy" (p @> p @> nil)              (fun src dst -> Copy (src, dst))
       (*
          (* We don't expose symlink to the user yet since this might complicate things *)
          ; cstr "symlink" (a @> a @> nil) (fun src dst -> Symlink (dst, Cat src))
       *)
-      ; cstr "copy-and-add-line-directive" (a @> a @> nil) (fun src dst ->
+      ; cstr "copy-and-add-line-directive" (p @> p @> nil) (fun src dst ->
           Copy_and_add_line_directive (src, dst))
       ; cstr "system" (a @> nil) (fun cmd -> System cmd)
       ]
       sexp
 
-  let rec expand dir t ~f =
+  let rec expand dir t ~f : (string, Path.t) t =
     match t with
     | Run (prog, args) ->
-      Run (f dir prog,
-           List.map args ~f:(fun arg -> f dir arg))
+      Run (expand_path ~dir ~f prog,
+           List.map args ~f:(fun arg -> expand_str ~dir ~f arg))
     | Chdir (fn, t) ->
-      let fn = f dir fn in
-      Chdir (fn, expand (Path.relative dir fn) t ~f)
+      let fn = expand_path ~dir ~f fn in
+      Chdir (fn, expand fn t ~f)
     | Setenv (var, value, t) ->
-      Setenv (f dir var, f dir value, expand dir t ~f)
+      Setenv (expand_str ~dir ~f var, expand_str ~dir ~f value,
+              expand dir t ~f)
     | With_stdout_to (fn, t) ->
-      With_stdout_to (f dir fn, expand dir t ~f)
+      With_stdout_to (expand_path ~dir ~f fn, expand dir t ~f)
     | Progn l -> Progn (List.map l ~f:(fun t -> expand dir t ~f))
-    | Echo x -> Echo (f dir x)
-    | Cat x -> Cat (f dir x)
+    | Echo x -> Echo (expand_str ~dir ~f x)
+    | Cat x -> Cat (expand_path ~dir ~f x)
+    | Create_file x -> Create_file (expand_path ~dir ~f x)
     | Copy (x, y) ->
-      Copy (f dir x, f dir y)
+      Copy (expand_path ~dir ~f x, expand_path ~dir ~f y)
     | Symlink (x, y) ->
-      Symlink (f dir x, f dir y)
+      Symlink (expand_path ~dir ~f x, expand_path ~dir ~f y)
     | Copy_and_add_line_directive (x, y) ->
-      Copy_and_add_line_directive (f dir x, f dir y)
-    | System x -> System (f dir x)
+      Copy_and_add_line_directive (expand_path ~dir ~f x, expand_path ~dir ~f y)
+    | System x -> System (expand_str ~dir ~f x)
 
   let rec fold t ~init:acc ~f =
     match t with
@@ -67,71 +98,218 @@ module Mini_shexp = struct
     | Progn l -> List.fold_left l ~init:acc ~f:(fun init t -> fold t ~init ~f)
     | Echo x -> f acc x
     | Cat x -> f acc x
+    | Create_file x -> f acc x
     | Copy (x, y) -> f (f acc x) y
     | Symlink (x, y) -> f (f acc x) y
     | Copy_and_add_line_directive (x, y) -> f (f acc x) y
     | System x -> f acc x
 
-  let rec sexp_of_t f : _ -> Sexp.t = function
-    | Run (a, xs) -> List (Atom "run" :: f a :: List.map xs ~f)
-    | Chdir (a, r) -> List [Atom "chdir" ; f a ; sexp_of_t f r]
-    | Setenv (k, v, r) -> List [Atom "setenv" ; f k ; f v ; sexp_of_t f r]
-    | With_stdout_to (fn, r) -> List [Atom "with-stdout-to"; f fn; sexp_of_t f r]
-    | Progn l -> List (Atom "progn" :: List.map l ~f:(sexp_of_t f))
+  let rec sexp_of_t f g : _ -> Sexp.t = function
+    | Run (a, xs) -> List (Atom "run" :: g a :: List.map xs ~f)
+    | Chdir (a, r) -> List [Atom "chdir" ; g a ; sexp_of_t f g r]
+    | Setenv (k, v, r) -> List [Atom "setenv" ; f k ; f v ; sexp_of_t f g r]
+    | With_stdout_to (fn, r) -> List [Atom "with-stdout-to"; g fn; sexp_of_t f g r]
+    | Progn l -> List (Atom "progn" :: List.map l ~f:(sexp_of_t f g))
     | Echo x -> List [Atom "echo"; f x]
-    | Cat x -> List [Atom "cat"; f x]
+    | Cat x -> List [Atom "cat"; g x]
+    | Create_file x -> List [Atom "create-file"; g x]
     | Copy (x, y) ->
-      List [Atom "copy"; f x; f y]
+      List [Atom "copy"; g x; g y]
     | Symlink (x, y) ->
-      List [Atom "symlink"; f x; f y]
+      List [Atom "symlink"; g x; g y]
     | Copy_and_add_line_directive (x, y) ->
-      List [Atom "copy-and-add-line-directive"; f x; f y]
+      List [Atom "copy-and-add-line-directive"; g x; g y]
     | System x -> List [Atom "system"; f x]
+
+  open Future
+
+  let run ~dir ~env ~env_extra ~stdout_to ~tail prog args =
+    let stdout_to : Future.stdout_to =
+      match stdout_to with
+      | None          -> Terminal
+      | Some (fn, oc) -> Opened_file { filename = fn; tail; desc = Channel oc }
+    in
+    let env = Context.extend_env ~vars:env_extra ~env in
+    Future.run Strict ~dir:(Path.to_string dir) ~env ~stdout_to
+      (Path.reach ~from:dir prog) args
+
+  let rec exec t ~dir ~env ~env_extra ~stdout_to ~tail =
+    match t with
+    | Run (prog, args) ->
+      run ~dir ~env ~env_extra ~stdout_to ~tail prog args
+    | Chdir (dir, t) ->
+      exec t ~env ~env_extra ~stdout_to ~tail ~dir
+    | Setenv (var, value, t) ->
+      exec t ~dir ~env ~stdout_to ~tail
+        ~env_extra:(String_map.add env_extra ~key:var ~data:value)
+    | With_stdout_to (fn, t) ->
+      if tail then Option.iter stdout_to ~f:(fun (_, oc) -> close_out oc);
+      let fn = Path.to_string fn in
+      exec t ~dir ~env ~env_extra ~tail
+        ~stdout_to:(Some (fn, open_out_bin fn))
+    | Progn l ->
+      exec_list l ~dir ~env ~env_extra ~stdout_to ~tail
+    | Echo str ->
+      return
+        (match stdout_to with
+         | None -> print_string str; flush stdout
+         | Some (_, oc) ->
+           output_string oc str;
+           if tail then close_out oc)
+    | Cat fn ->
+      with_file_in (Path.to_string fn) ~f:(fun ic ->
+        match stdout_to with
+        | None -> copy_channels ic stdout
+        | Some (_, oc) ->
+          copy_channels ic oc;
+          if tail then close_out oc);
+      return ()
+    | Create_file fn ->
+      let fn = Path.to_string fn in
+      if Sys.file_exists fn then Sys.remove fn;
+      Unix.close (Unix.openfile fn [O_CREAT; O_TRUNC; O_WRONLY] 0o666);
+      return ()
+    | Copy (src, dst) ->
+      copy_file ~src:(Path.to_string src) ~dst:(Path.to_string dst);
+      return ()
+    | Symlink (src, dst) ->
+      if Sys.win32 then
+        copy_file ~src:(Path.to_string src) ~dst:(Path.to_string dst)
+      else begin
+        let src =
+          if Path.is_root dst then
+            Path.to_string src
+          else
+            Path.reach ~from:(Path.parent dst) src
+        in
+        let dst = Path.to_string dst in
+        match Unix.readlink dst with
+        | target ->
+          if target <> src then begin
+            Unix.unlink dst;
+            Unix.symlink src dst
+          end
+        | exception _ ->
+          Unix.symlink src dst
+      end;
+      return ()
+    | Copy_and_add_line_directive (src, dst) ->
+      with_file_in (Path.to_string src) ~f:(fun ic ->
+        with_file_out (Path.to_string dst) ~f:(fun oc ->
+          let fn =
+            match Path.extract_build_context src with
+            | None -> src
+            | Some (_, rem) -> rem
+          in
+          Printf.fprintf oc "# 1 %S\n" (Path.to_string fn);
+          copy_channels ic oc));
+      return ()
+    | System cmd ->
+      let path, arg, err =
+        Utils.system_shell ~needed_to:"interpret (system ...) actions"
+      in
+      match err with
+      | Some err -> err.fail ()
+      | None ->
+        run ~dir ~env ~env_extra ~stdout_to ~tail path [arg; cmd]
+
+  and exec_list l ~dir ~env ~env_extra ~stdout_to ~tail =
+    match l with
+    | [] ->
+      if tail then Option.iter stdout_to ~f:(fun (_, oc) -> close_out oc);
+      Future.return ()
+    | [t] ->
+      exec t ~dir ~env ~env_extra ~stdout_to ~tail
+    | t :: rest ->
+      exec t ~dir ~env ~env_extra ~stdout_to ~tail:false >>= fun () ->
+      exec_list rest ~dir ~env ~env_extra ~stdout_to ~tail
 end
 
