In the shell, what does " 2>&1 " mean?

File descriptor 1 is the standard output (stdout).
File descriptor 2 is the standard error (stderr).

Here is one way to remember this construct (although it is not entirely accurate): at first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1". & indicates that what follows and precedes is a file descriptor and not a filename. So the construct becomes: 2>&1.

Consider >& as redirect merger operator.

echo test > afile.txt

redirects stdout to afile.txt. This is the same as doing

echo test 1> afile.txt

To redirect stderr, you do:

echo test 2> afile.txt

>& is the syntax to redirect a stream to another file descriptor - 0 is stdin, 1 is stdout, and 2 is stderr.

You can redirect stdout to stderr by doing:

echo test 1>&2 # or echo test >&2

Or vice versa:

echo test 2>&1

So, in short... 2> redirects stderr to an (unspecified) file, appending &1 redirects stderr to stdout.

Some tricks about redirection

Some syntax particularity about this may have important behaviours. There is some little samples about redirections, STDERR, STDOUT, and arguments ordering.

1 - Overwriting or appending?

Symbol > means redirection.

• > means send to as a whole completed file, overwriting target if exist (see noclobber bash feature at #3 later).
• >> means send in addition to would append to target if exist.

In any case, the file would be created if they not exist.

2 - The shell command line is order dependent!!

For testing this, we need a simple command which will send something on both outputs:

$ ls -ld /tmp /tntls: cannot access /tnt: No such file or directorydrwxrwxrwt 118 root root 196608 Jan  7 11:49 /tmp$ ls -ld /tmp /tnt >/dev/nullls: cannot access /tnt: No such file or directory$ ls -ld /tmp /tnt 2>/dev/nulldrwxrwxrwt 118 root root 196608 Jan  7 11:49 /tmp

(Expecting you don't have a directory named /tnt, of course ;). Well, we have it!!

So, let's see:

$ ls -ld /tmp /tnt >/dev/nullls: cannot access /tnt: No such file or directory$ ls -ld /tmp /tnt >/dev/null 2>&1$ ls -ld /tmp /tnt 2>&1 >/dev/nullls: cannot access /tnt: No such file or directory

The last command line dumps STDERR to the console, and it seem not to be the expected behaviour... But...

If you want to make some post filtering about standard output, error output or both:

$ ls -ld /tmp /tnt | sed 's/^.*$/<-- & --->/'ls: cannot access /tnt: No such file or directory<-- drwxrwxrwt 118 root root 196608 Jan  7 12:02 /tmp --->$ ls -ld /tmp /tnt 2>&1 | sed 's/^.*$/<-- & --->/'<-- ls: cannot access /tnt: No such file or directory ---><-- drwxrwxrwt 118 root root 196608 Jan  7 12:02 /tmp --->$ ls -ld /tmp /tnt >/dev/null | sed 's/^.*$/<-- & --->/'ls: cannot access /tnt: No such file or directory$ ls -ld /tmp /tnt >/dev/null 2>&1 | sed 's/^.*$/<-- & --->/'$ ls -ld /tmp /tnt 2>&1 >/dev/null | sed 's/^.*$/<-- & --->/'<-- ls: cannot access /tnt: No such file or directory --->

Notice that the last command line in this paragraph is exactly same as in previous paragraph, where I wrote seem not to be the expected behaviour (so, this could even be an expected behaviour).

Well, there is a little tricks about redirections, for doing different operation on both outputs:

$ ( ls -ld /tmp /tnt | sed 's/^/O: /' >&9 ) 9>&2  2>&1  | sed 's/^/E: /'O: drwxrwxrwt 118 root root 196608 Jan  7 12:13 /tmpE: ls: cannot access /tnt: No such file or directory

Note: &9 descriptor would occur spontaneously because of ) 9>&2.

Addendum: nota! With the new version of bash (>4.0) there is a new feature and more sexy syntax for doing this kind of things:

$ ls -ld /tmp /tnt 2> >(sed 's/^/E: /') > >(sed 's/^/O: /')O: drwxrwxrwt 17 root root 28672 Nov  5 23:00 /tmpE: ls: cannot access /tnt: No such file or directory

And finally for such a cascading output formatting:

$ ((ls -ld /tmp /tnt |sed 's/^/O: /' >&9 ) 2>&1 |sed 's/^/E: /') 9>&1| cat -n     1  O: drwxrwxrwt 118 root root 196608 Jan  7 12:29 /tmp     2  E: ls: cannot access /tnt: No such file or directory

Addendum: nota! Same new syntax, in both ways:

$ cat -n <(ls -ld /tmp /tnt 2> >(sed 's/^/E: /') > >(sed 's/^/O: /'))     1  O: drwxrwxrwt 17 root root 28672 Nov  5 23:00 /tmp     2  E: ls: cannot access /tnt: No such file or directory

Where STDOUT go through a specific filter, STDERR to another and finally both outputs merged go through a third command filter.

3 - A word about noclobber option and >| syntax

That's about overwriting:

While set -o noclobber instruct bash to not overwrite any existing file, the >| syntax let you pass through this limitation:

$ testfile=$(mktemp /tmp/testNoClobberDate-XXXXXX)$ date > $testfile ; cat $testfileMon Jan  7 13:18:15 CET 2013$ date > $testfile ; cat $testfileMon Jan  7 13:18:19 CET 2013$ date > $testfile ; cat $testfileMon Jan  7 13:18:21 CET 2013

The file is overwritten each time, well now:

$ set -o noclobber$ date > $testfile ; cat $testfilebash: /tmp/testNoClobberDate-WW1xi9: cannot overwrite existing fileMon Jan  7 13:18:21 CET 2013$ date > $testfile ; cat $testfilebash: /tmp/testNoClobberDate-WW1xi9: cannot overwrite existing fileMon Jan  7 13:18:21 CET 2013

Pass through with >|:

$ date >| $testfile ; cat $testfileMon Jan  7 13:18:58 CET 2013$ date >| $testfile ; cat $testfileMon Jan  7 13:19:01 CET 2013

Unsetting this option and/or inquiring if already set.

$ set -o | grep noclobbernoclobber           on$ set +o noclobber$ set -o | grep noclobbernoclobber           off$ date > $testfile ; cat $testfileMon Jan  7 13:24:27 CET 2013$ rm $testfile

4 - Last trick and more...

For redirecting both output from a given command, we see that a right syntax could be:

$ ls -ld /tmp /tnt >/dev/null 2>&1

for this special case, there is a shortcut syntax: &> ... or >&

$ ls -ld /tmp /tnt &>/dev/null$ ls -ld /tmp /tnt >&/dev/null

Nota: if 2>&1 exist, 1>&2 is a correct syntax too:

$ ls -ld /tmp /tnt 2>/dev/null 1>&2

4b- Now, I will let you think about:

$ ls -ld /tmp /tnt 2>&1 1>&2  | sed -e s/^/++/++/bin/ls: cannot access /tnt: No such file or directory++drwxrwxrwt 193 root root 196608 Feb  9 11:08 /tmp/$ ls -ld /tmp /tnt 1>&2 2>&1  | sed -e s/^/++//bin/ls: cannot access /tnt: No such file or directorydrwxrwxrwt 193 root root 196608 Feb  9 11:08 /tmp/

4c- If you're interested in more information

You could read the fine manual by hitting:

man -Len -Pless\ +/^REDIRECTION bash

in a bash console ;-)