Windows

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1. Press Win + R
2. Type cmd
3. Enter this: wmic os get osarchitecture

Sample output:

OSArchitecture
64

Or click the Windows button, select Settings > System > About System type (Under Device specifications)

  Linux

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Open the CLI to a Linux server by SSH using Putty.
Type the following command line: uname –m

Sample output:

64 32

Second way:

$ getconf LONG_BIT

  Mac OS

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Execute this:

lipo -info /bin/ls

Sample output:

Non-fat file: /bin/ls is architecture: x86_64