I had no idea the Unix sort could do that. Thanks for posting. I've used GNU Utilities for Win32 before but it hasn't been updated since 2003 and is getting hard to find. I found a new project with the same objective called GNU on Windows that looks promising.

From reading the Wikipedia page for Sort it looks like the left angle bracket redirect, '<', isn't needed to read the input file. Also, I noticed that Windows Server 2012 includes a command called sort that doesn't have the -k option. That's a pity since not only does it have less functionality than the Unix version but also it complicates setting up a path to the GNU version with the same name.