Get Commad Line Output

Sometimes we need get the output from a cmd tool. Generally , the cmd output infomation into stdout or stderr.

We use two method to get the output from the cmd line then used in our application.

First, assume we have a cmd tool called cmdtool, the following command we can run on the termianl:

./cmdtool -i input_file -o output_file -a 1

1, popen

 FILE *f_p = popen(cmd_str.toStdString().c_str(), "r");
 QString p_output = "";
 char buf[1024];
 if (f_p != NULL)
 {
      while (fgets(buf, 1024, f_p) != NULL)
      {
            // DO SOMETHING
      }        
}
        

2, system

 int PIPEDES[2];
    /* Create a one-way communication channel (pipe).
       If successful, two file descriptors are stored in PIPEDES;
       bytes written on PIPEDES[1] can be read from PIPEDES[0].
       Returns 0 if successful, -1 if not.  */
    if(pipe(PIPEDES)) // PIPEDES[0] read pipe, PIPEDES[1], write pipe
    {
        printf("pipe error, exit!");
        return -1;
    }
    /* Duplicate FD, returning a new file descriptor on the same file.  */
    int bak_fd = dup(STDOUT_FILENO);
    /* Duplicate FD to FD2, closing FD2 and making it open on the same file.  */
    int new_fd = dup2(PIPEDES[1], STDOUT_FILENO);

    // output is write into fd[1]
    system(pComd);
    //write(PIPEDES[1], c, sizeof(c) / sizeof(c[0]) - 1);
    read(PIPEDES[0], result, size);
    dup2(bak_fd, new_fd);
    close(PIPEDES[0]);
    close(PIPEDES[1]);
    return 0;