Bash 中的嵌套条件（如果 [[...]
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Nested conditions in Bash (if [[...)
提问by SonicGold
I have several questions I would like to be answered since I'm not a Bash expert so far... There are several conditions I need to check before performing a specific task, but some of them are related and cannot be detached : I would like the following statement to be checked :
我有几个问题想要回答，因为到目前为止我还不是 Bash 专家......在执行特定任务之前我需要检查几个条件，但其中一些是相关的并且无法分离：我会像下面要检查的语句：
if ((1 OR 2) AND (3 OR 4))
then
blabla...
fi
I made this so far,
到目前为止，我做了这个，
if [[ "[ `echo "" | cut -c 1-2` -lt 0 || `echo "" | cut -c 1-2` -gt 23 ]" && "[ `echo "" | cut -c 4-5` -lt 0 || `echo "" | cut -c 4-5` -gt 23 ]" ]]
then
echo "La plage horaire indiquée n'est pas valide ! Format = HH-HH"
exit 3
fi
and it works properly. I have the feeling that there's something much easier that can be done in order to perform this check, but I cannot see how...
它工作正常。我有一种感觉，为了执行这项检查，可以做一些更容易的事情，但我看不出如何......
And second thing, in the code I wrote above, you can see this :
第二件事，在我上面写的代码中，你可以看到：
"[ `echo "" | cut -c 1-2` -lt 0 || `echo "" | cut -c 1-2` -gt 23 ]"
The simple square-bracket corresponds to the test function, right ? So why is it properly called when putting double-quotes ? I understand I could not put any backquote there, it would not make any sense with my echo "$5"... thing, but double-quotes are supposed to replace special characters such as $ and prevent from ignoring single spaces, so why is it replacing the [ character ? Is it considered as one of these special characters ?
简单的方括号对应于测试函数，对吗？那么为什么在放置双引号时正确调用它呢？我知道我不能在那里放任何反引号，这对我的 echo "$5"... 没有任何意义，但是双引号应该替换特殊字符，例如 $ 并防止忽略单个空格，那么为什么它取代了 [ 字符 ? 它是否被视为这些特殊字符之一？
Thank you in advance for your answers !
预先感谢您的回答！
回答by glenn Hymanman
Don't repeat yourself:
不要重复自己：
a=$(echo "" | cut -c 1-2)
b=$(echo "" | cut -c 4-5)
or use bash parameter expansion syntax:
或使用 bash 参数扩展语法：
a=${5:0:2}
b=%{5:3:2}
And you can use arithmetic substitution:
您可以使用算术替换：
if (( a < 0 || a > 23)) && (( b < 0 || b > 23 )); then
echo "La plage horaire indiquée n'est pas valide ! Format = HH-HH"
exit 3
fi
回答by Ignacio Vazquez-Abrams
From help [[:
来自help [[：
( EXPRESSION ) Returns the value of EXPRESSION
( EXPRESSION ) Returns the value of EXPRESSION