 module Desc = struct
-  module T = struct
-    type 'a t =
+  module Ast = struct
+    type ('a, 'path) t =
       | Bash of 'a
-      | Shexp of 'a Mini_shexp.t
+      | Shexp of ('a, 'path) Mini_shexp.t
 
-    let t a sexp =
+    let t a b sexp =
       match sexp with
-      | Atom _ -> Bash  (a              sexp)
-      | List _ -> Shexp (Mini_shexp.t a sexp)
+      | Atom _ -> Bash  (a                sexp)
+      | List _ -> Shexp (Mini_shexp.t a b sexp)
 
-    type context = Path.t
-
-    let expand dir t ~f =
-      match t with
-      | Bash x -> Bash (f dir x)
-      | Shexp x -> Shexp (Mini_shexp.expand dir x ~f)
+    let sexp_of_t f g : _ -> Sexp.t = function
+      | Bash a -> List [Atom "bash" ; f a]
+      | Shexp a -> List [Atom "shexp" ; Mini_shexp.sexp_of_t f g a]
 
     let fold t ~init ~f =
       match t with
       | Bash x -> f init x
       | Shexp x -> Mini_shexp.fold x ~init ~f
-
-    let sexp_of_t f : _ -> Sexp.t = function
-      | Bash a -> List [Atom "bash" ; f a]
-      | Shexp a -> List [Atom "shexp" ; Mini_shexp.sexp_of_t f a]
   end
 
-  include T
+  type t = (string, Path.t) Ast.t
+  let t = Ast.t string Path.t
+  let sexp_of_t = Ast.sexp_of_t Sexp.To_sexp.string Path.sexp_of_t
 
-  module Unexpanded = String_with_vars.Lift(T)
+  module Unexpanded = struct
+    type t = (String_with_vars.t, String_with_vars.t) Ast.t
+    let t = Ast.t String_with_vars.t String_with_vars.t
+    let sexp_of_t = Ast.sexp_of_t String_with_vars.sexp_of_t String_with_vars.sexp_of_t
+
+    let fold_vars t ~init ~f =
+      Ast.fold t ~init ~f:(fun acc pat ->
+        String_with_vars.fold ~init:acc pat ~f)
+
+    let expand dir (t : t) ~f : (_, _) Ast.t =
+      match t with
+      | Bash  x -> Bash  (expand_str ~dir ~f x)
+      | Shexp x -> Shexp (Mini_shexp.expand dir x ~f)
+  end
 end
 
 type t =
-  { env    : string array
-  ; dir    : Path.t
-  ; action : string Desc.t
+  { context : Context.t option
+  ; dir     : Path.t
+  ; action  : Desc.t
   }
 
