管道、状态码、循环

#!/bin/bash
#

# ；then是为了整齐

# &>/dev/null 丢到垃圾箱里
if ps -ef | grep mysqld | grep -v grep &>/dev/null;then
echo "MySQL is running"
else
echo "MySQL is STOPPED"
fi

#!/bin/bash
#
#注意这里[]前后必须有空格
if [ $1 -eq $2 ];then
        echo "$1 = $2"
elif [ $1 -gt $2 ];then
        echo "$1 > $2"
elif [ $1 -lt $2 ];then
        echo "$1 < $2"
fi

#!/bin/bash
#

var1="hello"
var2="world"

#因为 这里的 < 是重定向
if [ $var1 \< $var2 ];then
        echo "<"
else
        echo ">"
fi

#!/bin/bash
#

var1=""
var2="world"

if [  -n "$var1"  ];then
        echo "not null"
else
        echo "null"
fi