intro
- program use IO to interact with outside world.
standard io
- ruby defines standard io constant: STDIN, STDOUT, STDERR
- they are same as global variables: $stdin, $stdout, $stderr
- which pointing to program's input, output and error streams.
- you can play with it through your terminal simply.
- method 'gets' capture STDIN string.
- method 'puts' to sent STDOUT string.
- method 'warn' to direct to STDERR string.
