2>&1

      # Redirects stderr to stdout.

      # Error messages get sent to same place as standard output.

        >>filename 2>&1

            bad_command >>filename 2>&1

            # Appends both stdout and stderr to the file "filename" ...

        2>&1 | [command(s)]

            bad_command 2>&1 | awk '{print $5}'   # found

            # Sends stderr through a pipe.

            # |& was added to Bash 4 as an abbreviation for 2>&1 |.

   i>&j

      # Redirects file descriptor i to j.

      # All output of file pointed to by i gets sent to file pointed to by j.

   >&j

      # Redirects, by default, file descriptor 1 (stdout) to j.

      # All stdout gets sent to file pointed to by j.