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I know that the > sign is used for output redirection in the command line, but I'm having trouble finding something that explains the use of 2>&1 in the command line. For example:

curl http://www.google.com > /dev/null 2>&1 &
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3 Answers 3

up vote 53 down vote accepted

The 1 denotes standard output (stdout). The 2 denotes standard error (stderr).

So 2>&1 says to send standard error to where ever standard output is being redirected as well. Which since it's being sent to /dev/null is akin to ignoring any output at all.

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Is there a reason that an ampersand appears before the 1, but not before the 2? I thought the & was a reserved character for running a job in the background, but I guess that's only if the ampersand appears as a long character at the end of a command...? –  Matt Huggins Nov 16 '09 at 23:24
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Because 0 (stdin), 1 (stdout) and 2 (stderr) are actually file descriptors the shell requires an ampersand put in front of them for redirection. It duplicates the file descriptor in this case effectively merging the two streams of information together. –  Chealion Nov 16 '09 at 23:36
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Think of it this way: if you just had "1" with no ampersand, the shell would create a file named "1" and redirect stderr output to it. –  CarlF Nov 17 '09 at 6:05
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Okay, would this alone be valid then? curl http://www.google.com 2>/dev/null How does the command line know that the "2" here is intended to mean stderr and isn't actually the second parameter that I'm passing to the curl command? –  Matt Huggins Nov 17 '09 at 18:21
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@Matt Huggins: Yes. That would send all output from stderr straight to /dev/null instead. You can see it in practice by trying curl, curl 1>/dev/null and curl 2>/dev/null just to see the output change. Again the ampersand is only needed for the file descriptor being redirected to. –  Chealion Nov 18 '09 at 0:34

My understand as follow:

If you only want to read the Output and Error information of the command on the screen, then just write: curl http://www.google.com

And some times you want to save the Output information to a file instead of terminal screen for later review, then you can write: curl http://www.google.com > logfile

But in this way, the StdErr information will be omitted, since > only redirect the StdOut to logfile.

So if you care about the error information of the command once it fail to execute, then you need to combine StdOut with StdErr by using 2>&1 (which means fold StdErr into StdOut), so the following command line can be written: curl http://www.google.com > logfile 2>&1

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2 refers to STDERR. 2>&1 will send STDERR to the same location as 1 (STDOUT).

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