bash: while: command not found? ??

[Linda Walsh]

At the # prompt, if I type:

   # <filename cat

it takes filename and pipes it as input into cat.

then, why, when I type:

    # <filename while read ln; do echo $ln; done

do I get...

   bash: syntax error near unexpected token `do'

So I try putting do on a separate line:

   # <filename while read ln
   bash: while: command not found.

hmm...I thought 'while was a builtin?  why can't bash find it?