diff --git a/functions/backup_function.fish b/functions/backup_function.fish
deleted file mode 100644
index 57bd948..0000000
--- a/functions/backup_function.fish
+++ /dev/null
@@ -1,33 +0,0 @@
-function backup_function
-    set -l funcname $argv[1]
-
-    if functions -q -- $funcname
-        _create_tmpname
-        functions -- $funcname | sed -e s/^function\ $funcname/function\ old_$funcname/ > $tmpname
-        _source_old_function
-    else
-        _print_function_not_found $argv
-        remove_backup_function $argv
-        return 1
-    end
-end
-
-function _create_tmpname
-    set -l tmpdir /tmp
-
-    set -g tmpname (printf "$tmpdir/fish_funced_%d_%d.fish" %self (random))
-    while test -f $tmpname
-        set -g tmpname (printf "$tmpdir/fish_funced_%d_%d.fish" %self (random))
-    end
-end
-
-function _print_function_not_found
-    set_color red
-    _ "Function '$argv' not found.
-"
-    set_color normal
-end
-
-function _source_old_function
-    . $tmpname
-end
diff --git a/functions/redefine_function.fish b/functions/redefine_function.fish
new file mode 100644
index 0000000..50d3bfa
--- /dev/null
+++ b/functions/redefine_function.fish
@@ -0,0 +1,41 @@
+function redefine_function -d "Redefine a function and backup the old using the prefix 'old_'"
+  if not set -q argv[1]; _print_error 'Missing function name argument.'; return 1; end
+  if not set -q argv[2]; _print_error 'Missing original function file argument.'; return 1; end
+  if not set -q argv[3]; _print_error 'Missing function code argument.'; return 1; end
+
+  _backup_old_function $argv
+  _load_new_function $argv
+end
+
+function _backup_old_function
+  set -l funcname $argv[1]
+  set -l funcfile $argv[2]
+
+  if test -f $funcfile
+    . $funcfile
+    functions -c $funcname old_$funcname
+  else
+    _print_error "File '$funcfile' not found"
+    return 1
+  end
+end
+
+function _load_new_function
+  set -l func $argv[3]
+
+  set -q TMPDIR; or set -l TMPDIR /tmp
+  set -l tmpname (printf "$TMPDIR/fish_funced_%d_%d.fish" %self (random))
+  while test -f $tmpname
+    set tmpname (printf "$TMPDIR/fish_funced_%d_%d.fish" %self (random))
+  end
+
+  echo $func > $tmpname
+  . $tmpname
+  rm $tmpname >/dev/null
+end
+
+function _print_error
+  set_color red
+  echo $argv
+  set_color normal
+end