-let sexp_of_t { env; dir; action } =
-  let open Sexp.To_sexp in
-  Sexp.List
-    [ List [ Atom "env"   ; array string env             ]
-    ; List [ Atom "dir"   ; string (Path.to_string dir)  ]
-    ; List [ Atom "action"; Desc.sexp_of_t string action ]
+let t contexts sexp =
+  let open Sexp.Of_sexp in
+  let context sexp =
+    let name = string sexp in
+    match String_map.find name contexts with
+    | None   -> of_sexp_errorf sexp "Context %s not found" name
+    | Some c -> c
+  in
+  record
+    (field_o "context" context >>= fun context ->
+     field   "dir"     Path.t  >>= fun dir ->
+     field   "action"  Desc.t  >>= fun action ->
+     return { context; dir; action })
+    sexp
+
+let sexp_of_t { context; dir; action } =
+  let fields : Sexp.t list =
+    [ List [ Atom "dir"    ; Path.sexp_of_t dir    ]
+    ; List [ Atom "action" ; Desc.sexp_of_t action ]
     ]
+  in
+  let fields =
+    match context with
+    | None -> fields
+    | Some { name; _ } -> List [ Atom "context"; Atom name ] :: fields
+  in
+  Sexp.List fields
+
+let exec { action; dir; context } =
+  let env =
+    match context with
+    | None -> Lazy.force Context.initial_env
+    | Some c -> c.env
+  in
+  match action with
+  | Bash cmd ->
+    Future.run Strict ~dir:(Path.to_string dir) ~env
+      "/bin/bash" ["-e"; "-u"; "-o"; "pipefail"; "-c"; cmd]
+  | Shexp shexp ->
+    Mini_shexp.exec shexp ~dir ~env ~env_extra:String_map.empty
+      ~stdout_to:None ~tail:true
diff --git a/src/action.mli b/src/action.mli
new file mode 100644
index 00000000..8195a7bd
--- /dev/null
+++ b/src/action.mli
@@ -0,0 +1,60 @@
+open! Import
+
+type var_expansion =
+  | Not_found
+  | Path  of Path.t
+  | Paths of Path.t list
+  | Str   of string
+
+module Mini_shexp : sig
+  type ('a, 'path) t =
+    | Run            of 'path * 'a list
+    | Chdir          of 'path * ('a, 'path) t
+    | Setenv         of 'a * 'a * ('a, 'path) t
+    | With_stdout_to of 'path * ('a, 'path) t
+    | Progn          of ('a, 'path) t list
+    | Echo           of 'a
+    | Create_file    of 'path
+    | Cat            of 'path
+    | Copy           of 'path * 'path
+    | Symlink        of 'path * 'path
+    | Copy_and_add_line_directive of 'path * 'path
+    | System         of 'a
+
+  val t : 'a Sexp.Of_sexp.t -> 'b Sexp.Of_sexp.t -> ('a, 'b) t Sexp.Of_sexp.t
+  val sexp_of_t : 'a Sexp.To_sexp.t -> 'b Sexp.To_sexp.t -> ('a, 'b) t Sexp.To_sexp.t
+end
+
+module Desc : sig
+  module Ast : sig
+    type ('a, 'path) t =
+      | Bash  of 'a
+      | Shexp of ('a, 'path) Mini_shexp.t
+
+    val t : 'a Sexp.Of_sexp.t -> 'b Sexp.Of_sexp.t -> ('a, 'b) t Sexp.Of_sexp.t
+    val sexp_of_t : 'a Sexp.To_sexp.t -> 'b Sexp.To_sexp.t -> ('a, 'b) t Sexp.To_sexp.t
+  end
+
+  type t = (string, Path.t) Ast.t
+  val t : t Sexp.Of_sexp.t
+  val sexp_of_t : t Sexp.To_sexp.t
+
+  module Unexpanded : sig
+    type desc = t
+    type t = (String_with_vars.t, String_with_vars.t) Ast.t
+    val t : t Sexp.Of_sexp.t
+    val sexp_of_t : t Sexp.To_sexp.t
+    val fold_vars : t -> init:'a -> f:('a -> string -> 'a) -> 'a
+    val expand : Path.t -> t -> f:(string -> var_expansion) -> desc
+  end with type desc := t
+end
+
+type t =
+  { context : Context.t option
+  ; dir     : Path.t
+  ; action  : Desc.t
+  }
+
+val t : Context.t String_map.t -> t Sexp.Of_sexp.t
+val sexp_of_t : t Sexp.To_sexp.t
+val exec : t -> unit Future.t
diff --git a/src/alias.ml b/src/alias.ml
index e6a10435..16ac4df0 100644
--- a/src/alias.ml
+++ b/src/alias.ml
@@ -91,6 +91,6 @@ let rules store ~prefixes ~tree =
     let rule =
       Build_interpret.Rule.make
         (Build.path_set deps >>>
-         Build.touch alias.file)
+         Build.create_file alias.file)
     in
     rule :: acc)
diff --git a/src/build.ml b/src/build.ml
index 44e358e6..92d4eb40 100644
--- a/src/build.ml
+++ b/src/build.ml
@@ -18,14 +18,10 @@ type lib_dep_kind =
 type lib_deps = lib_dep_kind String_map.t
 
 module Repr = struct
-  type ('a, 'b) prim =
-    { targets : Path.t list
-    ; exec    : 'a -> 'b Future.t
-    }
   type ('a, 'b) t =
     | Arr : ('a -> 'b) -> ('a, 'b) t
-    | Prim : ('a, 'b) prim -> ('a, 'b) t
-    | Store_vfile : 'a Vspec.t -> ('a, unit) t
+    | Targets : Path.t list -> ('a, 'a) t
+    | Store_vfile : 'a Vspec.t -> ('a, Action.t) t
     | Compose : ('a, 'b) t * ('b, 'c) t -> ('a, 'c) t
     | First : ('a, 'b) t -> ('a * 'c, 'b * 'c) t
     | Second : ('a, 'b) t -> ('c * 'a, 'c * 'b) t
@@ -121,13 +117,6 @@ let files_recursively_in ~dir =
   in
   path_set (loop src_dir Pset.empty)
 
-let prim ~targets exec = Prim { targets; exec }
-
-let create_files ~targets exec =
-  prim ~targets (fun x -> Future.return (exec x))
-let create_file ~target exec =
-  create_files ~targets:[target] exec
-
 let store_vfile spec = Store_vfile spec
 
 let get_prog (prog : _ Prog_spec.t) =
@@ -145,7 +134,7 @@ let prog_and_args ~dir prog args =
     >>>
     arr fst))
 
-let run ?(dir=Path.root) ?stdout_to ?env ?(extra_targets=[]) prog args =
+let run ?(dir=Path.root) ?stdout_to ?context ?(extra_targets=[]) prog args =
   let extra_targets =
     match stdout_to with
     | None -> extra_targets
@@ -154,169 +143,59 @@ let run ?(dir=Path.root) ?stdout_to ?env ?(extra_targets=[]) prog args =
   let targets = Arg_spec.add_targets args extra_targets in
   prog_and_args ~dir prog args
   >>>
-  prim ~targets
-    (fun (prog, args) ->
-       let stdout_to =
-         match stdout_to with
-         | None -> Future.Terminal
-         | Some path -> File (Path.to_string path)
-       in
-       Future.run Strict ~dir:(Path.to_string dir) ~stdout_to ?env
-         (Path.reach prog ~from:dir) args)
-
-module Shexp = struct
-  open Future
-  open Action.Mini_shexp
-
-  let run ~dir ~env ~env_extra ~stdout_to ~tail prog args =
-    let stdout_to : Future.stdout_to =
+  Targets targets
+  >>^  (fun (prog, args) ->
+    let action : (_, _) Action.Mini_shexp.t = Run (prog, args) in
+    let action =
       match stdout_to with
-      | None          -> Terminal
-      | Some (fn, oc) -> Opened_file { filename = fn; tail; desc = Channel oc }
+      | None      -> action
+      | Some path -> With_stdout_to (path, action)
     in
-    let env = Context.extend_env ~vars:env_extra ~env in
-    Future.run Strict ~dir:(Path.to_string dir) ~env ~stdout_to prog args
+    { Action.
+      dir
+    ; context
+    ; action = Shexp action
+    })
 
