What is the `< <()` syntax?

Question

I've been using RVM for a while, and every time I just copied and pasted the following command to get it setup:

bash < <(curl -s https://rvm.beginrescueend.com/install/rvm)

It bugs me that I don't fully understand the syntax, and why we need the double <, and the parentheses. Can some one explain this or point me to the right reference?

Answer 1

The first one is input redirection. It feeds the contents of a file into the program as input. The second construct is <() and it's process redirection: it treats output of a process like a file. In this case, the effect is that you will run the contents of that url as though it was a bash script -- very dangerous! If you don't trust to source completely, don't do that. An attacker could use this method to have you run commands that would compromise your system.

Answer 2

Just my 2 cents. Bashs structure <() as @Daenyth stated "treats output of a process like a file". This structure may be very useful. Just consider following:

 diff <(ls dir1) <(ls dir2)

This will use vimdiff to show differences between contents of dir1 and dir2. Using vimdiff instead diff will even cooler.