2017年3月9日 请解释下面Shell脚本中if开头的整行代码的意思及应用场景吗? - xhj8816216/linux- GitHub Wiki

请解释下面Shell脚本中 if开头的整行代码的意思,你见过它的应用场景么? 1 2 3 4 5 6

if(kill -0 $pid 2>/dev/du11) then echo"oldboy" else echo"oldgirl" fi

面试题:请解释if (kill -0 $pid 2>/dev/null)代码的意思? 1 2 3 4 5 6

if(kill -0 $pid 2>/dev/null) then echo "oldboy" else echo "oldgirl" fi

老男孩解答要点:

 kill -0 $pid中的-0表示不发送任何信号给PID对应的进程,但是仍会对变量值PID对应的进程是否存在进行检查,如果$pid对应的进程存在,则返回0,不存在返回1。

 2>/dev/null不输出错误信息。

 系统里的应用场景是MySQL的/etc/init.d/mysqld脚本中停止MySQL服务的脚本代码段。

 使用/etc/init.d/mysqld stop命令执行脚本关闭数据库的程序代码如下:

'stop')

# Stop daemon. We usea signal here to avoid having to know the

# root password.

iftest-s"$mysqld_pid_file_path"

then

  mysqld_pid=`cat "$mysqld_pid_file_path"`

  if (kill-0$mysqld_pid2>/dev/null)

  then

    echo$echo_n"Shutting down MySQL"

    kill$mysqld_pid

    # mysqld shouldremove the pid file when it exits, so wait for it.

    wait_for_pid removed "$mysqld_pid""$mysqld_pid_file_path"; return_value=$?

  else

    log_failure_msg "MySQL server process #$mysqld_pidis not running!"

    rm"$mysqld_pid_file_path"

  fi

  # Delete lock forRedHat / SuSE

  if test-f"$lock_file_path"

  then

    rm-f"$lock_file_path"

  fi

  exit$return_value

else

  log_failure_msg "MySQLserver PID file could not be found!"

fi

;;