-  let rec exec t ~dir ~env ~env_extra ~stdout_to ~tail =
-    match t with
-    | Run (prog, args) ->
-      run ~dir ~env ~env_extra ~stdout_to ~tail prog args
-    | Chdir (fn, t) ->
-      exec t ~env ~env_extra ~stdout_to ~tail ~dir:(Path.relative dir fn)
-    | Setenv (var, value, t) ->
-      exec t ~dir ~env ~stdout_to ~tail
-        ~env_extra:(String_map.add env_extra ~key:var ~data:value)
-    | With_stdout_to (fn, t) ->
-      if tail then Option.iter stdout_to ~f:(fun (_, oc) -> close_out oc);
-      let fn = Path.to_string (Path.relative dir fn) in
-      exec t ~dir ~env ~env_extra ~tail
-        ~stdout_to:(Some (fn, open_out_bin fn))
-    | Progn l ->
-      exec_list l ~dir ~env ~env_extra ~stdout_to ~tail
-    | Echo str ->
-      return
-        (match stdout_to with
-         | None -> print_string str; flush stdout
-         | Some (_, oc) ->
-           output_string oc str;
-           if tail then close_out oc)
-    | Cat fn ->
-      let fn = Path.to_string (Path.relative dir fn) in
-      with_file_in fn ~f:(fun ic ->
-        match stdout_to with
-        | None -> copy_channels ic stdout
-        | Some (_, oc) ->
-          copy_channels ic oc;
-          if tail then close_out oc);
-      return ()
-    | Copy (src, dst) ->
-      let src = Path.relative dir src in
-      let dst = Path.relative dir dst in
-      copy_file ~src:(Path.to_string src) ~dst:(Path.to_string dst);
-      return ()
-    | Symlink (src, dst) ->
-      let src = Path.relative dir src in
-      let dst = Path.relative dir dst in
-      if Sys.win32 then
-        copy_file ~src:(Path.to_string src) ~dst:(Path.to_string dst)
-      else begin
-        let src =
-          if Path.is_root dst then
-            Path.to_string src
-          else
-            Path.reach ~from:(Path.parent dst) src
-        in
-        let dst = Path.to_string dst in
-        match Unix.readlink dst with
-        | target ->
-          if target <> src then begin
-            Unix.unlink dst;
-            Unix.symlink src dst
-          end
-        | exception _ ->
-          Unix.symlink src dst
-      end;
-      return ()
-    | Copy_and_add_line_directive (src, dst) ->
-      let src = Path.relative dir src in
-      let dst = Path.relative dir dst in
-      with_file_in (Path.to_string src) ~f:(fun ic ->
-        with_file_out (Path.to_string dst) ~f:(fun oc ->
-          let fn =
-            match Path.extract_build_context src with
-            | None -> src
-            | Some (_, rem) -> rem
-          in
-          Printf.fprintf oc "# 1 %S\n" (Path.to_string fn);
-          copy_channels ic oc));
-      return ()
-    | System cmd ->
-      let path, arg, err =
-        Utils.system_shell ~needed_to:"interpret (system ...) actions"
-      in
-      match err with
-      | Some err -> err.fail ()
-      | None ->
-        run ~dir ~env ~env_extra ~stdout_to ~tail
-          (Path.to_string path) [arg; cmd]
+let action ?(dir=Path.root) ?context ~targets action =
+  Targets targets
+  >>^ fun () ->
+  { Action. context; dir; action }
 
-  and exec_list l ~dir ~env ~env_extra ~stdout_to ~tail =
-    match l with
-    | [] ->
-      if tail then Option.iter stdout_to ~f:(fun (_, oc) -> close_out oc);
-      Future.return ()
-    | [t] ->
-      exec t ~dir ~env ~env_extra ~stdout_to ~tail
-    | t :: rest ->
-      exec t ~dir ~env ~env_extra ~stdout_to ~tail:false >>= fun () ->
-      exec_list rest ~dir ~env ~env_extra ~stdout_to ~tail
+let shexp ?dir ?context ~targets shexp =
+  action ?dir ?context ~targets (Shexp shexp)
 
-  let exec t ~dir ~env =
-    exec t ~dir ~env ~env_extra:String_map.empty ~stdout_to:None ~tail:true
-end
+let echo fn s =
+  shexp ~targets:[fn] (With_stdout_to (fn, Echo s))
 
-let action action ~dir ~env ~targets =
-  prim ~targets (fun () ->
-    match (action : _ Action.Desc.t) with
-    | Bash cmd ->
-      Future.run Strict ~dir:(Path.to_string dir) ~env
-        "/bin/bash" ["-e"; "-u"; "-o"; "pipefail"; "-c"; cmd]
-    | Shexp shexp ->
-      Shexp.exec ~dir ~env shexp)
-
-let echo fn =
-  create_file ~target:fn (fun data ->
-    with_file_out (Path.to_string fn) ~f:(fun oc -> output_string oc data))
+let echo_dyn fn =
+  Targets [fn]
+  >>^ fun s ->
+  { Action.
+    context = None
+  ; dir     = Path.root
+  ; action  = Shexp (With_stdout_to (fn, Echo s))
+  }
 
 let copy ~src ~dst =
   path src >>>
-  create_file ~target:dst (fun () ->
-    copy_file ~src:(Path.to_string src) ~dst:(Path.to_string dst))
+  shexp ~targets:[dst] (Copy (src, dst))
 
 let symlink ~src ~dst =
-  if Sys.win32 then
-    copy ~src ~dst
-  else
-    path src >>>
-    create_file ~target:dst (fun () ->
-      let src =
-        if Path.is_root dst then
-          Path.to_string src
-        else
-          Path.reach ~from:(Path.parent dst) src
-      in
-      let dst = Path.to_string dst in
-      match Unix.readlink dst with
-      | target ->
-        if target <> src then begin
-          Unix.unlink dst;
-          Unix.symlink src dst
-        end
-      | exception _ ->
-        Unix.symlink src dst)
+  path src >>>
+  shexp ~targets:[dst] (Symlink (src, dst))
 
-let touch target =
-  create_file ~target (fun _ ->
-    Unix.close
-      (Unix.openfile (Path.to_string target)
-         [O_CREAT; O_TRUNC; O_WRONLY] 0o666))
+let create_file fn =
+  shexp ~targets:[fn] (Create_file fn)
+
+let and_create_file fn =
+  arr (fun (action : Action.t) ->
+    { action with
+      action =
+        match action.action with
+        | Bash cmd ->
+          let fn = quote_for_shell (Path.to_string fn) in
+          Bash (sprintf "(%s); rm -f %s; touch %s" cmd fn fn)
+        | Shexp shexp ->
+          Shexp (Progn [shexp; Create_file fn])
+    })
diff --git a/src/build.mli b/src/build.mli
index c0f39f09..885d9c0b 100644
--- a/src/build.mli
+++ b/src/build.mli
@@ -8,14 +8,11 @@ val arr : ('a -> 'b) -> ('a, 'b) t
 
 val return : 'a -> (unit, 'a) t
 
-val create_file : target:Path.t -> ('a -> 'b) -> ('a, 'b) t
-val create_files : targets:Path.t list -> ('a -> 'b) -> ('a, 'b) t
-
 module Vspec : sig
   type 'a t = T : Path.t * 'a Vfile_kind.t -> 'a t
 end
 
