http://lua-users.org/wiki/IoLibraryTutorial

Reading input from the terminal

The following code will create a prompt at the terminal allowing the user to type in anything they please and have it stored in the variable called name:

name = io.read()

Reading from a file

Line by line

lines = io.lines('file.txt')
for line in lines do print(line) end

All at once

file = io.open('file.txt')
contents = file:read("a") -- a means all, other options are n, l, and L.
file:close()

print(contents)