"An operating system is similar to a government. Like a government, it performs no useful function by itself."

"An operating system is similar to a government. Like a government, it performs no useful function by itself. It simply provides an environment within which other programs can do useful work."

Cited from:
Silberschatz, A., G. Gagne, and P. B. Galvin. Operating system concepts . 8th ed. Hoboken, N.J.: Wiley & Sons, 2010. Print.

复习时看到了这个比喻句。