What does "> /dev/null 2>&1" means in a shell program ?

What does "> /dev/null 2>&1" means in a shell program ? 

REDIRECTION : 

"A > B" means, It truncates the output of A to B

"A >> B" means, It Appends the output of A to B

The above two redirections methods can be usually used to redirect to a file, but it can be a device as well. 

SYNTAX : command_A > /dev/null 2>&1

DESCRIPTION : The greater than symbol(>) in commands like these, writes the output somewhere. In this case, command_A is being redirected into /dev/null, and "2" is being redirected into &1.

STANDARD IN, OUT and ERR :

STANDARD INPUTS(STDIN/"0"/fd0) :  (i) usually gets input from (KEYBOARD), if it is an interactive program, (ii) or output of another program can be redirected as input

STANDARD OUTPUTS(STDOUT/"1"/fd1) : Input programs usually prints to standard output(SCREEN) if it doesn't get any compilation error

STANDAR ERROR(STDERR/"2"/fd2) : Used to write diagnostic or error messages(SCREEN)

STDIN = 0 or fd0

STDOUT = 1 or fd1

STDERR = 2 or fd2

where fd=file descriptor

P.S : By default, if you don’t name or number one explicitly, It will be considered as 1

/dev/null : The null device in any Operating System is typically used for disposing of unwanted output streams of a process. The /dev/null device is a special file, not a directory, so one cannot move a whole directory into it with the Unix mv command. Data written to this file gets discarded. It is similar to the file call NUL on Windows machines.

You can see the command_A above is redirecting standard output of command into /dev/null, then redirecting standard error(2) into standard output. & in front of the destination is must. The & informs the shell to put the command in the background. '&' indirectly indicates that what follows is a file descriptor and not a filename. So the construct becomes: 2>&1.

In a single statement : "All output from this command_A should gets discarded except the error messages"