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Top Forums Shell Programming and Scripting Meaning of "> /dev/null 2>&1" Post 302100938 by tayyabq8 on Tuesday 26th of December 2006 01:50:45 AM
Old 12-26-2006
Code:
> /dev/null 2>&1

You need to understand the theory first and then its upto you how and where you want to apply that theory. I'll try to explain above to you.

The greater-than (>) in commands like these redirect the program's output somewhere. In this case, something is being redirected into /dev/null, and something is being redirected into &1.

Standard in, out and error:

There are three standard sources of input and output for a program. Standard input usually comes from the keyboard if it's an interactive program, or from another program if it's processing the other program's output. The program usually prints to standard output, and sometimes prints to standard error. These three file descriptors (you can think of them as “data pipes”) are often called STDIN, STDOUT, and STDERR.

Sometimes they're not named, they're numbered! The built-in numberings for them are 0, 1, and 2, in that order. By default, if you don't name or number one explicitly, you're talking about STDOUT.

That means file descriptor 0 or fd0 denotes STDIN or standard input and file descriptor 1 or fd1 denotes STDOUT or standard output and file descriptor 2 or fd2 denotes STDERR or standard error.

You can see the command above is redirecting standard output into /dev/null, which is a place you can dump anything you don't want (often called the bit-bucket), then redirecting standard error into standard output (you have to put an & in front of the destination when you do this).

The short explanation, therefore, is “all output from this command should be shoved into a black hole.” That's one good way to make a program be really quiet!

Regards,
Tayyab
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DAEMON(3)						   BSD Library Functions Manual 						 DAEMON(3)

NAME
daemon -- run in the background LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <stdlib.h> int daemon(int nochdir, int noclose); DESCRIPTION
The daemon() function is for programs wishing to detach themselves from the controlling terminal and run in the background as system daemons. Unless the argument nochdir is non-zero, daemon() changes the current working directory to the root (/). Unless the argument noclose is non-zero, daemon() will redirect standard input, standard output and standard error to /dev/null. RETURN VALUES
On return 0 indicates success with -1 indicating error. ERRORS
The function daemon() may fail and set errno for any of the errors specified for the library functions fork(2) and setsid(2). SEE ALSO
fork(2), setsid(2) HISTORY
The daemon() function first appeared in 4.4BSD. CAVEATS
Unless the noclose argument is non-zero, daemon() will close the first three file descriptors and redirect them to /dev/null. Normally, these correspond to standard input, standard output and standard error. However, if any of those file descriptors refer to something else they will still be closed, resulting in incorrect behavior of the calling program. This can happen if any of standard input, standard output or standard error have been closed before the program was run. Programs using daemon() should therefore make sure to either call daemon() before opening any files or sockets or, alternately, verifying that any file descriptors obtained have a value greater than 2. BUGS
daemon() uses fork() as part of its tty detachment mechanism. Consequently the process id changes when daemon() is invoked. Processes employing daemon() can not be reliably waited upon until daemon() has been invoked. BSD
September 3, 1999 BSD
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