BASH 脚本：当索引是变量时 $@ 的第 n 个参数？
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BASH scripting: n-th parameter of $@ when the index is a variable?
提问by nosid
I want to retrieve the n-th parameter of $@ (the list of command line parameters passed to the script), where n is stored in a variable.
我想检索 $@ 的第 n 个参数（传递给脚本的命令行参数列表），其中 n 存储在一个变量中。
I tried ${$n}.
我试过 ${$n}。
For example, I want to get the 2nd command line parameter of an invocation:
例如，我想获取调用的第二个命令行参数：
./my_script.sh alpha beta gamma
And the index should not be explicit but stored in a variable n.
并且索引不应该是显式的，而是存储在变量 n 中。
Sourcecode:
源代码：
n=2
echo ${$n}
I would expect the output to be "beta", but I get the error:
我希望输出为“测试版”，但出现错误：
./my_script.sh: line 2: ${$n}: bad substitution
What am I doing wrong?
我究竟做错了什么？
采纳答案by konqi
Try this:
尝试这个：
#!/bin/bash
args=("$@")
echo ${args[1]}
okay replace the "1" with some $n or something...
好的，用一些 $n 或其他东西替换“1”...
回答by nosid
You can use variable indirection. It is independent of arrays, and works fine in your example:
您可以使用变量间接。它独立于数组，在您的示例中工作正常：
n=2
echo "${!n}"
Edit:Variable Indirectioncan be used in a lot of situations. If there is a variable foobar, then the following two variable expansions produce the same result:
编辑：变量间接可以在很多情况下使用。如果存在变量foobar，则以下两个变量扩展会产生相同的结果：
$foobar
name=foobar
${!name}
回答by Smi
The following works too:
以下也有效：
#!/bin/bash
n=2
echo ${@:$n:1}
回答by Jens
The portable (non-bash specific) solution is
便携式（非 bash 特定）解决方案是
$ set a b c d
$ n=2
$ eval echo ${$n}
b
回答by Summer_More_More_Tea
evalcan help you access the variable indirectly, which means evaluate the expression twice.
eval可以帮助您间接访问变量，这意味着对表达式求值两次。
You can do like this eval alph=\$$n; echo $alph
你可以这样做 eval alph=\$$n; echo $alph
