How to get input from URL and perform a corresponding javascript operation

I am working on a file sharing site. You will type in the file ID in a part of the URL, and the website will pull it from IPFS. Example (doesnt necessarily have to be in this format):

http://bit.ly/2IMqDGV

This will open a page, in which some code will extract asdefg and download the corresponding file. The file is hosted on IPFS, and I wish to extract it from there.

How can this be achieved?