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 (seenoclobberbash 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 /tnt
ls: cannot access /tnt: No such file or directory
drwxrwxrwt 118 root root 196608 Jan 7 11:49 /tmp
$ ls -ld /tmp /tnt >/dev/null
ls: cannot access /tnt: No such file or directory
$ ls -ld /tmp /tnt 2>/dev/null
drwxrwxrwt 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/null
ls: cannot access /tnt: No such file or directory
$ ls -ld /tmp /tnt >/dev/null 2>&1
$ ls -ld /tmp /tnt 2>&1 >/dev/null
ls: 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 /tmp
E: 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 /tmp
E: 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 $testfile
Mon Jan 7 13:18:15 CET 2013
$ date > $testfile ; cat $testfile
Mon Jan 7 13:18:19 CET 2013
$ date > $testfile ; cat $testfile
Mon Jan 7 13:18:21 CET 2013
The file is overwritten each time, well now:
$ set -o noclobber
$ date > $testfile ; cat $testfile
bash: /tmp/testNoClobberDate-WW1xi9: cannot overwrite existing file
Mon Jan 7 13:18:21 CET 2013
$ date > $testfile ; cat $testfile
bash: /tmp/testNoClobberDate-WW1xi9: cannot overwrite existing file
Mon Jan 7 13:18:21 CET 2013
Pass through with >|:
$ date >| $testfile ; cat $testfile
Mon Jan 7 13:18:58 CET 2013
$ date >| $testfile ; cat $testfile
Mon Jan 7 13:19:01 CET 2013
Unsetting this option and/or inquiring if already set.
$ set -o | grep noclobber
noclobber on
$ set +o noclobber
$ set -o | grep noclobber
noclobber off
$ date > $testfile ; cat $testfile
Mon 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 directory
drwxrwxrwt 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 ;-)