I am trying to build a class that would behave is a different way if run using a shell or from a GUI.
It could be included in both forms using #include "myclass.h"...
However, in the constructor I would like to differentiate between Shell runs and GUI runs.
I can easily achieve it using a parameter that would be passed to the constructor when declaring it but I want to explore my options.
I am using C++ on ubuntu and my GUI is using Qt.
The standard C way of determining whether X Window is present:
#include <stdlib.h>
if (NULL == getenv("DISPLAY")) is_gui_present = false;
else is_gui_present = true;
If you want to determine if there is a shell at all, or the application was run from, say, a file manager, then it's not easy: both cases are just call of exec system call from a shell or a file manager/GUI program runner (often with the same parameters), you need to pass a flag explicitly to see that.
P.S. I've just found a way to do that: check the environment for variable "TERM" - it is set for a shell and is inherited to Qt program, it is often not set in a GUI program. But don't take this as an accurate solution!
Launching programs from the desktop (double click or from a desktop file/start menu) will usually redirect their stdin file descriptor to a pipe. You can detect this:
#include <cstdio> // fileno()
#include <unistd.h> // isatty()
if (isatty(fileno(stdin)))
// We were launched from the command line.
else
// We were launched from inside the desktop
