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If you're having trouble getting QuakeSpasm to run your purchased (non-shareware) Quake files on Linux, try this:

Go into your QuakeSpasm folder.
Go into your id1 folder (where you've copied the pak0.pak and PAK1.PAK files) from your purchased copy of Quake.
Change the upper-case letters in "PAK1.PAK" to lower-case letters; pak1.pak.

From there, QuakeSpasm will recognize your legally purchased version of Quake and grant you access to all games in "Quake: The Offering".