blocks|key|2864730|text|${parameter:-word}是parameter+expansion的一种|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|2864731|2864732|+${parameter:-word}|blockquote|BOLD|2864733|2864734|如果参数未设置或为null，则替换word的扩展。否则，将替换参数的值。|2864735|2864736|2864737|因此，${@:-1}被解释为：|2864738|1|2864739|Else+|unordered-list-item|2864740|
|2864741|如果$@未设置或为null，则替换为将$@保留为|2864742|2864743|因为后者是真的，所以echo打印$@是非常出色的|2864744|2864745|解决方法：|2864746|在:和-之间留出一些空间|2864747|$+echo+${@:+-1}
f|code-block|syntax|javascript|2864748|entityMap|0|LINK|mutability|MUTABLE|url|https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion^0|0|I|J|J|0|0|0|1|I|1|I|0|0|0|0|0|3|7|0|0|1|0|0|0|2|2|J|2|0|0|A|4|G|2|0|0|0|1|1|3|1|0|0^^$0|@$1|2|3|4|5|6|7|1L|8|@$9|1M|A|1N|B|C]]|D|@$9|1O|A|1P|1|1Q]]|E|$]]|$1|F|3|-4|5|6|7|1R|8|@]|D|@]|E|$]]|$1|G|3|H|5|I|7|1S|8|@$9|1T|A|1U|B|J]|$9|1V|A|1W|B|C]]|D|@]|E|$]]|$1|K|3|-4|5|6|7|1X|8|@]|D|@]|E|$]]|$1|L|3|M|5|6|7|1Y|8|@]|D|@]|E|$]]|$1|N|3|-4|5|6|7|1Z|8|@]|D|@]|E|$]]|$1|O|3|-4|5|6|7|20|8|@]|D|@]|E|$]]|$1|P|3|Q|5|6|7|21|8|@$9|22|A|23|B|C]]|D|@]|E|$]]|$1|R|3|S|5|6|7|24|8|@$9|25|A|26|B|C]]|D|@]|E|$]]|$1|T|3|U|5|V|7|27|8|@]|D|@]|E|$]]|$1|W|3|X|5|6|7|28|8|@]|D|@]|E|$]]|$1|Y|3|Z|5|V|7|29|8|@$9|2A|A|2B|B|C]|$9|2C|A|2D|B|C]]|D|@]|E|$]]|$1|10|3|-4|5|6|7|2E|8|@]|D|@]|E|$]]|$1|11|3|12|5|6|7|2F|8|@$9|2G|A|2H|B|C]|$9|2I|A|2J|B|C]]|D|@]|E|$]]|$1|13|3|-4|5|6|7|2K|8|@]|D|@]|E|$]]|$1|14|3|15|5|6|7|2L|8|@]|D|@]|E|$]]|$1|16|3|17|5|6|7|2M|8|@$9|2N|A|2O|B|C]|$9|2P|A|2Q|B|C]]|D|@]|E|$]]|$1|18|3|19|5|1A|7|2R|8|@]|D|@]|E|$1B|1C]]|$1|1D|3|-4|5|6|7|2S|8|@]|D|@]|E|$]]]|1E|$1F|$5|1G|1H|1I|E|$1J|1K]]]]

<h3><code>${parameter:-word}</code> is a type of <a href="https://www.gnu.org/software/bash/manual/bash.html#Shell-Parameter-Expansion">parameter expansion</a>:</h3>

<blockquote>
 <code>${parameter:-word}</code>
 
 If parameter is unset or null, the expansion of word is substituted.
 Otherwise, the value of parameter is substituted.
</blockquote>

<hr>

So <code>${@:-1}</code> is interpreted as:

<ul>
<li>If <code>$@</code> is unset or null, substitute with <code>1</code></li>
<li>Else leave <code>$@</code> as it is</li>
</ul>

Since the latter is true, <code>echo</code> prints <code>$@</code> in all its glory

<hr>

<h3>The Fix:</h3>

Put some space between <code>:</code> and <code>-</code>:

<pre><code>$ echo ${@: -1}
f
</code></pre>

blocks|key|109142|text|作为Zaid建议的空格的替代语法，您还可以使用圆括号来获得所需的功能：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|109143|echo+${@:(-1)}|code-block|syntax|javascript|109144|109145|+f+|blockquote|109146|109147|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|O|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|P|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|Q|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|R|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|M|$]]

As an alternative syntax to the space suggested by Zaid, you can also use round brackets to get the functionality you want:

