Multiple standard input? How?

Question

From this commandlinefu post:

$ diff <(wget -q -O - URL1) <(wget -q -O - URL2)

Seems like multiple standard inputs but---if I remember my Linux Porgramming course correctly---this can't be it. I thought that, by definition, standard input was one stream.

Perhaps this has zilch to do with stdin? Can someone explain this to me; maybe provide links to documentation.

Bonus: How would a script handle these multiple streams? If someone could provide a Python or Perl example, that would be very helpful.

Answer 1

This isn't multiple standard input. This is a bash'ism that called 'Process Substitution' http://tldp.org/LDP/abs/html/process-sub.html

It creates a pseudo file (/dev/fd/something) for each substitution. It's pretty useful. The command can only read as a stream, meaning it can not go back and forth with fseek. It needs to read it as a stream of bytes, like a pipe.

Answer 2

<(...) does process substitution in bash. The output of the process in the parens is sent to an additional file descriptor beyond the normal 3, and a filename is returned corresponding to that file descriptor. In this way the output of a command can be treated as a filename to be passed to another command.