为什么Bash模式匹配?(* [[:: class:]])foobar很慢? [英] Why is Bash pattern match for ?(*[[:class:]])foobar slow?

问题描述

我有一个文本文件foobar.txt，大约10KB，不长.但是，在高性能Linux计算机上，以下匹配搜索命令大约需要10秒钟.

I have a text file foobar.txt which is around 10KB, not that long. Yet the following match search command takes about 10 seconds on a high-performance Linux machine.

bash>shopt -s extglob
bash>[[ `cat foobar.txt` == ?(*[[:print:]])foobar ]]

没有匹配项:foobar.txt中的所有字符都是可打印的，但是没有字符串"foobar".

There is no match: all the characters in foobar.txt are printable but there is no string "foobar".

搜索应尝试匹配两个备选方案，每个备选方案均不匹配:

The search should try to match two alternatives, each of them will not match:

"foobar"

那是瞬间

*[[:print:]]foobar

-应该这样:

应该逐个字符逐个扫描文件，每次检查下一个字符是否为

should scan the file character by character in one pass, each time, check if the next characters are

[[:print:]]foobar

这也应该很快，每个字符绝不能花费毫秒.

this should also be fast, no way should take a millisecond per character.

实际上，如果我放?，那就做

In fact, if I drop ?, that is, do

bash>[[ `cat foobar.txt` == *[[:print:]]foobar ]]

此是瞬时的.但这仅仅是上面的第二种选择，没有第一种.

this is instantaneous. But this is simply the second alternative above, without the first.

那为什么这么长?

推荐答案

正如其他人所指出的，最好使用 grep .

As others have noted, you're probably better off using grep.

也就是说，如果您希望坚持使用 [[]条件-结合@konsolebox和@rici的建议-您将得到:

That said, if you wanted to stick with a [[ conditional - combining @konsolebox and @rici's advice - you'd get:

[[ $(<foobar.txt)  =~ (^|[[:print:]])foobar$ ]]

正则表达式已更新为符合OP的要求-谢谢@rici.

Regex updated to match the OP's requirements - thanks, @rici.

通常来说，最好使用正则表达式进行字符串匹配(在这种情况下，通过 =〜运算符)，而不是[globbing] 模式(通过 == 运算符)，其主要目的是匹配文件名和文件夹名.

Generally speaking, it is preferable to use regular expressions for string matching (via the =~ operator, in this case), rather than [globbing] patterns (via the == operator), whose primary purpose is matching file- and folder names.

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