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

systemd-cat

SYSTEMD-CAT(1)							    systemd-cat 						    SYSTEMD-CAT(1)

NAME

       systemd-cat - Connect a pipeline or program's output with the journal

SYNOPSIS

       systemd-cat [OPTIONS...] [COMMAND] [ARGUMENTS...]

       systemd-cat [OPTIONS...]

DESCRIPTION

       systemd-cat may be used to connect STDOUT and STDERR of a process with the journal, or as a filter tool in a shell pipeline to pass the
       output the previous pipeline element generates to the journal.

       If no parameter is passed systemd-command will write everything it reads from standard input (STDIN) to the journal.

       If parameters are passed they are executed as command line with standard output (STDOUT) and standard error output (STDERR) connected to
       the journal, so that all it writes is stored in the journal.

OPTIONS

       The following options are understood:

       --h, --help
	   Prints a short help text and exits.

       --version
	   Prints a short version string and exits.

       -t, --identifier=
	   Specify a short string that is used to identify the logging tool. If not specified no identifying string is written to the journal.

       -p, --priority=
	   Specify the default priority level for the logged messages. Pass one of emerg, alert, crit, err, warning, notice, info, debug, resp. a
	   value between 0 and 7 (corresponding to the same named levels). These priority values are the same as defined by syslog(3). Defaults to
	   info. Note that this simply controls the default, individual lines may be logged with different levels if they are prefixed
	   accordingly. For details see --level-prefix= below.

       --level-prefix=
	   Controls whether lines read are parsed for syslog priority level prefixes. If enabled (the default) a line prefixed with a priority
	   prefix such as <5> is logged at priority 5 (notice), and similar for the other priority levels. Takes a boolean argument.

EXIT STATUS

       On success 0 is returned, a non-zero failure code otherwise.

EXAMPLES

       Example 1. Invoke a program

       This calls /bin/ls with STDOUT/STDERR connected to the journal:

	   # systemd-cat ls

       Example 2. Usage in a shell pipeline

       This builds a shell pipeline also invoking /bin/ls and writes the output it generates to the journal:

	   # ls | systemd-cat

       Even though the two examples have very similar effects the first is preferable since only one process is running at a time, and both STDOUT
       and STDERR are captured while in the second example only STDOUT is captured.

SEE ALSO

       systemd(1), systemctl(1), logger(1)

AUTHOR

       Lennart Poettering <lennart@poettering.net>
	   Developer

systemd 							    10/07/2013							    SYSTEMD-CAT(1)

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