vi -- handy tips

gistfile1.txt

Copy lines 1-2 /paste after 3 
:1,2t3

Move lines 4-5 /paste after 6 
:4,5m6

Save lines to new file
:10,15w file

Delete blanke lines
:g/^$/d

Making a list of numbers
:put =range(1,20)

Add line numbers to source code in vim
:%!nl

Start numbering with n
:%nl -v n

Replace all characters up to first comma
:%s/^[^,]*,//

Show Hidden Characters
:set list