-val store_vfile : 'a Vspec.t -> ('a, unit) t
+val store_vfile : 'a Vspec.t -> ('a, Action.t) t
 
 module O : sig
   val ( >>> ) : ('a, 'b) t -> ('b, 'c) t -> ('a, 'c) t
@@ -58,27 +55,37 @@ end
 val run
   :  ?dir:Path.t
   -> ?stdout_to:Path.t
-  -> ?env:string array
+  -> ?context:Context.t
   -> ?extra_targets:Path.t list
   -> 'a Prog_spec.t
   -> 'a Arg_spec.t list
-  -> ('a, unit) t
+  -> ('a, Action.t) t
 
 val action
-  :  string Action.Desc.t
-  -> dir:Path.t
-  -> env:string array
+  :  ?dir:Path.t
+  -> ?context:Context.t
   -> targets:Path.t list
-  -> (unit, unit) t
+  -> Action.Desc.t
+  -> (unit, Action.t) t
+
+val shexp
+  :  ?dir:Path.t
+  -> ?context:Context.t
+  -> targets:Path.t list
+  -> (string, Path.t) Action.Mini_shexp.t
+  -> (unit, Action.t) t
 
 (** Create a file with the given contents. *)
-val echo : Path.t -> (string, unit) t
+val echo : Path.t -> string -> (unit, Action.t) t
+val echo_dyn : Path.t -> (string, Action.t) t
 
-val copy : src:Path.t -> dst:Path.t -> (unit, unit) t
+val copy : src:Path.t -> dst:Path.t -> (unit, Action.t) t
 
-val symlink : src:Path.t -> dst:Path.t -> (unit, unit) t
+val symlink : src:Path.t -> dst:Path.t -> (unit, Action.t) t
 
-val touch : Path.t -> (unit, unit) t
+val create_file : Path.t -> (unit, Action.t) t
+
+val and_create_file : Path.t -> (Action.t, Action.t) t
 
 type lib_dep_kind =
   | Optional
@@ -96,14 +103,10 @@ type lib_deps = lib_dep_kind String_map.t
 
 
 module Repr : sig
-  type ('a, 'b) prim =
-    { targets : Path.t list
-    ; exec    : 'a -> 'b Future.t
-    }
   type ('a, 'b) t =
     | Arr : ('a -> 'b) -> ('a, 'b) t
-    | Prim : ('a, 'b) prim -> ('a, 'b) t
-    | Store_vfile : 'a Vspec.t -> ('a, unit) t
+    | Targets : Path.t list -> ('a, 'a) t
+    | Store_vfile : 'a Vspec.t -> ('a, Action.t) t
     | Compose : ('a, 'b) t * ('b, 'c) t -> ('a, 'c) t
     | First : ('a, 'b) t -> ('a * 'c, 'b * 'c) t
     | Second : ('a, 'b) t -> ('c * 'a, 'c * 'b) t
diff --git a/src/build_interpret.ml b/src/build_interpret.ml
index 5899f177..b016acaf 100644
--- a/src/build_interpret.ml
+++ b/src/build_interpret.ml
@@ -23,7 +23,7 @@ let deps t ~all_targets_by_dir =
   let rec loop : type a b. (a, b) t -> Pset.t -> Pset.t = fun t acc ->
     match t with
     | Arr _ -> acc
-    | Prim _ -> acc
+    | Targets _ -> acc
     | Store_vfile _ -> acc
     | Compose (a, b) -> loop a (loop b acc)
     | First t -> loop t acc
@@ -50,7 +50,7 @@ let lib_deps =
     = fun t acc ->
       match t with
       | Arr _ -> acc
-      | Prim _ -> acc
+      | Targets _ -> acc
       | Store_vfile _ -> acc
       | Compose (a, b) -> loop a (loop b acc)
       | First t -> loop t acc
@@ -76,7 +76,7 @@ let targets =
   let rec loop : type a b. (a, b) t -> Target.t list -> Target.t list = fun t acc ->
     match t with
     | Arr _ -> acc
-    | Prim { targets; _ } ->
+    | Targets targets ->
       List.fold_left targets ~init:acc ~f:(fun acc fn -> Target.Normal fn :: acc)
     | Store_vfile spec -> Vfile spec :: acc
     | Compose (a, b) -> loop a (loop b acc)
@@ -95,7 +95,7 @@ let targets =
 
 module Rule = struct
   type t =
-    { build   : (unit, unit) Build.t
+    { build   : (unit, Action.t) Build.t
     ; targets : Target.t list
     }
 
diff --git a/src/build_interpret.mli b/src/build_interpret.mli
index 60aead68..025454eb 100644
--- a/src/build_interpret.mli
+++ b/src/build_interpret.mli
@@ -11,11 +11,11 @@ end
 
 module Rule : sig
   type t =
-    { build   : (unit, unit) Build.t
+    { build   : (unit, Action.t) Build.t
     ; targets : Target.t list
     }
 
-  val make : (unit, unit) Build.t -> t
+  val make : (unit, Action.t) Build.t -> t
 end
 
 val deps
diff --git a/src/build_system.ml b/src/build_system.ml
index 8e723d7d..e3277491 100644
--- a/src/build_system.ml
+++ b/src/build_system.ml
@@ -22,9 +22,7 @@ module Rule = struct
   type t =
     { deps         : Pset.t
     ; targets      : Pset.t
-    ; (* Keep the arrow around so that we can do more query, such as for finding external
-         library dependencies *)
-      build        : (unit, unit) Build.t
+    ; build        : (unit, Action.t) Build.t
     ; mutable exec : Exec_status.t
     }
 end
@@ -154,19 +152,26 @@ let save_vfile (type a) (module K : Vfile_kind.S with type t = a) fn x =
 module Build_exec = struct
   open Build.Repr
 
