里面是进程的并发执行，父进程创建子进程，。父进

程每隔 3 秒重复建立两个子进程，首先创建的让其执行 ls 命令，之后创建执行让其

执行 ps 命令，并控制 ps 命令总在 ls 命令之前执行。-Which is the process of concurrent execution, the parent process creates the child process,. Parent process repeated every 3 seconds to establish two sub-processes, first created allowed the implementation of ls command, and then let the implementation of ps command to create the implementation, and control the ps command are always executed before the ls command.