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Darwin and case sensitivity in filenames

I normally use a Mac as a pretty interface to a unix (POSIX) system and don't run into problems, but every now and then my whole workflow will come to a stop because Darwin did that one thing slightly different from everyone else and I have to run that down. This time it was case sensitive file names.

POSIX file systems are case sensitive. So a file called Test is different from TEST is different from tesT (Though one is recommended not to torture users in that fashion).

So under a proper POSIX system you can do 

mkdir Test
mkdir tesT
mkdir TEST

and end up with three different directories.

Darwin lulls you into a false sense of security by accepting all three forms but behind the scenes it simply maps it all to the same form. So you get a nonsensical complaint as you do this

mkdir Test  # OK
mkdir tesT  # Directory already exists? WTF?
mkdir TEST  # Directory already exists? Whaaa?

Sigh.
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