Something closely related to command substitution is process substitution. The syntax is as follows:

<(command)

It works very similarly to command substitution, but instead of sending the output of a command as a string somewhere, you can reference the output as a file. This means that some commands, which do not expect a string but instead a reference to a file, can be used with dynamic input as well.

While too advanced to discuss in great detail, here's a simple example that should get the point across:

reader@ubuntu:~/scripts/chapter_12$ diff <(ls /tmp/) <(ls /home/)
1,11c1
< directory-file
< grep-file
< grep-file-stdout
< passwd
< shadow
---
> reader

The diff command normally compares two files and prints their differences. Now, instead of files, we used process substitution to have diff compare the results from ls /tmp/ and ls /home/, using the <(ls /tmp/) syntax.