In Bash, inserting 1>&2 after a command redirects

Standard error (fd 2) is not involved as a destination in 1>&2; this redirection only moves stdout (fd 1) to wherever stderr (fd 2) currently points.

Redirecting standard input to standard error would require 0>&2, not 1>&2; the source descriptor here is 1 (stdout), not 0 (stdin).

Bash assigns file descriptor 1 to standard output and file descriptor 2 to standard error. The redirection 1>&2 uses the file descriptor duplication operator >&, causing the kernel to make fd 1 point to the same underlying stream as fd 2. Any data subsequently written to stdout is therefore delivered to the standard error stream, which is useful for ensuring diagnostic output appears even when stdout is redirected elsewhere.

Redirecting standard error to standard output is the opposite operation, expressed as 2>&1, not 1>&2.

Redirecting standard output to standard input would require 1>&0 and is not a valid interpretation of 1>&2.