Update Dockerfile-No-Jdk

[anjia0532]

fixed when $MIRROR is false but allways use mirror addr

[hengyunabc]

没看懂这个，为什么加了个 x ？

[anjia0532]

一般写bash脚本的一个常用写法

export MIRROR=false
if [ $MIRROR ];  then echo 'true' ; fi

打印的还是true 一般需要判断字符串

export MIRROR=false
if [ "$MIRROR" = "false" ];  then echo "false" ; else echo "true" ; fi
>> false

export MIRROR=true
if [ "$MIRROR" = "false" ];  then echo "false" ; else echo "true" ; fi
>> true

说明符合预期

但是如果MIRROR是空值,就很尴尬。

export MIRROR=true
echo $MIRROR
>> true
unset MIRROR
echo $MIRROR
>> 空
if [ "$MIRROR" = "false" ];  then echo "false" ; else echo "true" ; fi
>> true

if [ "$MIRROR" = "true" ];  then echo "true" ; else echo "false" ; fi
>> false

而加上x，或者别的字母，起码能保证，是两个有值的字符串在比对

export MIRROR=true
“x$MIRROR” = "xtrue" -> xtrue = xtrue -> true
unset MIRROR
“x$MIRROR” = "xtrue" -> x = xtrue -> false

虽然上述步骤，可以通过调整分支语句的顺序，来得到预期效果。但是容易导致意料之外的结果，比如，把 true/false一颠倒，走的都是else，这种问题很隐蔽。很容易成坑。

另外还有种更好理解的一种写法，允许MIRROR是true/y/yes/1 都是使用镜像源(但是在alpine里默认的/bin/sh不支持)

MIRROR=false
if [[ "$MIRROR" =~ ^(true|y|yes|1)$ ]]; then
    echo "$MIRROR is in the list"
else
    echo "$MIRROR is not in the list"
fi

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