3.7)  How can a process detect if it's running in the background?

      First of all: do you want to know if you're running in the
      background, or if you're running interactively? If you're
      deciding whether or not you should print prompts and the like,
      that's probably a better criterion. Check if standard input
      is a terminal:

            sh: if [ -t 0 ]; then ... fi
            C: if(isatty(0)) { ... }

      In general, you can't tell if you're running in the background.
      The fundamental problem is that different shells and different
      versions of UNIX have different notions of what "foreground" and
      "background" mean - and on the most common type of system with a
      better-defined notion of what they mean, programs can be moved
      arbitrarily between foreground and background!

      UNIX systems without job control typically put a process into the
      background by ignoring SIGINT and SIGQUIT and redirecting the
      standard input to "/dev/null"; this is done by the shell.

      Shells that support job control, on UNIX systems that support job
      control, put a process into the background by giving it a process
      group ID different from the process group to which the terminal
      belongs.  They move it back into the foreground by setting the
      terminal's process group ID to that of the process.  Shells that
      do *not* support job control, on UNIX systems that support job
      control, typically do what shells do on systems that don't
      support job control.