应用层如何执行shell脚本，通过启动一个shell脚本服务执行
private fun execCmd(cmd: String, listener: (isExe: Boolean, result: String?) -> Unit) {
 Thread(Runnable {
 val cmdPath = "/storage/emulated/legacy/cmd"
 val retPath = "/storage/emulated/legacy/ret"
 FileIOUtils.writeFileFromString(cmdPath, cmd)
 FileIOUtils.writeFileFromString(retPath, "")
 SystemProperties.setProp("ctl.stop", "sdcard_it")
 SystemProperties.setProp("persist.sys.ret.status", "0")
 SystemProperties.setProp("persist.sys.cmd.file", cmdPath)
 SystemProperties.setProp("persist.sys.ret.file", retPath)
 SystemProperties.setProp("ctl.start", "sdcard_it")

 var retryCount = 5
 var isExe = false
 do {
 isExe = SystemProperties.getProp("persist.sys.ret.status") == "1"
 Thread.sleep(200)
 } while (!isExe && retryCount-- > 0)
 SystemProperties.setProp("persist.sys.ret.status", "0")
 val result = FileIOUtils.readFile2String(retPath)
 listener(isExe, result)
 }).start()
 } 
 
sdcard_it为一个服务，但是不启动，在应用层启动
service sdcard_it/system/bin/sh /system/etc/back.sh
     user root
     group root
     disabled
     oneshot