+  let nop =
+    { Action.
+      context = None
+    ; dir     = Path.root
+    ; action  = Shexp (Progn [])
+    }
+
   let exec bs t x ~targeting =
     let rec exec
       : type a b. (a, b) t -> a -> b Future.t = fun t x ->
       let return = Future.return in
       match t with
       | Arr f -> return (f x)
-      | Prim { exec; _ } -> exec x
+      | Targets _ -> return x
       | Store_vfile (Vspec.T (fn, kind)) ->
         let file = get_file bs fn (Sexp_file kind) in
         assert (file.data = None);
         file.data <- Some x;
         save_vfile kind fn x;
-        Future.return ()
+        Future.return nop
       | Compose (a, b) ->
         exec a x >>= exec b
       | First t ->
@@ -248,6 +253,8 @@ let compile_rule t ~all_targets_by_dir ?(allow_override=false) pre_rule =
       (Pset.fold deps ~init:[] ~f:(fun fn acc -> wait_for_file t fn ~targeting :: acc))
     >>= fun () ->
     Build_exec.exec t build () ~targeting
+    >>= fun action ->
+    Action.exec action
   ) in
   let rule =
     { Rule.
diff --git a/src/context.mli b/src/context.mli
index 33578fe6..5b1003d1 100644
--- a/src/context.mli
+++ b/src/context.mli
@@ -129,3 +129,5 @@ val opam_config_var : t -> string -> string option Future.t
 val install_prefix : t -> Path.t Future.t
 
 val env_for_exec : t -> string array
+
+val initial_env : string array Lazy.t
diff --git a/src/gen_rules.ml b/src/gen_rules.ml
index 1001066c..b4ebf75b 100644
--- a/src/gen_rules.ml
+++ b/src/gen_rules.ml
@@ -241,15 +241,8 @@ module Gen(P : Params) = struct
       List.map (Lib_db.resolve_selects t ~from:dir lib_deps) ~f:(fun { dst_fn; src_fn } ->
         let src = Path.relative dir src_fn in
         let dst = Path.relative dir dst_fn in
-        Build.path src
-        >>>
-        Build.create_files ~targets:[dst] (fun () ->
-          let src_fn = Path.to_string src in
-          let dst_fn = Path.to_string dst in
-          with_file_in src_fn ~f:(fun ic ->
-            with_file_out dst_fn ~f:(fun oc ->
-              Printf.fprintf oc "# 1 \"%s\"\n" src_fn;
-              copy_channels ic oc))))
+        Build.shexp ~targets:[dst]
+          (Copy_and_add_line_directive (src, dst)))
 
     (* Hides [t] so that we don't resolve things statically *)
     let t = ()
@@ -286,22 +279,31 @@ module Gen(P : Params) = struct
 
     [@@@warning "-32"]
 
-    let run ?(dir=ctx.build_dir) ?stdout_to ?(env=ctx.env) ?extra_targets prog args =
-      Build.run ~dir ?stdout_to ~env ?extra_targets prog args
+    let run ?(dir=ctx.build_dir) ?stdout_to ?extra_targets prog args =
+      Build.run ~dir ?stdout_to ~context:ctx ?extra_targets prog args
 
-    let bash ?dir ?stdout_to ?env ?extra_targets cmd =
-      run (Dep (Path.absolute "/bin/bash")) ?dir ?stdout_to ?env ?extra_targets
+    let bash ?dir ?stdout_to ?extra_targets cmd =
+      run (Dep (Path.absolute "/bin/bash")) ?dir ?stdout_to ?extra_targets
         [ As ["-e"; "-u"; "-o"; "pipefail"; "-c"; cmd] ]
 
-    let system ?dir ?stdout_to ?env ?extra_targets cmd ~needed_to =
+    let system ?dir ?stdout_to ?extra_targets cmd ~needed_to =
       let path, arg, fail = Utils.system_shell ~needed_to in
       let build =
-        run (Dep path) ?dir ?stdout_to ?env ?extra_targets
+        run (Dep path) ?dir ?stdout_to ?extra_targets
           [ As [arg; cmd] ]
       in
       match fail with
       | None -> build
       | Some fail -> Build.fail fail >>> build
+
+    let action ?dir ~targets action =
+      Build.action ?dir ~context:ctx ~targets action
+
+    let shexp ?dir ~targets shexp =
+      Build.shexp ?dir ~context:ctx ~targets shexp
+
+    let shexp_context_independent ?dir ~targets shexp =
+      Build.shexp ?dir ~targets shexp
   end
 
   module Alias = struct
@@ -745,7 +747,7 @@ module Gen(P : Params) = struct
               |> List.map ~f:(Printf.sprintf "%s\n")
               |> String.concat ~sep:"")
           >>>
-          Build.echo path
+          Build.echo_dyn path
         )
       | _ ->
         ()
@@ -978,8 +980,7 @@ module Gen(P : Params) = struct
   let mk_lib_cm_all (lib : Library.t) ~dir ~modules cm_kind =
     let deps = cm_files ~dir (String_map.values modules) ~cm_kind in
     add_rule (Build.paths deps >>>
-            Build.return "" >>>
-            Build.echo (lib_cm_all lib ~dir cm_kind))
+              Build.create_file (lib_cm_all lib ~dir cm_kind))
 
   let expand_includes ~dir includes =
     Arg_spec.As (List.concat_map includes ~f:(fun s ->
@@ -1109,7 +1110,7 @@ module Gen(P : Params) = struct
             |> List.map ~f:(fun (m : Module.t) ->
               sprintf "module %s = %s\n" m.name (Module.real_unit_name m))
             |> String.concat ~sep:"")
-         >>> Build.echo (Path.relative dir m.ml_fname)));
+         >>> Build.echo_dyn (Path.relative dir m.ml_fname)));
 
     let requires, real_requires =
       requires ~dir ~dep_kind ~item:lib.name
@@ -1300,7 +1301,7 @@ module Gen(P : Params) = struct
       -> dep_kind:Build.lib_dep_kind
       -> targets:Path.t list
       -> deps:Dep_conf.t list
-      -> (unit, unit) Build.t
+      -> (unit, Action.t) Build.t
   end = struct
     module U = Action.Desc.Unexpanded
 
@@ -1338,7 +1339,7 @@ module Gen(P : Params) = struct
         ; lib_deps  = String_set.empty
         }
       in
-      U.fold t ~init ~f:(fun acc var ->
+      U.fold_vars t ~init ~f:(fun acc var ->
         let module A = Artifacts in
         match String.lsplit2 var ~on:':' with
         | Some ("exe"     , s) -> add_artifact acc ~var (Ok (Path.relative dir s))
@@ -1354,23 +1355,27 @@ module Gen(P : Params) = struct
           add_artifact acc ~var ~lib_dep res
         | _ -> acc)
 
