How to use the duplication redirection to combine one output stream into another output stream? I want to combine STDERR and STDIN together and send them to a file.

To combine one output stream into another output stream, you can use the duplication redirection operator as described below:

command options redirections x>&y
  Combines output stream handler x into y

For example, the following combines handler 2 (STDERR) into handler 1 (STDOUT). So info.txt will get both outputs and error.txt will be empty.

C:\fyicenter>dir junk 1> info.txt 2>error.txt 2>&1

For example, the following combines handler 1 (STDOUT) into handler 2 (STDERR). So error.txt will get both outputs and info.txt will be empty.

C:\fyicenter>dir junk 1> info.txt 2>error.txt 1>&2

Note that duplication redirection must be specified after other redirections. The following will not work:

C:\fyicenter>dir junk 1>&2 1> info.txt 2> error.txt

⇒ Pipe STDOUT of Previous Command as STDIN of Next

⇐ Use Handler Numbers with <, >, >>

⇑ Windows Command Syntax

⇑⇑ Windows Batch File Tutorials