What does the dollar sign mean in robots.txt

问题:

I am curious about a website and want to do some web crawling at the /s path. Its robots.txt:

User-Agent: *
Allow: /$
Allow: /debug/
Allow: /qa/
Allow: /wiki/
Allow: /cgi-bin/loginpage
Disallow: /

My questions are:

• What does the dollar-sign mean in this case?

• And is it appropriate to crawl the URL /s? with respect to the robots.txt file?

回答1:

If you follow the original robots.txt specification, $ has no special meaning, and there is no Allow field defined. A conforming bot would have to ignore fields it does not know, therefore such a bot would actually see this record:

User-Agent: *
Disallow: /

However, the original robots.txt specification has been extended by various parties. But as the authors of the robots.txt in question did not target a specific bot, we don’t know which "extension" they had in mind.

Typically (but not necessarily, as it’s not formally specified), Allow overwrites rules specified in Disallow, and $ represents the end of the URL path.

Following this interpretation (it’s, for example, used by Google), Allow: /$ would mean: You may crawl /, but you may not crawl /a, /b and so on.

So crawling of URLs whose path starts with /s would not be allowed (neither according to the original spec, thanks to Disallow: /, nor according to Google’s extension).