Create a Sentinel-1 Scientific Data Hub account
Go here: https://scihub.esa.int/dhus/ and look for a "Register" link. Follow the registration steps.
aria2 is a lightweight multi-protocol & multi-source command-line download utility. It supports HTTP/HTTPS, FTP, BitTorrent and Metalink. You will use aria2 to download the Sentinel-1 files from your Sentinel-1 Scientific Data Hub shop carts, they are exported as metalink.
Check how to install aria2 on your platform: http://aria2.sourceforge.net/
Create a Sentinel-1 Scientific Data Hub shop cart
Search for Sentinel-1 datasets and add them to a "Download cart"
Download the cart. You'll have a metalink called products.meta4
Download the datasets
aria2c --http-user='<your username>' --http-passwd='<your password>' --check-certificate=false --max-concurrent-downloads=2 -M products.meta4
The max concurrent downloads are limited by ESA to two parallel downloads, increasing this number will only download two products of the cart