可用于检测所指定的进程是否存在。原理为：一个进程运行后会在/proc下创建以其进程号为名的目录，此目录下的cmdline文件中保存有进程名。-Can be used to detect the existence of the specified process. Principle is: will create its process ID in the name of the directory under/proc after a process running, cmdline files in this directory name in the preservation process.