bash eval as if 语句条件
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eval as if statement condition
提问by AlphaBeta
Why is the output of the following statement true?
为什么以下语句的输出为真？
VAR="[[ "x"=="y" ]]"
if eval $VAR ; then
echo "true"
else
echo "false"
fi
Is there any other way to use a variable as condition for an if statement in bash?
有没有其他方法可以使用变量作为 bash 中 if 语句的条件？
采纳答案by anubhava
Your quoting is an issue and you need spaces around ==:
您的引用是一个问题，您需要在周围留出空格==：
VAR='[[ "x" == "y" ]]'
if eval $VAR ; then
echo "true"
else
echo "false"
fi
However you must avoid using evalas far as possible. You can use a function here:
但是，您必须eval尽可能避免使用。您可以在此处使用函数：
fvar() { [[ "x" == "y" ]]; }
Then call it as:
然后将其称为：
if fvar; then
echo "true"
else
echo "false"
fi
Reasonswhy your evalcondition is evaluating to truebecause you have VAR="[[ "x"=="y" ]]"where the text inside [[and ]]is being treated as a single string. Which is same as evaluating [[ a ]]or [[ foo ]]
原因为什么你的eval病情正在评估到true，因为你VAR="[[ "x"=="y" ]]"里面的文字，其中[[和]]被视为一个字符串。这与评估[[ a ]]或[[ foo ]]
回答by Scott Plante
My first answer attempt was voted down, so I'll try to clarify in hopes it at least makes it back to 0 and add something of value considering the other answer. I'll admit I was a bit unclear what you were asking.
我的第一次回答尝试被否决了，所以我会尝试澄清，希望它至少让它回到 0 并考虑其他答案添加一些有价值的东西。我承认我有点不清楚你在问什么。
First, are your x and y supposed to be literals or variables? Part of the reason I'm uncertain is the extra quoting in the assignment. Note these are equivalent:
首先，你的 x 和 y 应该是文字还是变量？我不确定的部分原因是作业中的额外引用。请注意，这些是等效的：
$ VAR="[[ "x"=="y" ]]"
$ echo $VAR
[[ x==y ]]
$ VAR="[[ x==y ]]"
$ echo $VAR
[[ x==y ]]
So in this case, saying
所以在这种情况下，说
if eval $VAR ; then
is the same as saying
和说一样
if [[ x==y ]]
When you're having trouble with eval, you'll want to print out the value of your variable like this to see what's being executed.
当您在使用 eval 时遇到问题时，您需要像这样打印出变量的值以查看正在执行的内容。
The x==y is one argument, and the [[ operator on a single string argument says, "is this a non-zero length string?" For example:
x==y 是一个参数，单个字符串参数上的 [[ 运算符表示，“这是一个非零长度的字符串吗？” 例如：
$ if [[ dkfjek ]]; then echo true; else echo false; fi
true
$ if [[ any_non_zero_length_string ]]; then echo true; else echo false; fi
true
$ if [[ "" ]]; then echo true; else echo false; fi
false
To use the == you need whitespace. For example, without and with eval:
要使用 == 您需要空格。例如，没有和有 eval：
$ if [[ x == x ]]; then echo true; else echo false; fi
true
$ if [[ x == y ]]; then echo true; else echo false; fi
false
$ VAR="[[ x == x ]]"; if eval $VAR; then echo true; else echo false; fi
true
$ VAR="[[ x == y ]]"; if eval $VAR; then echo true; else echo false; fi
false
Usually though you really want to avoid eval unless you've really nailed down what the value of the variable can be. Consider a code snippet where you let the user decide the condition to test: you might write something like this in a script:
通常虽然你真的想避免 eval ，除非你真的确定了变量的值可以是什么。考虑一个让用户决定要测试的条件的代码片段：您可以在脚本中编写如下内容：
read -p "type condition> " COND;
VAR="[[ $COND ]]";
if eval $VAR;
then
echo true;
else
echo false;
fi
You might run it expecting input/output like:
您可能会在期望输入/输出的情况下运行它，例如：
type condition> x == x
true
But what if someone entered this?
但是如果有人输入了这个怎么办？
type condition> x == x ]]; touch /tmp/oh-no; [[ true
true
What was $VAR? What about /tmp/oh-no?
什么是 $VAR？/tmp/oh-no 怎么样？
$ echo $VAR
[[ x == x ]]; touch /tmp/oh-no; [[ true ]]
$ ls -l /tmp/oh-no
-rw-r--r-- 1 splante isiusers 0 Nov 3 15:17 /tmp/oh-no
Remember the syntax is "if <command>". The "[[" is just one possible command, but any compound command can be executed following if. Now suppose instead of touch /tmp/oh-nothe use entered some other more destructive command? It would have executed too.
记住语法是“if <command>”。"[[" 只是一个可能的命令，但任何复合命令都可以在 if 之后执行。现在假设不是touch /tmp/oh-no使用输入了一些其他更具破坏性的命令？它也会被执行。
You asked "Is there any other way to use a variable as condition for an if statement in bash?" This is the typical way to compare two variables for string equivalence in bash:
您问“有没有其他方法可以将变量用作 bash 中 if 语句的条件？” 这是在 bash 中比较字符串等价的两个变量的典型方法：
if [[ "$x" = "$y" ]]; then ...
I assume now you already knew this, but I almost never use eval and just try to work the problem out differently so you don't need to have the whole condition be a variable.
我假设现在您已经知道这一点，但我几乎从不使用 eval，只是尝试以不同的方式解决问题，因此您不需要将整个条件设为变量。
