This question is an exact duplicate of:
I have a file with many lines, and I want to trim each line to be 80 characters in length. How could I do this?
I have already filtered out lines shorter than 80 characters, so now I'm left with a file that has lines 80+ characters in length and I want to trim each line so that all are exactly 80. In other words I want to preserve the first 80 characters in each line and remove the rest of the line.