Shell判断字符串（变量）是否为空

您可以将 -z 选项传递给 if 命令或条件表达式。如果STRING的长度为0,variable ($var)为空。test命令用于检查文件类型并比较值。这个页面显示了如何使用test命令确定bash shell变量是否为空。

判断 bash 变量是否为空：

要点：变量两端用双引号括起来

1. 如果bash变量未设置或设置为空字符串，则返回true: if [ -z "$var" ];
2. 另一个选择: [ -z "$var" ] && echo "Empty"
3. 确定bash变量是否为空: [[ ! -z "$var" ]] && echo "Not empty" || echo "Empty"

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Bash Shell 判断一个变量是否为空

让我们看看语法和例子的细节。if命令的语法如下:

if [ -z "$var" ]
then
      echo "\$var is empty"
else
      echo "\$var is NOT empty"
fi

OR

if test -z "$var" 
then
      echo "\$var is empty"
else
      echo "\$var is NOT empty"
fi

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检查bash shell变量是否为空的另一个方式

您还可以尝试控制操作符。语法如下:

[ -z "$var" ] && echo "Empty"
[ -z "$var" ] && echo "Empty" || echo "Not empty"

OR

[[ -z "$var" ]] && echo "Empty"
[[ -z "$var" ]] && echo "Empty" || echo "Not empty"

OR

## 检查 $var 是否设置使用! 即检查 expr 是否为假 ##
[ ! -z "$var" ] || echo "Empty"
[ ! -z "$var" ] && echo "Not empty" || echo "Empty"
 
[[ ! -z "$var" ]] || echo "Empty"
[[ ! -z "$var" ]] && echo "Not empty" || echo "Empty"

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