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

According to my knowledge :
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.

4 years ago

2 is the console standard error.

1 is the console standard output.

This is the standard Unix, and Windows also follows the POSIX.

E.g. when you run

perl test.pl 2>&1

the standard error is redirected to standard output, so you can see both outputs together:

perl test.pl > debug.log 2>&1

After execution, you can see all the output, including errors, in the debug.log.

perl test.pl 1>out.log 2>err.log

Then standard output goes to out.log, and standard error to err.log.

I suggest you to try to understand these.

4 years ago

The numbers refer to the file descriptors (fd).

Zero is stdin
One is stdout
Two is stderr
2>&1 redirects fd 2 to 1.

This works for any number of file descriptors if the program uses them.

You can look at /usr/include/unistd.h if you forget them:

/ Standard file descriptors.  */

#define STDIN_FILENO    0   /* Standard input.  */

#define STDOUT_FILENO   1   /* Standard output.  */

#define STDERR_FILENO   2   /* Standard error output.  /

That said I have written C tools that use non-standard file descriptors for custom logging so you don't see it unless you redirect it to a file or something.

4 years ago