The open source movement changed how companies build software. Facebook, Twitter, and Yahoo employees pitched in during the early days of the data-crunching software Hadoop. Even after the relationship between Apple and Google soured, the companies' coders kept working together on an obscure but important piece of software called LLVM. Microsoft now uses and contributes to the Linux operating system, even though it competes with Windows. The embrace of open source isn't about altruism.

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