On my Mac running Yosemite(10.10.2) I tried:

$ mkdir a
$ mkdir A
mkdir: A: File exists

Coming from a case sensitive *nix world, this was strange. Why does Apple need a case insensitive file system?

Who knows, but here’s A Linus Rant on it.

HFS+ (the Mac filesystem) is usually configured to be case insensitive but case preserving. Case insensitive means that this works:

 $ ls -ld a
drwxr-xr-x  2 ddigges  ddigges 68 Nov 11 18:04 a
 $ ls -ld A
drwxr-xr-x  2 ddigges  ddigges 68 Nov 11 18:04 A

However, when you create a new file it will remember which letters were capitalized and which were not.

$ mkdir aBc
$ ls -ld aBc
drwxr-xr-x  2 ddigges  ddigges  68 Nov 11 18:27 aBc

HFS is a bit of an oddity, having the ability to ignore and recognize case at the same time.

To know whether your Mac filesystem is case sensitive or not, use the diskutil comand:

$ diskutil info /
   File System Personality:  Journaled HFS+
   Type (Bundle):            hfs
   Name (User Visible):      Mac OS Extended (Journaled)

Look for the File System Personality and Name fields in the output. If the file system is case sensitive, you will see Case-sensitive Journaled HFS in the File System Personality and Mac OS Extended (Case-sensitive, Journaled) in the name.

Say you need to copy files from a *nix system, or do something else that requires a case sensitive file system, you can create a case sensitive disk image on your Mac as follows:

$ hdiutil create -type SPARSE -fs 'Case-sensitive Journaled HFS+' -size 60g -volname workspace $WHERE_TO_STORE_THE_IMAGE

$ hdiutil attach $WHERE_TO_STORE_THE_IMAGE

$ cd /Volumes/workspace

/Volumes/workspace $ mkdir a
/Volumes/workspace $ mkdir A
/Volumes/workspace $ ls
A   a

Linus would be proud.