[sadfnjksdf]

grep is even shorter:

    grep $

That reads lines from stdin and echoes to stdout.

[dvdkhlng]

Grep expressions are not a turing-complete language, so I guess that won't count. You could claim that you solved the task using a shell script (that invoked the grep "function") but if you programmed in shell, the shortest solution AFAICS is

  #!/bin/sh
  cat