Returning output from bash script to calling C++ function

Question

I am writing a baby program for practice. What I am trying to accomplish is basically a simple little GUI which displays services (for Linux); with buttons to start, stop, enable, and disable services (Much like the msconfig application "Services" tab in Windows). I am using C++ with Qt Creator on Fedora 21.

I want to create the GUI with C++, and populating the GUI with the list of services by calling bash scripts, and calling bash scripts on button clicks to do the appropriate action (enable, disable, etc.)

But when the C++ GUI calls the bash script (using system("path/to/script.sh")) the return value is only for exit success. How do I receive the output of the script itself, so that I can in turn use it to display on the GUI?

For conceptual example: if I were trying to display the output of (systemctl --type service | cut -d " " -f 1) into a GUI I have created in C++, how would I go about doing that? Is this even the correct way to do what I am trying to accomplish? If not,

1. What is the right way? and
2. Is there still a way to do it using my current method?

I have looked for a solution to this problem but I can't find information on how to return values from Bash to C++, only how to call Bash scripts from C++.

Answer 1

We're going to take advantage of the popen function, here.

std::string exec(char* cmd) {
    FILE* pipe = popen(cmd, "r");
    if (!pipe) return "ERROR";
    char buffer[128];
    std::string result = "";
    while(!feof(pipe)) {
        if(fgets(buffer, 128, pipe) != NULL)
            result += buffer;
    }
    pclose(pipe);
    return result;
}

This function takes a command as an argument, and returns the output as a string.

NOTE: this will not capture stderr! A quick and easy workaround is to redirect stderr to stdout, with 2>&1 at the end of your command.

Here is documentation on popen. Happy coding :)