-    let expand_string_with_vars =
-      let dep_exn ~dir name = function
-        | Some dep -> Path.reach ~from:dir dep
+    let expand_var =
+      let dep_exn name = function
+        | Some dep -> dep
         | None -> die "cannot use ${%s} with files_recursively_in" name
       in
-      fun  ~artifacts ~targets ~deps dir var_name ->
+      fun ~artifacts ~targets ~deps var_name ->
         match String_map.find var_name artifacts with
-        | Some path -> Some (Path.reach ~from:dir path)
+        | Some path -> Action.Path path
         | None ->
           match var_name with
-          | "@" -> Some (String.concat ~sep:" "
-                           (List.map targets ~f:(Path.reach ~from:dir)))
-          | "<" -> Some (match deps with [] -> "" | dep1::_ -> dep_exn ~dir var_name dep1)
+          | "@" -> Paths targets
+          | "<" -> (match deps with
+            | []        -> Str ""
+            | dep1 :: _ -> Path (dep_exn var_name dep1))
           | "^" ->
-            let deps = List.map deps ~f:(dep_exn ~dir var_name) in
-            Some (String.concat ~sep:" " deps)
-          | _ -> root_var_lookup ~dir var_name
+            Paths (List.map deps ~f:(dep_exn var_name))
+          | "ROOT" -> Path Path.root
+          | _ ->
+            match String_map.find var_name dollar_var_map with
+            | Some s -> Str s
+            | _ -> Not_found
 
     let run t ~dir ~dep_kind ~targets ~deps =
       let deps =
@@ -1381,7 +1386,7 @@ module Gen(P : Params) = struct
       let forms = extract_artifacts ~dir t in
       let t =
         U.expand dir t
-          ~f:(expand_string_with_vars ~artifacts:forms.artifacts ~targets ~deps)
+          ~f:(expand_var ~artifacts:forms.artifacts ~targets ~deps)
       in
       let build =
         Build.record_lib_deps ~dir ~kind:dep_kind
@@ -1390,7 +1395,7 @@ module Gen(P : Params) = struct
         >>>
         Build.paths (String_map.values forms.artifacts)
         >>>
-        Build.action t ~dir ~env:ctx.env ~targets
+        Build.action t ~dir ~targets
       in
       match forms.failures with
       | [] -> build
@@ -1427,22 +1432,24 @@ module Gen(P : Params) = struct
       |> Digest.to_hex in
     let alias = Alias.make alias_conf.name ~dir in
     let digest_path = Path.extend_basename (Alias.file alias) ~suffix:("-" ^ digest) in
-    let dummy = Build.touch digest_path in
     Alias.add_deps alias [digest_path];
-    let deps =
-      let deps = Dep_conf_interpret.dep_of_list ~dir alias_conf.deps in
-      match alias_conf.action with
-      | None -> deps
-      | Some action ->
-        deps
-        >>> Action_interpret.run
-          action
-          ~dir
-          ~dep_kind:Required
-          ~targets:[]
-          ~deps:alias_conf.deps
-    in
-    add_rule (deps >>> dummy)
+    let deps = Dep_conf_interpret.dep_of_list ~dir alias_conf.deps in
+    add_rule
+      (match alias_conf.action with
+       | None ->
+         deps
+         >>>
+         Build.create_file digest_path
+       | Some action ->
+         deps
+         >>> Action_interpret.run
+               action
+               ~dir
+               ~dep_kind:Required
+               ~targets:[]
+               ~deps:alias_conf.deps
+         >>>
+         Build.and_create_file digest_path)
 
   (* +-----------------------------------------------------------------+
      | Modules listing                                                 |
@@ -1517,7 +1524,7 @@ module Gen(P : Params) = struct
     List.iter stanzas ~f:(fun stanza ->
       let dir = ctx_dir in
       match (stanza : Stanza.t) with
-      | Rule         rule -> user_rule         rule ~dir
+      | Rule         rule  -> user_rule   rule  ~dir
       | Alias        alias -> alias_rules alias ~dir
       | Library _ | Executables _ | Provides _ | Install _ -> ());
     let files = lazy (
@@ -1645,22 +1652,24 @@ module Gen(P : Params) = struct
       in
       add_rule
         (Build.fanout meta template
+         >>^ (fun ((meta : Meta.t), template) ->
+           let buf = Buffer.create 1024 in
+           let ppf = Format.formatter_of_buffer buf in
+           Format.pp_open_vbox ppf 0;
+           List.iter template ~f:(fun s ->
+             if String.is_prefix s ~prefix:"#" then
+               match
+                 String.split_words (String.sub s ~pos:1 ~len:(String.length s - 1))
+               with
+               | ["JBUILDER_GEN"] -> Format.fprintf ppf "%a@," Meta.pp meta.entries
+               | _ -> Format.fprintf ppf "%s@," s
+             else
+               Format.fprintf ppf "%s@," s);
+           Format.pp_close_box ppf ();
+           Format.pp_print_flush ppf ();
+           Buffer.contents buf)
          >>>
-         Build.create_file ~target:meta_path (fun ((meta : Meta.t), template) ->
-           with_file_out (Path.to_string meta_path) ~f:(fun oc ->
-             let ppf = Format.formatter_of_out_channel oc in
-             Format.pp_open_vbox ppf 0;
-             List.iter template ~f:(fun s ->
-               if String.is_prefix s ~prefix:"#" then
-                 match
-                   String.split_words (String.sub s ~pos:1 ~len:(String.length s - 1))
-                 with
-                 | ["JBUILDER_GEN"] -> Format.fprintf ppf "%a@," Meta.pp meta.entries
-                 | _ -> Format.fprintf ppf "%s@," s
-               else
-                 Format.fprintf ppf "%s@," s);
-             Format.pp_close_box ppf ();
-             Format.pp_print_flush ppf ())));
+         Build.echo_dyn meta_path);
 
       if has_meta || has_meta_tmpl then
         Some pkg.name
@@ -1781,9 +1790,11 @@ module Gen(P : Params) = struct
     in
     let entries = local_install_rules entries ~package in
     add_rule
-      (Build.path_set (Install.files entries) >>>
-       Build.create_file ~target:fn (fun () ->
-         Install.write_install_file fn entries))
+      (Build.path_set (Install.files entries)
+       >>^ (fun () ->
+         Install.gen_install_file entries)
+       >>>
+       Build.echo_dyn fn)
 
   let () = String_map.iter P.packages ~f:(fun ~key:_ ~data:pkg ->
     install_file pkg.Package.path pkg.name)
diff --git a/src/install.ml b/src/install.ml
index 727a1ceb..f90c0585 100644
--- a/src/install.ml
+++ b/src/install.ml
@@ -119,14 +119,15 @@ let group entries =
   |> SMap.of_alist_multi
   |> SMap.bindings
 
