please give me answers using Linux commands. thanks

1. How do you find lines in a file that do not contain the string abc.
2. How would you change the contents of a file
3. How would you save an edited file in vi to another file?
4. what patterns does

$1 < 20

match in awk?

And

What does the following awk line do?

$1 == "ATOM" {na += 1; print }