A cursed file on macOS

September 16, 2022

I like to think that I mostly understand how computers work. But those darn computers still find ways to surprise me.

For example, I thought I had a pretty good mental model of how files worked, e.g. they represent data, usually on disk, so you wouldn't expect your shell environment to be able to affect their contents. But you'd be wrong!

λ SYSTEM_VERSION_COMPAT=0 cat /System/Library/CoreServices/SystemVersion.plist
...
	<key>ProductVersion</key>
	<string>12.6.3</string>
...

as opposed to:

λ SYSTEM_VERSION_COMPAT=1 cat /System/Library/CoreServices/SystemVersion.plist
...
	<key>ProductVersion</key>
	<string>10.16</string>
...

It looks like Apple apparently hardcodes this behaviour into open: https://opensource.apple.com/source/xnu/xnu-7195.81.3/libsyscall/wrappers/open-base.c