bash - Redirection in join and cat

Question

This question already has answers here: question from:https://stackoverflow.com/questions/65622861/redirection-in-join-and-cat

Answer 1

<(cmd) is a process substitution. It shares a character with -- but is entirely unrelated to -- the < used for redirection:

• <(cmd) expands to a filename that you can open and read to get the program's output. You can pass as many filenames as you want to a command.
• < filename opens a filename as file descriptor 0. Opening a new file on the same file descriptor overwrites and closes the previous one.

Your process substitution example simply passes two separate filenames:

$ echo join -1 2 -2 1 <(sort -k 2 wine.txt) <(sort reviews.txt)
join -1 2 -2 1 /dev/fd/63 /dev/fd/6

If you similarly pass two filenames to cat, you get the same kind of result:

$ cat file1 file2
Hello
World