Can't execute simple Shell Script/Commonads with exec - c++

Question

I designed a GUI with Qt5 and with the click of a button I want to start a python software. At first it tried writing a simple shell script which looks like this:
#!/bin/bash
cd ~/Printrun
python pronterface.py
The script works fine when executed directly in the shell but now i want a c++ code that execute the script. I already found some useful stuff here, but i cant seem to get it to work on my project. Maybe it is because I'm running the code from Qt?
Here is the code:
execlp("home/user/", "./my_shell_script.sh", (char *)0);
and
execlp("home/user/Printrun", "python pronterface.py", (char *)0);
both dont return any errors but dont seem to work.
I am sorry if I am missing something obvious I pretty new to stuff like this.
EDIT: I also tried system() with no success and i read that exec is a much cleaner solution

Answer 1

You are misinterpreting the specifications of execlp(). The first argument, sometimes documented as path (but more often so for different exec functions), is expected to be a path all the way to the executable, not stopping short at the directory containing it. The variable arguments then designate all elements to be provided as process's argv vector, including the zeroth, which conventionally names the program being executed. Thus:
execlp("home/user/my_shell_script.sh", "my_shell_script.sh", (char *)0);
Note, however, that when the path you give contains any slash characters, as yours does, execlp() is functionally equivalent to execl().
Moreover, your python example (at least) seems to assume, incorrectly, that the first argument gives the initial working directory for the process. If you want to change the working directory then you must do so manually. For example,
chdir("home/user/Printrun");
execlp("python", "pronterface.py", (char *)0);
Furthermore, do be sure to check function return values for errors, and to handle them appropriately (not demonstrated above), though in the case of exec-family functions, returning at all indicates an error.
In that light however, you should recognize that if you're using this in the handler function for a GUI widget then you have a potential problem. Very likely in that case you want to fork() first, so that the new process runs alongside the GUI instead of replacing it, or to use system() instead, which handles all the forking, execing, and waiting for you. Note also that unlike the exec functions, system() runs your command via a shell. For example, then:
system("cd ~/Printrun; python pronterface.py");