Robots.txtの理解

When building a web scraper or bot, it's crucial to be a good internet citizen. The robots.txt file is a key part of this.

It's a text file that websites use to communicate with web crawlers and other bots. It tells them which parts of the site they are allowed to access and which parts they should avoid.

Every website that uses a robots.txt file places it in a standard location: the root directory of its domain.

This means you can always find it by adding /robots.txt to the end of the website's main URL. For example:

• https://www.example.com/robots.txt
• https://www.google.com/robots.txt

You can simply type this into your browser to view a site's rules.

The User-agent directive specifies which bot the following rules apply to. Think of it as addressing a specific bot or all bots.

• User-agent: *: This applies to ALL web crawlers and bots.
• User-agent: Googlebot: This applies only to Google's specific web crawler.
• User-agent: MyCustomBot: You can even specify rules for your own bot if the website owner knows its name.

Each set of rules starts with a User-agent line.

The Disallow directive is used to tell bots which URLs or directories they should NOT access. It's the primary way to restrict crawling.

Here are some examples:

• Disallow: /: Disallows access to the entire website (except for robots.txt itself).
• Disallow: /private/: Disallows access to the /private/ directory and everything within it.
• Disallow: /search?: Disallows URLs starting with /search?, often used for search results pages.

Always respect these rules!

Sometimes, a website might want to disallow a whole directory but allow access to a specific file or sub-directory within it. This is where the Allow directive comes in.

Allow rules override Disallow rules for more specific paths.

For example:

User-agent: *
Disallow: /images/
Allow: /images/public/

This means all bots should avoid the /images/ folder, but they ARE allowed to access content within /images/public/.

The Sitemap directive isn't about restricting access; it's about helping bots discover content.

It points to the XML Sitemap file(s) for the website. A sitemap lists all the pages and files a website owner wants search engines to crawl and index.

Example:

Sitemap: https://www.example.com/sitemap.xml

This helps well-behaved bots find your content more efficiently.

You can easily fetch a website's robots.txt file using Python's requests library. This allows your script to programmatically read and interpret the rules.

Try running this example to see the robots.txt for Wikipedia:

Let's look at a combined example to understand how rules interact:

User-agent: *
Disallow: /temp/
Disallow: /admin/
Allow: /admin/public/

User-agent: MyBot
Disallow: /

• A general bot (*) cannot access /temp/ or /admin/, but it CAN access /admin/public/.
• A bot named MyBot cannot access ANYTHING on the site.

The most specific rule usually wins, especially Allow over Disallow for sub-paths.

It's crucial to understand that robots.txt is a voluntary agreement for well-behaved bots. It's not a security mechanism!

• Malicious bots can (and often will) ignore these rules.
• The content of robots.txt itself is public. Don't put sensitive information there.
• It's for managing server load and respecting content preferences, not hiding data.

Always scrape ethically and respect website policies.

In this lesson, we explored the robots.txt file, a fundamental component of ethical web scraping.

• You learned how to locate it and its core directives: User-agent, Disallow, Allow, and Sitemap.
• We saw how to fetch and interpret these rules using Python.
• Crucially, we emphasized that robots.txt is a guideline for respectful bots, not a security measure.

Always check and respect a website's robots.txt before scraping!