how to free up disk space after running bitcoind for the first time? I am following Antonopoulos' Mastering Bitcoin.  I have installed Bitcoin core and ran bitcoind for the first time. Although the book said it will remind me to create a configuration file, it did not.  It rapidly used up the disk space (I am running it on VM Ubuntu) and I had to interrupt it.  

I want to free up the used space and try again with configuration file, but I do not know what to do.
Find the data directory: https://bitcoin.stackexchange.com/a/11210

Create a file named "bitcoin.conf"

Add
    prune=550
to that file.

Restart Bitcoin Core.