[n]<&word
n
is made to be a copy of that file descriptor.
If the digits in word do not specify a file descriptor open for
input, a redirection error occurs.
If word
evaluates to `-', file descriptor n
is closed. If
n
is not specified, the standard input (file descriptor 0) is used.
The operator
[n]>&word
n
is not specified, the standard output (file descriptor 1) is used.
If the digits in word do not specify a file descriptor open for
output, a redirection error occurs.
As a special case, if n
is omitted, and word does not
expand to one or more digits, the standard output and standard
error are redirected as described previously.