external head_n : t -> int -> string -> string array = "ocaml_guestfs_head_n"
external tail : t -> string -> string array = "ocaml_guestfs_tail"
external tail_n : t -> int -> string -> string array = "ocaml_guestfs_tail_n"
+external df : t -> string = "ocaml_guestfs_df"
+external df_h : t -> string = "ocaml_guestfs_df_h"