The Unix philosophy is a set of cultural norms and philosophical approaches to developing software based on the experience of leading developers of the Unix operating system.
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Doug McIlroy, the inventor of Unix pipes and one of the founders of the Unix tradition, summarized the philosophy as follows:[1]
This is the Unix philosophy: Write programs that do one thing and do it well. Write programs to work together. Write programs to handle text streams, because that is a universal interface.
This is usually abridged to "Write programs that do one thing and do it well."
Eric S. Raymond, in his book The Art of Unix Programming,[2] summarizes the Unix philosophy as the widely-used KISS Principle of "Keep it Simple, Stupid."[3] He also provides a series of design rules:
In 1994 Mike Gancarz (a member of the team that designed the X Window System), drew on his own experience with Unix, as well as discussions with fellow programmers and people in other fields who depended on Unix, to produce The UNIX Philosophy which sums it up in 9 paramount precepts:
Richard P. Gabriel suggests that a key advantage of Unix was that it embodied a design philosophy he termed "worse is better", in which simplicity of both the interface and the implementation are more important than any other attributes of the system—including correctness, consistency, and completeness. Gabriel argues that this design style has key evolutionary advantages, though he questions the quality of some results.
For example, in the early days Unix was a monolithic kernel (which means that user processes carried out kernel system calls all on the user stack). If a signal was delivered to a process while it was blocked on a long-term I/O in the kernel, then what should be done? Should the signal be delayed, possibly for a long time (maybe indefinitely) while the I/O completed? The signal handler could not be executed when the process was in kernel mode, with sensitive kernel data on the stack. Should the kernel back-out the system call, and store it, for replay and restart later, assuming that the signal handler completes successfully?
In these cases Ken Thompson and Dennis Ritchie favored simplicity over perfection. The Unix system would occasionally return early from a system call with an error stating that it had done nothing—the "Interrupted System Call", or an error number 4 (EINTR) in today's systems. Of course the call had been aborted in order to call the signal handler. This could only happen for a handful of long-running system calls, i.e. read(), write(), open(), select(), etc. On the plus side, this made the I/O system many times simpler to design and understand. The vast majority of user programs were never affected because they didn't handle or experience signals other than SIGINT or Control-C and would die right away if one was raised. For the few other programs—things like shells or text editors that respond to job control key presses—small wrappers could be added to system calls so as to retry the call right away if this EINTR error was raised. Thus, the problem was solved in a simple manner.