的变量为空白谓词相当于变量没有定义,例如在命令行方面:
==>set Softupgradeversion
Environment variable Softupgradeversion not defined
要检查的批处理脚本谓:
if "%Softupgradeversion%"=="" (
echo Softupgradeversion is empty
) else (
echo "%Softupgradeversion%"
)
或
if not defined Softupgradeversion (
echo Softupgradeversion is empty
) else (
echo "%Softupgradeversion%"
)
注意在if "%Softupgradeversion%"==""
适当双引号。
实际上,if €%Softupgradeversion%€==€€
甚至if €%Softupgradeversion%==€
与几乎任何字符代替€
的效果几乎相同。然而,双引号是为变量的最安全的方法可以包含空格,甚至一些CLI-特殊字符应该是escaped如下:
==>set "unusual=a space"
==>if "%unusual%"=="a space" echo good
good
==>if [%unusual%]==[a space] echo good
space]==[a was unexpected at this time.
甚至
==>set "unusual=& > < |"
==>set unusual
unusual=& > < |
==>echo "%unusual%"
"& > < |"
添加的语言标记。 –
使用'IF'%Softupgradeversion%“==”“('..也就是说,用可能为空的值的双引号括起来,并且将来你应该发布你实际得到的内容,而不仅仅是”它不会“ t“工作” – TripeHound