Meaning of "> /dev/null 2>&1" Post: 302100938

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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

12-26-2006

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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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LEARN ABOUT DEBIAN

netkit-rsh

RSH(1)							    BSD General Commands Manual 						    RSH(1)

NAME

     rsh -- remote shell

SYNOPSIS

     rsh [-Kdnx] [-k realm] [-l username] host [command]

DESCRIPTION

     Rsh executes command on host.

     Rsh copies its standard input to the remote command, the standard output of the remote command to its standard output, and the standard error
     of the remote command to its standard error.  Interrupt, quit and terminate signals are propagated to the remote command; rsh normally termi-
     nates when the remote command does.  The options are as follows:

     -K    The -K option turns off all Kerberos authentication.

     -d    The -d option turns on socket debugging (using setsockopt(2)) on the TCP sockets used for communication with the remote host.

     -l    By default, the remote username is the same as the local username.  The -l option allows the remote name to be specified.  Kerberos
	   authentication is used, and authorization is determined as in rlogin(1).

     -n    The -n option redirects input from the special device /dev/null (see the BUGS section of this manual page).

     If no command is specified, you will be logged in on the remote host using rlogin(1).

     Shell metacharacters which are not quoted are interpreted on local machine, while quoted metacharacters are interpreted on the remote
     machine.  For example, the command

	   rsh otherhost cat remotefile >> localfile

     appends the remote file remotefile to the local file localfile, while

	   rsh otherhost cat remotefile ">>" other_remotefile

     appends remotefile to other_remotefile.

FILES

     /etc/hosts

SEE ALSO

     rlogin(1), kerberos(3), krb_sendauth(3), krb_realmofhost(3)

HISTORY

     The rsh command appeared in 4.2BSD.

BUGS

     If you are using csh(1) and put a rsh in the background without redirecting its input away from the terminal, it will block even if no reads
     are posted by the remote command.	If no input is desired you should redirect the input of rsh to /dev/null using the -n option.

     You cannot run an interactive command (like rogue(6) or vi(1)) using rsh; use rlogin(1) instead.

     Stop signals stop the local rsh process only; this is arguably wrong, but currently hard to fix for reasons too complicated to explain here.

Linux NetKit (0.17)						  August 15, 1999					       Linux NetKit (0.17)

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