Question on frame title

[Benny Sum]

I try to run shell-command to retrieve some information on the file and put it 
in the title frame.  But I got some strange behavior:

(setq frame-title-format
 (list (format "%s" (shell-command-to-string(concat "ls -1 " " %b ")))))

But it says ls: cannot access filename: No such file or directory on the title 
frame.  

It seems the filename is correct, but not sure why cannot access it.  Is it due 
to the white space returned from the %b?  

Anyone has a example code to run a shell-command on %b in frame-title-format?