du | sort -n

du_sort.c

pid_t sortPID = fork();
if(sortPID < 0) 
    perror("cannot fork a process for sort");
else if(sortPID == 0){
    //Forked process --> child: sort.
 	int fd[2];
    //sort child opens the pipes.
    if (pipe(fd) < 0) 
        perror("cannot create pipe");
    close(fd[1]); //We're not going to write to the pipe, so close it. 
 	//Child forks to create grandchild.
    pid_t duPID = fork();
	if(duPID < 0) 
        perror("cannot fork for du");
    else if (duPID == 0) {
        //Forked process --> grand child: du
        close(fd[0]); //We won't read from the pipe, so close it.        
        dup2(fd[1], 1); //Since du will use the write-end of the pipe created (by the forked process of sort), we need to duplicate it to file descriptor 1.
        close(fd[1]);//We will no longer access the write end of the pipe through fd[1]; we can close it.
        execlp("du"); //This command should not return if there is no error
        perror("du failed to execute properly"); //Print error in execution of du; e.g. du could not be found in $PATH
    } else {
    //Forked process --> child: sort
        dup2(fd[0], 0);//The read end of the pipe will be used as standard input by sort
        execlp("sort", "-n");//Again this should not return unless there is an error
        perror();//Print error in executing of sort
    }
} else {
    int status;
    if (waitpid(c,&status, NULL) < 0)//waitpid returns a value less than 0; if it encounters an error. Here, we wait for sort. 
        perror("cannot wait for sort");
}