I like APIs. I like being able to call a function, give it some data, and tell it what to do with it, and have it do it without fuss or bother.
And I like interfaces that are stable and reliable.
However, on a typical unix-like system, certain common operations have no real API. For some operations, the normal way to do things is to run an external program.
Usually the system() call of the title is involved, but it could be popen() or some other variant. I'm using system() here as shorthand terminology for executing another program to do the work.
The two standard examples are mail and printing. You throw the data you want something done with at sendmail or lp.
This is atrocious. This isn't an API - this is merely convention that often works. Frankly, it worries me that there aren't standard programming APIs for such common tasks as sending mail and printing a file.
While mail appears to be standardized sufficiently to be largely hidden by applications, printing is another matter. The variety (and general disfunctionality) of print dialog boxes should be a clue here. Worst, of course, are those that simply have a text field that you type the print command into.
Apart from the inefficiency of launching external programs, the lack of genuine APIs limits the interactions a program can have, in particular the feedback it can get from the application it launched.
It's the 21st century. We deserve to have some decent 21st century APIs.