Bash 布尔测试
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Bash Boolean testing
提问by JLMarks
I am attempting to run a block of code if one flag is set to true and the other is set to false. ie
如果一个标志设置为 true 而另一个设置为 false，我将尝试运行一段代码。IE
var1=true
var2=false
if [[ $var1 && ! $var2 ]]; then var2="something"; fi
Since that did not evaluate the way that I expected I wrote several other test cases and I am having a hard time understanding how they are being evaluated.
由于这并没有按照我预期的方式进行评估，因此我编写了其他几个测试用例，并且我很难理解它们是如何被评估的。
aa=true
bb=false
cc="python"
if [[ "$aa" ]]; then echo "Test0" ; fi
if [[ "$bb" ]]; then echo "Test0.1" ; fi
if [[ !"$aa" ]]; then echo "Test0.2" ; fi
if [[ ! "$aa" ]]; then echo "Test0.3" ; fi
if [[ "$aa" && ! "$bb" ]]; then echo "Test1" ; fi
if [[ "$aa" && ! "$aa" ]]; then echo "Test2" ; fi
if [[ "$aa" ]] && ! [[ "$bb" ]]; then echo "test3" ; fi
if [[ "$aa" ]] && ! [[ "$cc" ]]; then echo "test4" ; fi
if [[ $aa && ! $bb ]]; then echo "Test5" ; fi
if [[ $aa && ! $aa ]]; then echo "Test6" ; fi
if [[ $aa ]] && ! [[ $bb ]]; then echo "test7" ; fi
if [[ $aa ]] && ! [[ $cc ]]; then echo "test8" ; fi
When I run the preceding codeblock the only output I get is
当我运行前面的代码块时，我得到的唯一输出是
Test0
Test0.1
Test0.2
Test0
Test0.1
Test0.2
however, my expectation is that I would get
然而，我的期望是我会得到
Test0
Test1
Test3
Test5
Test7
TEST0
的Test1
Test3的
TEST5
TEST7
I have tried to understand the best way to run similar tests, however most examples I have found are set up in the format of
if [[ "$aa" == true ]];
which is not quite what I want to do. So my question is what is the best way to make comparisons like this, and why do several of the test cases that I would expect to pass simply not?
我试图了解运行类似测试的最佳方法，但是我发现的大多数示例都是以
if [[ "$aa" == true ]];格式设置的。
这不是我想要做的。所以我的问题是进行这种比较的最佳方法是什么，为什么我希望通过的几个测试用例根本没有通过？
Thank you!
谢谢！
回答by Ignacio Vazquez-Abrams
Without any operators, [[only checks if the variable is empty. If it is, then it is considered false, otherwise it is considered true. The contents of the variables do not matter.
没有任何运算符，[[只检查变量是否为空。如果是，那么它被认为是假的，否则它被认为是真的。变量的内容无关紧要。
回答by devnull
Your understanding of booleansin shell context is incorrect.
您对shell 上下文中布尔值的理解是不正确的。
var1=true
var2=false
Both the above variables are truesince those are non-empty strings.
上述两个变量都为真，因为它们是非空字符串。
You could instead make use of arithmetic context:
您可以改为使用算术上下文：
$ a=1
$ b=0
$ ((a==1 && b==0)) && echo y
y
$ ((a==0 && b==0)) && echo y
$
$ ((a && !(b))) && echo y; # This seems to be analogous to what you were attempting
y
回答by Idriss Neumann
trueand falseare evaluated as strings ;)
true并被false评估为字符串 ;)
[[ $var ]]is an equivalent of [[ -n $var ]]that check if $varis empty or not.
[[ $var ]]相当于[[ -n $var ]]检查是否$var为空。
Then, no need to quote your variables inside [[. See this reminder.
然后，无需在[[. 看到这个提醒。
Finally, here is an explication of the difference between &&inside brackets and outside.
回答by chepner
The shell does not have Boolean variables, per se. However, there are commandsnamed trueand falsewhose exit statuses are 0 and 1, respectively, and so can be used similarly to Boolean values.
外壳本身没有布尔变量。但是，有一些命令命名true和false退出状态分别为 0 和 1，因此可以与布尔值类似地使用。
var1=true
var2=false
if $var1 && ! $var2; then var2="something"; fi
The difference is that instead of testing if var1is set to a true value, you expand it to the name of a command, which runs and succeeds. Likewise, var2is expanded to a command name which runs and fails, but because it is prefixed with !the exit status is inverted to indicate success.
不同之处在于，不是测试 ifvar1是否设置为真值，而是将其扩展为命令的名称，该命令运行并成功。同样，var2扩展为运行并失败的命令名称，但由于它以!退出状态为前缀，因此被反转以指示成功。
(Note that unlike most programming languages, an exit status of 0 indicates success because while most commands have 1 way to succeed, there are many different ways they could fail, so different non-zero values can be assigned different meanings.)
（请注意，与大多数编程语言不同，退出状态 0 表示成功，因为虽然大多数命令有 1 种成功方式，但它们可能会以多种不同方式失败，因此可以为不同的非零值分配不同的含义。）
回答by Brandonian
The closest you can come seems to be use functions instead of variables because you can use their return status in conditionals.
最接近的似乎是使用函数而不是变量，因为您可以在条件中使用它们的返回状态。
$ var1() { return 0; }
$ var2() { return 1; } # !0 = failure ~ false
and we can test this way
我们可以这样测试
$ var1 && echo "it's true" || echo "it's false"
it's true
$ var2 && echo "it's true" || echo "it's false"
it's false
or this way
或者这样
$ if var1; then echo "it's true"; else echo "it's false"; fi
it's true
$ if var2; then echo "it's true"; else echo "it's false"; fi
it's false
Hope this helps.
希望这可以帮助。
