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

 

In a Unix shell, if I want to combine stderr and stdout into the stdout stream for further manipulation, I can append the following on the end of my command: 

2>&1

So, if I want to use "head" on the output from g++, I can do something like this: 

g++ lots_of_errors 2>&1 | head

so I can see only the first few errors.

I always have trouble remembering this, and I constantly have to go look it up, and it is mainly because I don't fully understand the syntax of this particular trick.

Can someone break this up and explain character by character what "2>&1" means?

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 is a file descriptor and not a filename. So the construct becomes: 2>&1 . 

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. 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 - Overwritting or appending?

Symbole > mean redirection.

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

    • Any case, the file would be created if they not exist.

    2 - The shell command line is order dependant!!

    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 lets 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 dump STDERR to the console, it seem not to be the expected behaviour... But...

    If you want to make some post filtering about one ouput, the other 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 paraghaph, 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 ouputs : 

    $ ( 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

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

    Addendum: nota! With 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 finaly 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 overwritting :

    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

    File is overwritted 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 syntaxe 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 informations

    you could Read The Fine Manual by hitting: 

    man -Len -Pless +/^REDIRECTION bash

    in a bash console ;-)

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