NuLib2 is a command-line file archiver for Apple II archives. It can operate on ShrinkIt and Binary II files (.shk, .sdk, .bxy, .bse, .bny, .bqy).
NufxLib is a library of code that supports ShrinkIt archives. It's used by NuLib2 and CiderPress.
A pre-built NuLib2 binary is available for x86 Windows. For Linux and Mac OS X systems, you can download the source code and build it.
More information, including full documentation for NuLib2 and NufxLib, can be found on https://nulib.com/ (or https://fadden.github.io/nulib2).