This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
ShrinkIt files are the Apple II world's answer to .ZIP files in PC-ville. An .SHK file is a file which contains one or more files which are almost always in compressed form. Usually, they are produced by GS-ShrinkIt (also called "GSHK" or "ShrinkIt-GS") or the Balloon NDA, or by 8-bit ShrinkIt. Some .SHK files are produced by Macs; these may not always be compatible with A2 ShrinkIt programs. An .SHK file can be unshrinked by ShrinkIt even if it shows up on the Apple II with a TXT or BIN filetype and even if the name does not end with ".SHK" or ".shk". If a ShrinkIt file does not show up as available for unshrinking, you can toggle an "All files" option to see the file and then select it. If an .SHK file has a Binary II header, ShrinkIt will automatically remove it and assign the correct filetype. (Of course, this will usually be SHK.) Other kinds of ShrinkIt files include .SEA and .SDK. An Apple II .SEA file is a IIgs executable self-extracting archive-- i.e. you can click it on the GS Finder and it will unShrink. There are also Mac .SEA files and these are not GS-compatible. A ShrinkIt whole-disk archive is an .SHK file which is usually labeled ".SDK" to show that it is a Shrinked diskette. An .SDK file can archive a 3.5" diskette (both sides) or 5.25" diskette (one side). Most are archives of 5.25" DOS 3.3 diskettes produced by 8-bit ShrinkIt. A whole-disk ShrinkIt archive retains all data bytes on a diskette, including files, Catalog/Directory sectors, empty tracks, and DOS if present. An .sdk file of a DOS 3.3 5.25" disk created by 8-bit ShrinkIt also preserves volume number-- important for some games and utilities which depend upon volume numbers to identify disks. (5.25" whole-disk archives created by GS-ShrinkIt do not preseve volume number.) 8-bit/IIe ShrinkIt can be used to fully unshrink any Apple II .SHK file except .SHK files which contain files with GS/OS resource forks and .SEA files. For this reason, 8-bit ShrinkIt should not be used to unshrink .SHK file archives containing GS programs unless you know that none of the contained files has a resource fork. GS-ShrinkIt can handle nearly all kinds of Apple II .SHK and .SDK files. It will not handle shrinked 5.25" DOS 3.3 .SDK files created by 8-bit ShrinkIt. In fact, most users automatically use 8-bit ShrinkIt to create and unshrink .SDK files of old 5.25" wares. (Balloon does not currently support whole-disk archives.) Naturally, things are somewhat more crowded on 64K Apple II's. On these machines, the functions are separated. SHRINK creates .SHK files and UNSHRINK unshrinks them. On a PC, the utility NuLib (v3.24) lets you view contents and unshrink most kinds of .SHK files. (There is a handy option to unshrink and convert Apple II text files to PC text format.) It will not unshrink IIgs files with resource forks. Here is a simple one-line batch (text) file program for easily viewing the contents of .shk files you download to a PC (just double-click on the file name): c:\nulib\nulib v %1 |more The above is for NuLib.exe located in folder c:\nulib . Save the text as nulibv.bat in c:\nulib and tell Windows to use c:\nulib\nulibv.bat as the 'application to perform action' for doing an Open. (You do this by selecting View--Options in the My Computer window and editing the file type info for .shk files.) NuLib can also convert 5.25" .SDK files into .PO (ProDOS order) disk images which can be used by Apple II emulators. This works for .SDK files produced by 8-bit ShrinkIt but not for those produced by GS-ShrinkIt. The unshrinking process is very speedy and the size of a compressed ShrinkIt file is, often, around half that of the original files it contains. This makes .SHK files very handy for archiving your software. And, since a ShrinkIt file also preserves filetype information of contained files, ShrinkIt has become the preferred format for uploading and storing Apple II files on the internet. ----------------------------