Also, you used single quotes, '' not backticks, `` Things in single quotes don't execute.
Also, if this "$by_ln" contains multiple lines, as seems likely, that's a very dangerous way to program because you're likely to hit the maximum size of a shell variable -- which is very small on many systems.
Also, you don't need to run cut so many times to get one field.
If you show me what your program's trying to do, I'll find a better way.