Like Corona688 already said: yes, precisely. The compatibility of UNIX is defined as the guarantee that you can compile the same source code on different systems with the same outcome. For instance, this means that regardless of how your terminal is constructed (in terms of real hardware) you can expect "printf()" to do the same/analogous on every one of them. You can look "printf()" up somewhere in the POSIX standard and will find a detailed "printf() is required to do X in case of Y, return A in case of B, ...".

Notice that this standard just describes what has to come out, not how this outcome is realized! This is why UNIX (and Unix.like systems) run on everything from small embedded systems in your washing machine over cell phones (Android is just a customized Linux kernel), most WLAN routers, NAS appliances to real big irons like the IBM p795. We have about a dozen p780s in our data centers, most of them 4TB of memory and 128 processors. They run dozens of LPARs each. Compare this, along with some 50-60 PCI busses in each I/O subsystem (each system can have up to 4 of them), with your typical PC-compatible server Windows runs on. In addition we have some z/Linux systems running on the mainframe, Linux on all sorts of hardware, a few Sun servers running Solaris (another Unix), etc., etc..

Yes, exactly. It is not only a kernel but also a standard library, which executes all the system calls in a standardized way. If you never have to execute interactive commands but only a fixed program you do not need all the utilities that come with a OS usually and which do things like create users, files, and so on. Most embeded systems are constructed this way: a Linux kernel, the standard library, the one program it is supposed to run and from the kernel and the library stripped everything not necessary to run that one program. Take apart your home WLAN router, telephone or similar device and you might probably find exactly this, burned into an EPROM.


For instance. He does not bite (well, not when the moon is not full, anyways) as he is a very friendly guy and by far the best expert in UNIX standards issues we have here you can ask him if you need to know details we can't provide. He won't answer via PM (because this would not contribute to the knowledge base we are building here), but he might write something into this thread.

I hope this helps.

bakunin