What bash < < syntax mean

bashsyntax

Ruby Version Manager (RVM) installed like so:

bash -s stable < <(curl -s https://raw.github.com/wayneeseguin/rvm/master/binscripts/rvm-installer)

I understand what first < mean (feeds bash script to bash interpreter), I'm confused with <(...) part. So, what parentheses do here and the less than sign.
In which cases we can use same syntax?

I tried to dig on the internet, found this SO question https://stackoverflow.com/questions/2188199/bash-double-or-single-bracket-parentheses-curly-braces and this question on ubuntuforums: http://ubuntuforums.org/showthread.php?p=7803008 But still have no idea why we use those parentheses and why we use input redirection twice.

bash < curl -s https://raw.github.com/wayneeseguin/rvm/master/binscripts/rvm-installer

ain't the same?

Thanks.

Best Answer

It's process substitution. It feeds the output of the command into a FIFO that can be read from like a normal file.

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Bash – Understanding Cryptic Bash Commands

It's, as you said, a forkbomb. What it does is define a function, then call it. The function is called :.

Let's name it forkbomb so we can better see what's going on:

forkbomb(){ forkbomb|forkbomb& };forkbomb

As you can see, and probably guess from your programming experience, the first part is the function definition (forkbomb(){ ... }), and the very last : is where the function gets called (the ; just separates statements in Bash).

Now, what does this function do? If you're familiar with Bash, you'll know that the | character pipes the standard output of one command/program to the standard input of another. So basically, :|: starts up two instances of the function (this is where it "forks").

And then the magic: the & puts those commands in the background, allowing the original function to return, while each instance forks 'til the cows come home in the background, thus using up all your resources and taking down the system (unless it has limits imposed on it).

Linux – What does the character , mean in bash

It has no special meaning. The command redirects to a file called ,update.

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