Am I running 32bit or 64bit on Mac OS?

Very simple command, but very useful sometimes.

$ uname -a

If you see something like this:

$ uname -a
Darwin naturewarm-lm 12.4.0 Darwin Kernel Version 12.4.0: Wed May  1 17:57:12 PDT 2013; root:xnu-2050.24.15~1/RELEASE_X86_64 x86_64

This means your are running on 64bit. If you are running on 32bit, you will get something end up with i386, just like this:

$ uname -a
Darwin naturewarm-lm 12.4.0 Darwin Kernel Version 12.4.0: Wed May  1 17:57:12 PDT 2013; root:xnu-2050.24.15~1/RELEASE_I386 i386