为什么 9 之后的 bash 命令行参数需要大括号？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/18318716/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Why do bash command line arguments after 9 require curly brackets?
提问by Default
This may not be the most thought provoking question, but nevertheless has struck my curiosity. I have not been able to come across any answer (let alone a definitive one) on the web.
这可能不是最发人深省的问题，但仍然引起了我的好奇。我无法在网络上找到任何答案（更不用说确定的答案了）。
While reading Advanced Shell Scripting, I came across this sectionregarding command line positional arguments which states that anything after the the ninth argument must be surrounded by ${} (the longer form of variable referencing/substitution).
在阅读 Advanced Shell Scripting 时，我遇到了有关命令行位置参数的这一部分，该部分指出第九个参数之后的任何内容都必须用 ${}（变量引用/替换的较长形式）括起来。
Simply put, why must you reference command line argument ten (and beyond) as ${10}, ${11}...instead of $10, $11, ...?
简而言之，为什么必须引用命令行参数十（及以后）作为${10}, ${11}...而不是$10, $11, ...？
采纳答案by dtorgo
Specifically, your question relates to "positional parameters." Using $var instead of ${var} is shorthand in bash. In most cases it works well. Bash variables must start with a letter or underscore. It internally treats variables that start with a digit as a "positional parameter." When bash detects a positional parameter it only looks at the first digit, which is why $10 returns $1"0". By calling ${10} you are instructing bash to look at the complete variable instead of its built-in default of the first digit.
具体来说，您的问题与“位置参数”有关。使用 $var 而不是 ${var} 是 bash 中的简写。在大多数情况下，它运行良好。Bash 变量必须以字母或下划线开头。它在内部将以数字开头的变量视为“位置参数”。当 bash 检测到位置参数时，它只查看第一位数字，这就是 $10 返回 $1"0" 的原因。通过调用 ${10}，您是在指示 bash 查看完整的变量，而不是它的第一个数字的内置默认值。
As to why it is this way? I have no idea. Legacy implementation which has been expanded upon is my guess. "Who would ever need more than....?"
至于为什么会这样？我不知道。已扩展的遗留实现是我的猜测。“谁会需要比……更多的东西？”
