003-老男孩教育每日一题-kill-0判断的意思及应用场景

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?

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

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

if(kill -0 $pid 2>/dev/null)

then

echo "oldboy"

else

echo "oldgirl"

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.

if test-s"$mysqld_pid_file_path"

then

mysqld_pid=`cat "$mysqld_pid_file_path"`

if (kill -0 $mysqld_pid 2>/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

;;

'stop')

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

# root password.

if test-s"$mysqld_pid_file_path"

then

mysqld_pid=`cat "$mysqld_pid_file_path"`

if (kill -0 $mysqld_pid 2>/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"

# Delete lock forRedHat / SuSE

if test-f"$lock_file_path"

then

rm-f"$lock_file_path"

exit$return_value

else

log_failure_msg "MySQLserver PID file could not be found!"

;;

李泳谊李导的博客请解释下面Shell脚本

今天是老男孩教育每日一题陪伴大家的第3天。

对于题目和答案的任何疑问，请在博客评论区留言。

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