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What is a robots.txt file?

What is a robots.txt file?

Robots.txt is a text file webmasters create to instruct web robots (typically search engine robots) how to crawl pages on their website. The robots.txt file is part of the the robots exclusion protocol (REP), a group of web standards that regulate how robots crawl the web, access and index content, and serve that content up to users. The REP also includes directives like meta robots, as well as page-, subdirectory-, or site-wide instructions for how search engines should treat links (such as “follow” or “nofollow”).

In practice, robots.txt files indicate whether certain user agents (web-crawling software) can or cannot crawl parts of a website. These crawl instructions are specified by “disallowing” or “allowing” the behavior of certain (or all) user agents.

Basic format:
User-agent: [user-agent name]Disallow: [URL string not to be crawled]

Together, these two lines are considered a complete robots.txt file — though one robots file can contain multiple lines of user agents and directives (i.e., disallows, allows, crawl-delays, etc.).

Within a robots.txt file, each set of user-agent directives appear as a discrete set, separated by a line break:

In a robots.txt file with multiple user-agent directives, each disallow or allow rule only applies to the useragent(s) specified in that particular line break-separated set. If the file contains a rule that applies to more than one user-agent, a crawler will only pay attention to (and follow the directives in) the most specific group of instructions.

Msnbot, discobot, and Slurp are all called out specifically, so those user-agents will only pay attention to the directives in their sections of the robots.txt file. All other user-agents will follow the directives in the user-agent: * group.

Example robots.txt:

Here are a few examples of robots.txt in action for a www.example.com site:

Robots.txt file URL: www.example.com/robots.txt
Blocking all web crawlers from all content
User-agent: * Disallow: /
Using this syntax in a robots.txt file would tell all web crawlers not to crawl any pages on www.example.com, including the homepage.

Allowing all web crawlers access to all content
User-agent: * Disallow:
Using this syntax in a robots.txt file tells web crawlers to crawl all pages on www.example.com, including the homepage.

Blocking a specific web crawler from a specific folder
User-agent: Googlebot Disallow: /example-subfolder/
This syntax tells only Google’s crawler (user-agent name Googlebot) not to crawl any pages that contain the URL string www.example.com/example-subfolder/.

Blocking a specific web crawler from a specific web page
User-agent: Bingbot Disallow: /example-subfolder/blocked-page.html
This syntax tells only Bing’s crawler (user-agent name Bing) to avoid crawling the specific page at www.example.com/example-subfolder/blocked-page.html.

How does robots.txt work?
Search engines have two main jobs:

Crawling the web to discover content;
Indexing that content so that it can be served up to searchers who are looking for information.
To crawl sites, search engines follow links to get from one site to another — ultimately, crawling across many billions of links and websites. This crawling behavior is sometimes known as “spidering.”

After arriving at a website but before spidering it, the search crawler will look for a robots.txt file. If it finds one, the crawler will read that file first before continuing through the page. Because the robots.txt file contains information about how the search engine should crawl, the information found there will instruct further crawler action on this particular site. If the robots.txt file does not contain any directives that disallow a user-agent’s activity (or if the site doesn’t have a robots.txt file), it will proceed to crawl other information on the site.

Other quick robots.txt must-knows:
(discussed in more detail below)

In order to be found, a robots.txt file must be placed in a website’s top-level directory.

Robots.txt is case sensitive: the file must be named “robots.txt” (not Robots.txt, robots.TXT, or otherwise).

Some user agents (robots) may choose to ignore your robots.txt file. This is especially common with more nefarious crawlers like malware robots or email address scrapers.

The /robots.txt file is a publicly available: just add /robots.txt to the end of any root domain to see that website’s directives (if that site has a robots.txt file!). This means that anyone can see what pages you do or don’t want to be crawled, so don’t use them to hide private user information.

Each subdomain on a root domain uses separate robots.txt files. This means that both blog.example.com and example.com should have their own robots.txt files (at blog.example.com/robots.txt and example.com/robots.txt).

It’s generally a best practice to indicate the location of any sitemaps associated with this domain at the bottom of the robots.txt file. Here’s an example:

Where does robots.txt go on a site?

Whenever they come to a site, search engines and other web-crawling robots (like Facebook’s crawler, Facebot) know to look for a robots.txt file. But, they’ll only look for that file in one specific place: the main directory (typically your root domain or homepage). If a user agent visits www.example.com/robots.txt and does not find a robots file there, it will assume the site does not have one and proceed with crawling everything on the page (and maybe even on the entire site). Even if the robots.txt page did exist at, say, example.com/index/robots.txt or www.example.com/homepage/robots.txt, it would not be discovered by user agents and thus the site would be treated as if it had no robots file at all.

In order to ensure your robots.txt file is found, always include it in your main directory or root domain.

Why do you need robots.txt?
Robots.txt files control crawler access to certain areas of your site. While this can be very dangerous if you accidentally disallow Googlebot from crawling your entire site (!!), there are some situations in which a robots.txt file can be very handy.

Some common use cases include:

  • Preventing duplicate content from appearing in SERPs (note that meta robots is often a better choice for this)
  • Keeping entire sections of a website private (for instance, your engineering team’s staging site)
  • Keeping internal search results pages from showing up on a public SERP
  • Specifying the location of sitemap(s)
  • Preventing search engines from indexing certain files on your website (images, PDFs, etc.)
  • Specifying a crawl delay in order to prevent your servers from being overloaded when crawlers load multiple pieces of content at once
  • If there are no areas on your site to which you want to control user-agent access, you may not need a robots.txt file at all.

    Checking if you have a robots.txt file
    Not sure if you have a robots.txt file? Simply type in your root domain, then add /robots.txt to the end of the URL. For instance, Moz’s robots file is located at moz.com/robots.txt.

    If no .txt page appears, you do not currently have a (live) robots.txt page.

    How to create a robots.txt file
    If you found you didn’t have a robots.txt file or want to alter yours, creating one is a simple process. This article from Google walks through the robots.txt file creation process, and this tool allows you to test whether your file is set up correctly.

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