-let write_install_file file entries =
-  with_file_out (Path.to_string file) ~f:(fun oc ->
-    let pr fmt = Printf.fprintf oc (fmt ^^ "\n") in
-    List.iter (group entries) ~f:(fun (section, entries) ->
-      pr "%s: [" (Section.to_string section);
+let gen_install_file entries =
+  let buf = Buffer.create 4096 in
+  let pr fmt = Printf.bprintf buf (fmt ^^ "\n") in
+  List.iter (group entries) ~f:(fun (section, entries) ->
+    pr "%s: [" (Section.to_string section);
       List.iter entries ~f:(fun (e : Entry.t) ->
         let src = Path.to_string e.src in
         match e.dst with
         | None     -> pr "  %S"      src
         | Some dst -> pr "  %S {%S}" src dst);
-      pr "]"))
+    pr "]");
+  Buffer.contents buf
diff --git a/src/install.mli b/src/install.mli
index aa5272eb..84102daa 100644
--- a/src/install.mli
+++ b/src/install.mli
@@ -31,4 +31,4 @@ module Entry : sig
 end
 
 val files : Entry.t list -> Path.Set.t
-val write_install_file : Path.t -> Entry.t list -> unit
+val gen_install_file : Entry.t list -> string
diff --git a/src/path.ml b/src/path.ml
index 511be60c..69a78e06 100644
--- a/src/path.ml
+++ b/src/path.ml
@@ -104,6 +104,51 @@ module Local = struct
     in
     loop initial_t (explode_path path)
 
+  let is_canonicalized =
+    let rec before_slash s i =
+      if i < 0 then
+        false
+      else
+        match s.[i] with
+        | '/' -> false
+        | '.' -> before_dot_slash s (i - 1)
+        | _   -> in_component     s (i - 1)
+    and before_dot_slash s i =
+      if i < 0 then
+        false
+      else
+        match s.[i] with
+        | '/' -> false
+        | '.' -> before_dot_dot_slash s (i - 1)
+        | _   -> in_component         s (i - 1)
+    and before_dot_dot_slash s i =
+      if i < 0 then
+        false
+      else
+        match s.[i] with
+        | '/' -> false
+        | _   -> in_component s (i - 1)
+    and in_component s i =
+      if i < 0 then
+        true
+      else
+        match s.[i] with
+        | '/' -> before_slash s (i - 1)
+        | _   -> in_component s (i - 1)
+    in
+    fun s ->
+      let len = String.length s in
+      if len = 0 then
+        true
+      else
+        before_slash s (len - 1)
+
+  let of_string s =
+    if is_canonicalized s then
+      s
+    else
+      relative "" s
+
   let rec mkdir_p = function
     | "" -> ()
     | t ->
@@ -176,8 +221,6 @@ let to_string = function
   | "" -> "."
   | t  -> t
 
-let sexp_of_t t = Sexp.Atom (to_string t)
-
 let root = ""
 
 let relative t fn =
@@ -189,7 +232,16 @@ let relative t fn =
     | _   , false -> fn
     | false, true -> External.relative t fn
 
-let of_string t =  relative "" t
+let of_string = function
+  | "" -> ""
+  | s  ->
+    if Filename.is_relative s then
+      Local.of_string s
+    else
+      s
+
+let t sexp = of_string (Sexp.Of_sexp.string sexp)
+let sexp_of_t t = Sexp.Atom (to_string t)
 
 let absolute =
   let initial_dir = Sys.getcwd () in
@@ -289,6 +341,3 @@ let insert_after_build_dir_exn =
         sprintf "_build/%s/%s" b rest
     | _ ->
       error a b
-
-
-
diff --git a/src/path.mli b/src/path.mli
index b5610296..d92043d6 100644
--- a/src/path.mli
+++ b/src/path.mli
@@ -26,6 +26,9 @@ end
 
 type t
 
+val t : t Sexp.Of_sexp.t
+val sexp_of_t : t Sexp.To_sexp.t
+
 val compare : t -> t -> int
 
 module Set : Set.S with type elt = t
diff --git a/src/string_with_vars.ml b/src/string_with_vars.ml
index 83b12924..710183e2 100644
--- a/src/string_with_vars.ml
+++ b/src/string_with_vars.ml
@@ -57,6 +57,10 @@ let of_string s = of_tokens (Token.tokenise s)
 
 let t sexp = of_string (Sexp.Of_sexp.string sexp)
 
+let just_a_var = function
+  | [Var (_, s)] -> Some s
+  | _ -> None
+
 let sexp_of_var_syntax = function
   | Parens -> Sexp.Atom "parens"
   | Braces -> Sexp.Atom "braces"
@@ -88,7 +92,7 @@ let expand t ~f =
         | Parens -> sprintf "$(%s)" v
         | Braces -> sprintf "${%s}" v)
   |> String.concat ~sep:""
-
+(*
 let expand_with_context context t ~f =
   List.map t ~f:(function
     | Text s -> s
@@ -100,27 +104,4 @@ let expand_with_context context t ~f =
         | Parens -> sprintf "$(%s)" v
         | Braces -> sprintf "${%s}" v)
   |> String.concat ~sep:""
-
-module type Container = sig
-  type 'a t
-  val t : 'a Sexp.Of_sexp.t -> 'a t Sexp.Of_sexp.t
-  val sexp_of_t : ('a -> Sexp.t) -> 'a t -> Sexp.t
-
-  type context
-  val expand : context -> 'a t -> f:(context -> 'a -> string) -> string t
-  val fold : 'a t -> init:'b -> f:('b -> 'a -> 'b) -> 'b
-end
-
-module Lift(M : Container) = struct
-  type nonrec t = t M.t
-  let t sexp = M.t t sexp
-
-  let sexp_of_t a = M.sexp_of_t sexp_of_t a
-
-  let fold t ~init ~f =
-    M.fold t ~init ~f:(fun acc x -> fold x ~init:acc ~f)
-
-  let expand context (t : t) ~f =
-    M.expand context t ~f:(expand_with_context ~f)
-end
-
+*)
diff --git a/src/string_with_vars.mli b/src/string_with_vars.mli
index 54536141..230a66c3 100644
--- a/src/string_with_vars.mli
+++ b/src/string_with_vars.mli
@@ -11,33 +11,10 @@ val sexp_of_t : t -> Sexp.t
 
 val of_string : string -> t
 
+val just_a_var : t -> string option
+
 val vars : t -> String_set.t
 
 val fold : t -> init:'a -> f:('a -> string -> 'a) -> 'a
 
 val expand : t -> f:(string -> string option) -> string
-
-module type Container = sig
-  type 'a t
-  val t : 'a Sexp.Of_sexp.t -> 'a t Sexp.Of_sexp.t
-  val sexp_of_t : ('a -> Sexp.t) -> 'a t -> Sexp.t
-
-  type context
-  val expand : context -> 'a t -> f:(context -> 'a -> string) -> string t
-  val fold : 'a t -> init:'b -> f:('b -> 'a -> 'b) -> 'b
-end
-
-module Lift(M : Container) : sig
-  type nonrec t = t M.t
-  val t : t Sexp.Of_sexp.t
-
-  val sexp_of_t : t -> Sexp.t
-
-  val fold : t -> init:'a -> f:('a -> string -> 'a) -> 'a
-
-  val expand
-    :  M.context
-    -> t
-    -> f:(M.context -> string -> string option)
-    -> string M.t
-end