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

Note the "2>&1" sequence, please keep them there. This command will redirect stderr to stdout.

 

2. Output stdout and stderr to a file simultaneously

Think about this: you are compiling a huge project which uses Makefile, the compilation failed and a lot of error message shown on the scree. It could be easier for you to analyze these error messages in your favorite editor.

Here is the solution. Either of below ways will achieve this. And of course the show-up sequence is exactly the same as on the screen.

$ find /var -name run &>file

or

$ find /var -name run >file 2>&1

Please note:(1) The sequence of "&" and ">" on two ways is different, don't mix them up!
(2) For the second one, you must assign stdout first, then stderr.

3. Ignore stdout or stderr

Sometimes you don't want to see some message just because they are trifling or you want to keep a clean screen. You can put everything into the bottomless pit: /dev/null:

This example ignores stderr:

$ find /var -name run 2>/dev/null

This example ignores both stdout and stderr:

$ secret-app 1>/dev/null 2>/dev/null