Thread: double fork for implement background

Hi!
I've been tring to do backgrounding by having the first child process fork and then immediately exit, with the grandchild executing the requested program.

But I don't know why I keep getting two copies of output

my code for implementing background:

Code:

if(cmd->background == 1){
    pid_t childPid;

   
    
    if((childPid == fork()) < 0){  /* error */
      perror("BAD FORK");

    }else if(childPid == 0){ /* child process */
     
      exit(0);
    }
    else if((childPid = fork()) == 0){ /*grandchild process */

      execvp(cmd->argvs[0][0], cmd->argvs[0]);
	
    }else{ /* parent process */

      waitpid(childPid, NULL, WNOHANG);
      
    }

Then I ran the command like this "seq 1 5", the result I get was:

Code:

shell: seq 1 5 &
shell: shell: 1
2
3
4
5
1
2
3
4
5

Can anyone tell me what's wrong with my code?

It isn't a series of if / else if

Code:

if ( fork() == 0 ) {
  if ( fork() == 0 ) {
    // grandchild
  } else {
    // child
  }
} else {
  // parent
}