blocks|key|208791|text|如果您知道总是要使用[+...+]，那么使用双括号条件复合命令Bash要比使用兼容POSIX的单括号版本Bash容易得多。在[[+...+]]复合中，单词拆分和路径名扩展不适用于单词，因此您可以依赖于|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|208792|if+[[+$aug1+==+"and"+]];|code-block|syntax|javascript|208793|将$aug1的值与字符串and进行比较。|208794|如果使用[+...+]，则始终需要记住使用双引号将变量引起来，如下所示：|208795|if+[+"$aug1"+=+"and"+];|208796|如果没有引用变量扩展，并且变量未定义或为空，则它将从犯罪现场消失，只剩下|208797|if+[+=+"and"+];|208798|这不是有效的语法。(如果$aug1包含空白或外壳元字符，它也会失败，并显示不同的错误消息。)|208799|现代的[[运算符还有许多其他很好的特性，包括正则表达式匹配。|208800|entityMap^0|A|7|1Q|9|0|0|1|5|C|3|0|4|7|0|0|0|0|C|5|0|3|2|0^^$0|@$1|2|3|4|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|15|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|16|8|@$9|17|A|18|B|C]|$9|19|A|1A|B|C]]|D|@]|E|$]]|$1|M|3|N|5|6|7|1B|8|@$9|1C|A|1D|B|C]]|D|@]|E|$]]|$1|O|3|P|5|H|7|1E|8|@]|D|@]|E|$I|J]]|$1|Q|3|R|5|6|7|1F|8|@]|D|@]|E|$]]|$1|S|3|T|5|H|7|1G|8|@]|D|@]|E|$I|J]]|$1|U|3|V|5|6|7|1H|8|@$9|1I|A|1J|B|C]]|D|@]|E|$]]|$1|W|3|X|5|6|7|1K|8|@$9|1L|A|1M|B|C]]|D|@]|E|$]]|$1|Y|3|-4|5|6|7|1N|8|@]|D|@]|E|$]]]|Z|$]]

If you know you're always going to use Bash, it's much easier to always use the double bracket conditional compound command <code>[[ ... ]]</code>, instead of the POSIX-compatible single bracket version <code>[ ... ]</code>. Inside a <code>[[ ... ]]</code> compound, word-splitting and pathname expansion are not applied to words, so you can rely on
<pre><code>if [[ $aug1 == &quot;and&quot; ]];
</code></pre>
to compare the value of <code>$aug1</code> with the string <code>and</code>.
If you use <code>[ ... ]</code>, you always need to remember to double quote variables like this:
<pre><code>if [ &quot;$aug1&quot; = &quot;and&quot; ];
</code></pre>
If you don't quote the variable expansion and the variable is undefined or empty, it vanishes from the scene of the crime, leaving only
<pre><code>if [ = &quot;and&quot; ];
</code></pre>
which is not a valid syntax. (It would also fail with a different error message if <code>$aug1</code> included white space or shell metacharacters.)
The modern <code>[[</code> operator has lots of other nice features, including regular expression matching.

blocks|key|208851|text|您还可以为变量设置一个默认值，这样就不需要使用两个"["，这相当于两个进程("[“实际上是一个程序)，而不是一个进程)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|208852|它遵循以下语法：${VARIABLE:-default}。|208853|整个事情必须以这样一种方式来考虑，即这个“默认”值是与“有效”值/内容不同的东西。|208854|如果由于某种原因这是不可能的，你可能需要添加一个步骤，比如检查是否有值，就像"if+-z+$VARIABLE；then+echo“"the+variable+need+to+be+fine”或"if！-z+$VARIABLE；then+#everything+is+fine，then#+all+is+fine，继续执行脚本的其余部分“。|208855|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

You can also set a default value for the variable, so you don't need to use two "[", which amounts to two processes ("[" is actually a program) instead of one.

It goes by this syntax: ${VARIABLE:-default}.

The whole thing has to be thought in such a way that this "default" value is something distinct from a "valid" value/content.

If that's not possible for some reason you probably need to add a step like checking if there's a value at all, along the lines of "if [ -z $VARIABLE ] ; then echo "the variable needs to be filled"", or "if [ ! -z $VARIABLE ] ; then #everything is fine, proceed with the rest of the script".

blocks|key|208868|text|在if[]语句中使用$aug1之前，请尝试为其赋值；之后，错误消息将消失。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|208869|entityMap^0|1|4|A|5|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

Try assigning a value to <code>$aug1</code> before use it in <code>if[]</code> statements; the error message will disappear afterwards.

blocks|key|426988|text|我花了一段时间才找到它，但请注意，如果你有一个间距错误，你也会得到同样的错误：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|426989|[:+=:+unary+operator+expected|code-block|syntax|javascript|426990|正确：|426991|if+[+"$APP_ENV"+=+"staging"+]|426992|vs|426993|if+["$APP_ENV"+=+"staging"+]|426994|一如既往，设置-x调试变量有助于找到这些变量：|offset|length|style|CODE|426995|set+-x|426996|entityMap^0|0|0|0|0|0|0|7|2|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|11|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|12|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|13|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|14|8|@$Q|15|R|16|S|T]]|9|@]|A|$]]|$1|U|3|V|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|W|3|-4|5|6|7|18|8|@]|9|@]|A|$]]]|X|$]]

It took me a while to find this, but note that if you have a spacing error, you will also get the same error:
<pre><code>[: =: unary operator expected
</code></pre>
Correct:
<pre><code>if [ &quot;$APP_ENV&quot; = &quot;staging&quot; ]
</code></pre>
vs
<pre><code>if [&quot;$APP_ENV&quot; = &quot;staging&quot; ]
</code></pre>
As always, setting <code>-x</code> debug variable helps to find these:
<pre><code>set -x
</code></pre>

This script is getting an error:
<pre><code>elif [ $operation = &quot;man&quot; ]; then
 if [ $aug1 = &quot;add&quot; ]; then # &lt;- Line 75
 echo &quot;Man Page for: add&quot;
 echo &quot;&quot;
 echo &quot;Syntax: add [number 1] [number 2]&quot;
 echo &quot;&quot;
 echo &quot;Description:&quot;
 echo &quot;Add two different numbers together.&quot;
 echo &quot;&quot;
 echo &quot;Info:&quot;
 echo &quot;Added in v1.0&quot;
 echo &quot;&quot;
elif [ -z $aug1 ]; then
 echo &quot;Please specify a command to read the man page.&quot;
else
 echo &quot;There is no manual page for that command.&quot;
fi
</code></pre>
I get this error:
<pre><code>calc_1.2: line 75: [: =: unary operator expected
</code></pre>

"unary operator expected" error in Bash if condition

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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此脚本出现错误：elif [ $operation = "man" ]; then if [ $aug1 = "add" ]; then # <- Line 75 echo "Man Page for: add" echo "" echo "Syntax: add [number...

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