          cd ?dirName?
               Change the current working directory to dirName, or  to
               the   home   directory   (as   specified  in  the  HOME
               environment variable) if  dirName  is  not  given.   If
               dirName  starts  with  a tilde, then tilde-expansion is
               done as described for Tcl_TildeSubst.  Returns an empty
               string.   This command can potentially be disruptive to
               an  application,  so  it  may  be   removed   in   some
               applications.
