Linux shell IO redirect

IO redirect is fundamental in Linux shell -- one basic thing you should  pay your attention when learning/using Linux.

3 Basic File descriptors:

File Descriptor	Name	Description
0	stdin	standard input
1	stdout	standard output
2	stderr	standard error

Below I'll introduce 3 advanced usage, well they are also very common scenarios.

1. Redirect stderr to stdout

$ ls non-exist 2>&1