C programs as good OS citizens

• consistently indicate their success or failure with their exit status

• consistenly accept command-line arguments
  (options, then filenames or data)

• may receive more persistent configuration via environment variables
  (conveniently managed by interactive shells)

• consistent representation of system-specific errors setting errno and reporting of error messages using perror()

• consistent default data streams, stdin, stdout, stderr which may be redirected using shells

• many utility programs perform as filters independently of how their input and output is obtained
  (command-line, keyboard & screen, redirected files, or pipes)