Question

Is it possible to run a c++ application from a terminal and on certain conditions return a command back into the terminal from which it was called from? For instance, if I were to run an application within my terminal and after my selections; my application needs to change my PATH by running an export command such as:

(USING BASH)

export PATH=.:/home/User/application/bin:$PATH

After I’m done and before my application completely closes can I make the application change my terminals local environment variables with the above command? Does Qt offer a way of doing this? Thanks in advance for any help!

Answer 1

No, you cannot change parent application environment.

Why? When your parent app started yours (probably using system()), it actually fork()ed – child process was born as to be almost exact replica of parent, and then that child used execve() call, which completely replaced executable image of that process with executable image of your application (for scripts it would be image of interpreter like bash).

In addition to that, that process also prepared few more things. One is list of open files, starting with file handles 0,1,2 (stdin, stdout, stderr). Also, it created memory block (which belongs to child process address space) which contains environment variables (as key=value pairs).

Because environment block belongs to your process, you can change your own environment as you please. But, it is impossible for your process to change environment memory block of parent (or any other process for that matter). The only way to possibly achieve this would be to use IPC (inter-process communication) and gently ask parent to do this task inside of it, but parent must be actively listening (on local or network socket) and be willing to fulfill such request from somebody, and child is not any special compared to any other process in that regard.

This also reason why you can change environment in bash using some shell script, but ONLY using source or . bash macro – because it is processed by bash itself, without starting any external process.

However, you cannot change environment by executing any other program or script for reasons stated above.