How to catch the ouput from a execl command

c, linux, subprocess

Solution

To do this, you need to open a pipe. You then replace the child's stdout with the writing end of the pipe, and read from the reading end of the pipe in the parent. Like this modified version of your code:

int cmd_quem(void) {
  int result;
  int pipefd[2];
  FILE *cmd_output;
  char buf[1024];
  int status;

  result = pipe(pipefd);
  if (result < 0) {
    perror("pipe");
    exit(-1);
  }

  result = fork();
  if(result < 0) {
    exit(-1);
  }

  if (result == 0) {
    dup2(pipefd[1], STDOUT_FILENO); /* Duplicate writing end to stdout */
    close(pipefd[0]);
    close(pipefd[1]);

    execl("/usr/bin/who", "who", NULL);
    _exit(1);
  }

  /* Parent process */
  close(pipefd[1]); /* Close writing end of pipe */

  cmd_output = fdopen(pipefd[0], "r");

  if (fgets(buf, sizeof buf, cmd_output)) {
    printf("Data from who command: %s\n", buf);
  } else {
    printf("No data received.\n");
  }

  wait(&status);
  printf("Child exit status = %d\n", status);

  return 0;
}

Problem

I'm using the `execl` function to run a Linux process from C. When I do, for example: ``` int cmd_quem() { int result; result = fork(); if(result < 0) { exit(-1); } if (result == 0) { execl("/usr/bin/who", "who", NULL); sleep(4); //checking if father is being polite exit(1); } else { // father's time wait(); } return 0; } ``` I get on the console the result of doing "who" on the terminal. What I'd like to know is if there is any function to "catch" the output result from a command. What I mean is, if there is anyway to catch this: ``` feuplive tty5 2009-11-21 18:20 ``` Which is one of the lines resulting from the who command.

Original source