为什么bash的＆QUOT;回声[T]＆QUOT;导致＆QUOT; T＆QUOT;不＆QUOT; [T]＆QUOT; [英] Why does bash "echo [t]" result in "t" not "[t]"

问题描述

此情况用于字符t和的值根。令人困惑

  $回声[S]
[S]
$回声[T]
ŧ
$回声[T]
[T]
$回声[根]
ŧ
 

解决方案

作为一个没有外壳常客（并不愿意成为），我发现令人惊讶的扩展名是如何设计的时候没有找到匹配的行为。我会报告猛砸参考

  

猛砸扫描每个字的字符 * ，？和 [。如果一个
  这些字符出现，则该字被视为图案，并
  匹配文件名的字母顺序排序列表取代
  模式。如果没有找到匹配的文件名：

  
  

  
• 如果shell选项 nullglob 被禁用，这个词是不变

  
• 如果shell选项 nullglob 置一词被删除

  
• 如果在 failglob 外壳选项设置，错误信息被打印并且不执行命令

  

好消息是，这个东西是可配置的。坏的是脚本就可以在许多方面一个不希望失败 - 至少，我没有，我花了一些时间来了解为什么回声行为您发布的方式，只是为了找到那是因为怪异的文件名的组合（谁曾经想命名文件 T ？），隐藏的配置（ nullglob 禁用，默认选项，但仍隐藏）和无害的命令。

我说的无害的，因为这是你得到的，例如，当目标为 LS （失败了，因为找不到文件）：

拉菲尔@毕宿五：〜$的mkdir测试
拉菲尔@毕宿五：〜$ CD测试
拉菲尔@毕宿五：〜/测试$按t
拉菲尔@毕宿五：〜/测试$ LS [T]
ŧ
拉菲尔@毕宿五：〜/测试$ LS [V]
LS：无法访问[V]：没有这样的文件或目录


This happens for the character t, and the value root. Quite perplexing

$ echo [s]
[s]
$ echo [t]
t
$ echo [ t ]
[ t ]
$ echo [root]
t

解决方案

Being not a shell habitué (and not willing to become) I found surprising how filename expansion is designed to behave when no matches are found. I'll report the Bash reference

Bash scans each word for the characters *, ?, and [. If one of these characters appears, then the word is regarded as a pattern, and replaced with an alphabetically sorted list of file names matching the pattern. If no matching file names are found:

• if the shell option nullglob is disabled, the word is left unchanged
• if the shell option nullglob is set the word is removed
• If the failglob shell option is set, an error message is printed and the command is not executed

The good news is this thing is configurable. The bad one is a script can fail in a number of ways one doesn't expect - at least, I did not, and it took me some time to understand why echo behaves the way you posted, just to find that it's because of a combination of weird filenames (who ever wants to name a file t?), hidden configuration (nullglob disabled, default option but still hidden) and a harmless command.

I said harmless because this is what you get, for example, when the target is ls (a failure because the file is not found):

raffaele@Aldebaran:~$ mkdir test
raffaele@Aldebaran:~$ cd test
raffaele@Aldebaran:~/test$ touch t
raffaele@Aldebaran:~/test$ ls [t]
t
raffaele@Aldebaran:~/test$ ls [v]
ls: cannot access [v]: No such file or directory

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