<pre><code>echo ${@:(-1)}
</code></pre>

<blockquote>
 f 
</blockquote>

blocks|key|111208|text|一些解释|type|unstyled|depth|inlineStyleRanges|entityRanges|data|111209|请查看bash手册页中的参数扩展一节：|offset|length|style|BOLD|111210|111211|blockquote|111212|111213|111214|如果$@为空，语法${@:-1}将返回1+...|CODE|111215|echo+${@:-1}
a+b+c+d+e+f

set+--
echo+${@:-1}
1

echo+${@:-It+is.}
It+is.|code-block|syntax|javascript|111216|任何违反:-语法的问题都可以解决。|111217|set+--+alpha+beta+ceta+delta+enigma+foo
echo+${@:+-1}+
foo
echo+${@:0-1}
foo
echo+${@:(-1)}
foo
echo+${@:10#-1}
foo
echo+${@:1*-1}
foo|111218|所有阵列都以相同的方式受到影响。|111219|array=(Hello+good+world)

echo+${array[@]:-1}
Hello+good+world

echo+${array[@]:0-1}
world|111220|entityMap|0|LINK|mutability|MUTABLE|url|/questions/tagged/bash^0|0|C|4|3|4|0|0|0|0|0|0|2|2|9|7|J|1|0|0|4|2|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1B|8|@$D|1C|E|1D|F|G]]|9|@$D|1E|E|1F|1|1G]]|A|$]]|$1|H|3|-4|5|6|7|1H|8|@]|9|@]|A|$]]|$1|I|3|-4|5|J|7|1I|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|1J|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|1K|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|1L|8|@$D|1M|E|1N|F|O]|$D|1O|E|1P|F|O]|$D|1Q|E|1R|F|O]]|9|@]|A|$]]|$1|P|3|Q|5|R|7|1S|8|@]|9|@]|A|$S|T]]|$1|U|3|V|5|6|7|1T|8|@$D|1U|E|1V|F|O]]|9|@]|A|$]]|$1|W|3|X|5|R|7|1W|8|@]|9|@]|A|$S|T]]|$1|Y|3|Z|5|6|7|1X|8|@]|9|@]|A|$]]|$1|10|3|11|5|R|7|1Y|8|@]|9|@]|A|$S|T]]|$1|12|3|-4|5|6|7|1Z|8|@]|9|@]|A|$]]]|13|$14|$5|15|16|17|A|$18|19]]]]

<h3>Some explanations</h3>

Have a look under section Parameter Expansion in the <a href="/questions/tagged/bash" class="post-tag" title="show questions tagged &#39;bash&#39;" rel="tag">bash</a>'s man page:

<blockquote>
 <ul>
 <li><code>${parameter:-word}</code> -> Use Default Values. If parameter is unset or null, the expansion of word is substituted...</li>
 <li><code>${parameter:=word}</code> -> Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter...</li>
 <li><code>${parameter:?word}</code> -> Display Error if Null or Unset. If parameter is null or unset, the expansion of word is written to the standard error and the shell, ...</li>
 <li>...</li>
 </ul>
</blockquote>

Well, the syntax <code>${@:-1}</code> will return <code>1</code> if <code>$@</code> is empty...

<pre><code>echo ${@:-1}
a b c d e f

set --
echo ${@:-1}
1

echo ${@:-It is.}
It is.
</code></pre>

Working around could be done with anything that break <code>:-</code> syntax.

<pre><code>set -- alpha beta ceta delta enigma foo
echo ${@: -1} 
foo
echo ${@:0-1}
foo
echo ${@:(-1)}
foo
echo ${@:10#-1}
foo
echo ${@:1*-1}
foo
</code></pre>

<h3>All arrays are affected in same manner.</h3>

<pre><code>array=(Hello good world)

echo ${array[@]:-1}
Hello good world

echo ${array[@]:0-1}
world
</code></pre>

I thought to post up a Q&amp;A on this as I did not find anything similar. If it already exists, please mark this as a duplicate.

<hr>

The following code, running under Bash shell, doesn't work (should return just <code>f</code>, the last (-1-th) item in <code>$@</code>):

<pre><code>$ set -- a b c d e f
$ echo ${@:-1}
a b c d e f
</code></pre>

Why doesn't ${@:-1} return the last element of $@?

我想发布一个关于这个的问答，因为我没有找到任何类似的东西。如果已经存在，请将其标记为副本。在Bash shell下运行的以下代码不起作用(应该只返回$@中的最后一个(第-1个)项f )：$ set -- a b c d e f$ echo ${@:-1}a b c d e f

问为什么${@:-1}不返回$@的最后一个元素？
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么${@:-1}不返回$@的最后一个元素？EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问为什么${@:-1}不返回$@的最后一个元素？
EN