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I have a C++ program from which I want to execute multiple commands in a shell. My current solution use the system() function and looks like this:

return_value = system(SETUP_ENVIRONMENT; RUN_USEFUL_APP_1);
... do_something_else ...
return_value = system(SETUP_ENVIRONMENT; RUN_USEFUL_APP_2);
... do_something_else ...
return_value = system(SETUP_ENVIRONMENT; RUN_USEFUL_APP_3);
...

It works, but SETUP_ENVIRONMENT takes a few seconds making the program really slow. But I have to run it every time since system() runs in a new shell each time. I want to be able to setup my shell once and then run all commands in it.

execute_in_shell(SETUP_ENVIRONMENT);
return_value = execute_in_shell(RUN_USEFUL_APP_1);
... do_something_else ...
return_value = execute_in_shell(RUN_USEFUL_APP_2);
... do_something_else ...
return_value = execute_in_shell(RUN_USEFUL_APP_3);
...

How do I do that?

I'm on Linux.

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9
  • 1
    You could open a shell in a pipe and send the commands through it. Commented Jan 7, 2016 at 10:32
  • 1
    stackoverflow.com/questions/245600/… Commented Jan 7, 2016 at 10:34
  • 1
    @mstruebing: That presumes that you know up front which programs to run, when, and with which arguments. The pipe is a better solution. Commented Jan 7, 2016 at 11:23
  • How do I open a shell in a pipe? Can I see return codes if I do that? Commented Jan 7, 2016 at 11:40
  • @oskla129 system's return value indicates whether the command could be executed at all, not the command's result. Commented Jan 7, 2016 at 12:26

2 Answers 2

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1

Alternatively to answer 1, you could also use your program to create a shell script which will run all your useful programs and execute this script at once. Then the shell won't be started each time for each particular useful program.

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1 Comment

I am not sure what the setting environment is, but as I understood, the most of time is consumed exactly by it, right? Perhaps then you could set the required environment for your main process, and then all spawned processes will inherit it automatically so, that you don't need to set it up for each particular useful program? or think over some other way to share the environment to them if possible (i.e. via a file or shared memory)
0

You have three reasonable options for doing this, depending on your specific need.

If the various calls you make to external tools are part of coherent routine, then you can – and probably should – follow @dmi's advice and write a short shell script that you can call from your C++ program.

If you instead need to start procedures here and there, you might be interested into running the shell as an inferior process and attaching your program to it – so that instead of talking with your terminal, the shell process talks to your C++ program.

This method is not very difficult but has a few gotchas (for instance, some programs like ssh, sudo or docker may expect to be attached to a tty). It is very well covered in most introductions to system programming (look for inter process communication and subprocesses) for any Unix variant. Let me outline that procedure:

  1. use the pipe system call to create pipes (stdin_r, stdin_w)
  2. use the pipe system call to create pipes (stdout_r, stdout_w)
  3. use the pipe system call to create pipes (stderr_r, stderr_w)
  4. use the fork system call to duplicate your program
  5. In the child, you close stdin_w, stdout_r, stderr_r, and use the exec system call parametrised by stdin_r, stdout_w, stderr_w to run the shell.
  6. In the parent, you close stdin_r, stdout_w, stderr_w, and you can now write commands in stdin_w, and read command output from stdout_r and stderr_r.

(This intentionally very sketchy, I included the outline only so that you are sure you found the right place in your favourite textbook).

There are third party libraries implementing all that low-level stuff for you. You can use boost::process (which is not yet an official part of boost now) whose usage is illustrated with a full tutorial. There are plenty of alternatives such as pstreams.

The third option would be to avoid using the shell and executing directly shell commands you use. This is the approach followed by Rashell, an OCaml library defining primitives allowing to reliably compose sub-processes, which you can use for your own inspiration.

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