vi question
 

if I have 2 files, how do I copy a section of one file to the other?

all I know is "yy" for copy and "p" to paste, but that can only be done in same file.

Also another question about replacement.

How do I replace all string in a file by the other.

right now I use :s1,lastline/oldstring/newstring/g

but the problem is I need to always know the number of line in the file before I can do replacement. Any faster way?

1. If you use vim, you can have multiple files opened in the same time and copying and pasting operations work fine between them.
2. :%s/oldstring/newstring/g
% means all lines in the file

If opening two (or more) files is not an option, you can do the following.

You have opened the 'master' file and are in command mode:

r!sed -n '2,4p' /etc/passwd

This will put lines 2,3 and 4 from the passwd file into the file you are editing, below the cursor position.

A great program that comes with vim is vimtutor,try it from the command line.