diff --git a/modules/lib/fs.lua b/modules/lib/fs.lua
index 0e6f100..9da5ca9 100644
--- a/modules/lib/fs.lua
+++ b/modules/lib/fs.lua
@@ -77,6 +77,7 @@ do
   return false
  end
  function fs.list(s)
+  s=s or ""
   local _,d,p = fs.resolve(s)
   if not d then
    local kt = {}
@@ -104,3 +105,14 @@ do
   return fT[d].isDirectory(p)
  end
 end
+function fs.cd(p)
+ if p:sub(1,1) ~= "/" then
+  p=(os.getenv("PWD") or "").."/"..p
+ end
+ p=fs.simplify(p)
+ if fs.exists(p) and fs.isdir(p) then
+  os.setenv("PWD",p)
+ else
+  error("non-existent/not a dir")
+ end
+end
diff --git a/modules/lib/shutil.lua b/modules/lib/shutil.lua
index a960940..38e5207 100644
--- a/modules/lib/shutil.lua
+++ b/modules/lib/shutil.lua
@@ -1,12 +1,6 @@
-function cd(p)
- os.setenv("PWD",fs.simplify((os.getenv("PWD") or "").."/"..p))
-end
+cd=fs.cd
+rm=fs.rm
+mkdir=fs.mkdir
 function ls(p)
  for k,v in ipairs(fs.list(p)) do print(v) end
 end
-function rm(p)
- fs.rm(p)
-end
-function mkdir(p)
- fs.mkdir(p)
-end