.xx "a.out" "assembler and link editor output" .xx "ar" "archive (library) file format" .xx "core" "format of core image file" .xx "dir" "format of directories" .xx "fs" "format of file system volume" .xx "passwd" "password file" .xx "tp" "DEC/mag tape formats" .xx "utmp" "user information" .xx "wtmp" "user login history"