Bash IO Redirection – What Does ‘3>&1 1>&2 2>&3’ Do in a Script?

You can do it like this:

./script 2>&1 1>/dev/null

This redirects fd 2 to what fd 1 points to (i.e. stdout), then redirects fd 1 to /dev/null. The second redirect doesn't affect the first one, output to fd 2 will be sent to stdout.

Order does matter though. With this:

./script 1>/dev/null 2>&1

will send all output to /dev/null.

Summary

Under bash, if that command is in a script, the script file will be overwritten with an error message.

Example

Consider the script:

$ cat test.sh
date 2&>$0

Now, run the script:

$ bash test.sh
test.sh: line 2: unexpected EOF while looking for matching ``'
test.sh: line 3: syntax error: unexpected end of file

Observe the new contents of the script:

$ cat test.sh
date: invalid date `2'

Explanation

The command, date 2&>$0, is interpreted as follows:

1. The date command is run with argument 2

2. All output, both stdout and stderr, from the date command is redirected to the file $0. $0 is the name of the current script.

   The symbol > indicates redirection of, by default, stdout. As a bash extension, the symbol &> is a shortcut indication redirection of both stdout and stderr. Consequently, both stdout and stderr are redirected to the file $0.

3. Once the script file is overwritten, it is no longer a valid script and bash will complain about the malformed commands.

Difference between bash and POSIX shells

With a simple POSIX shell, such as dash, the shortcut &> is not supported. Hence, the command date 2&>$0 will redirect only stdout to the file $0. In this case, that means that the script file is overwritten with an empty file while the date error message will appear on the terminal.