Question

I need help to get the following to work. I need to start a bash process from c++, this bash process needs to accept input from stdin and output as per normal it’s output to stdout.

From a different process I need to write commands to stdin which will then actually execute in bash as per above, then I’m interested in the result from stdout.

This is what I’ve tried so far, but the output does not make sense to me at all…

        if (pipe(pipeBashShell)) {
            fprintf(stderr, "Pipe error!\n");
            exit(1);
        }

        if ((pipePId = fork()) == -1) {
            fprintf(stderr, "Fork error. Exiting.\n"); /* something went wrong */
            exit(1);
        }

        if (pipePId == 0) { //this is the child process
            dup2(pipeBashShell[0], STDIN_FILENO);
            dup2(pipeBashShell[1], STDOUT_FILENO);
            dup2(pipeBashShell[1], STDERR_FILENO);

            static char* bash[] = {"/bin/bash", "-i", NULL};
            if (execv(*bash, bash) == -1) {
                fprintf(stderr, "execv Error!");
                exit(1);
            }
            exit(0);
        } else {
            char buf[512];
            memset(buf, 0x00, sizeof(buf));
            sprintf(buf, "ls\n");
            int byteswritten = write(pipeBashShell[1], buf, strlen(buf));
            int bytesRead = read(pipeBashShell[0], buf, sizeof(buf));

            write(STDOUT_FILENO, buf, strlen(buf));
            exit(0);
        }

.

The output of the result above is as follows:

‘ (main)
bash:: command not found gerhard@gerhard-work-pc:~/workspaces/si/si$ gerhard
orkspaces/si/si$ gerhard@ gerhard-work-pc:~/workspa
….

The command i’m trying to send to bash is “ls”, which should give me a directory listing

Am I missing something here?

Answer 1

You have created one pipe (with two ends) and you are trying to use it for bi-directional communication — from your main process to bash and vice versa. You need two separate pipes for that.

The way you have connected the file descriptors makes bash talk to itself — it interprets its prompt as a command which it cannot find, and then interprets the error messages as subsequend commands.

Edit:

The correct setup works as follows:

1. prepare two pipes:

   int parent2child[2], child2parent[2];
   pipe(parent2child);
   pipe(child2parent);
   

2. fork()

3. in the parent process:

   close(parent2child[0]);
   close(child2parent[1]);
   // write to parent2child[1], read from child2parent[0]
   

4. in the child process:

   close(parent2child[1]);
   close(child2parent[0]);
   dup2(parent2child[0], STDIN_FILENO);
   dup2(child2parent[1], STDOUT_FILENO);