if 操作符

[] 用于条件判断，() 用于数学运算：

num=10

if [ $num -eq 10 ]
then
    echo "$num equals to 10"
else
    echo "$num not equals to 10"
fi

if [ $num == 10 ]
then
    echo "$num equals to 10"
else
    echo "$num not euqals to 10"
fi

**(())用于数学运算和条件判断，[[]]**用于字符串比较：

num=9
if [[ $num == 10 ]]
then
    echo "$num equals to 10"
else
    echo "$num not equals to 10"
fi

if (( $num >= 10 ))
then
    echo "$num is not smaller than 10"
else
    echo "$num is smaller than 10"
fi

elif 用于多条件判断，也可以用于嵌套的条件判断：

num=8
if [[ $num = 10 ]]
then
    echo "$num equals to 10"
elif [[ $num < 10 ]]
then
    echo "$num less than 10"
else
    echo "$num bigger than 10"
fi