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What Is A Web Crawler? How Web Crawlers work?
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![](http://www.theitstuff.com/wp-content/uploads/2017/12/what-is-a-web-crawler-how-web-crawler-works.jpg)
As an avid Internet junkie, you must have for once in your life come across the word **Web Crawler.** So what is a web crawler, who uses web crawler? How does it work? Let us talk about all of these things in this article.

###  **What is a Web Crawler?**

[![web crawler source code sync][1]][1]

A **web crawler** also known as a **web-spider** is an internet software or bot that browses the internet by visiting different pages of many websites. The web crawler retrieves various information from those web pages and stores them in its records. These crawlers are mostly used to gather content from websites to improve searches in a search engine.

###  **Who uses Web Crawlers?**

Most search engines use crawlers to gather more and more content from publicly available websites so that they can provide more relevant content to their users.

[![search engines use web crawlers][2]][2]

A lot of commercial organizations use Web Crawlers to specifically search for email addresses and phone numbers of people so that they can later send them promotional offers and other schemes. This is basically spam, but that is how most companies create their mailing list.

Hackers use Web Crawlers to find out all the files in a website's folder mostly HTML and Javascript files. They then try to exploit the website by using XSS.

###  **How does a Web Crawler work?**

A Web-Crawler is an automated script which means all of its actions are predefined. A Crawler first begins with an initial list of URLs to visit, these URLs are called seeds. Then it identifies all the hyperlinks to other pages that are listed on the initial seed page. The web crawler then saves these web pages in form of HTML documents which are later worked upon by the search engine and an index is created.

###  **Web Crawler and SEO**

Web Crawling affects SEO i.e Search Engine Optimization in a big way. With a major chunk of the users using Google, it is important to get the Google crawlers to index most of your site. This can be done in many ways including not using repeated content and having as many backlinks on other websites. A lot of websites have been seen to abuse these tricks and they eventually get blacklisted by the Engine.

###  **Robots.txt**

The robots.txt file is a very special type of file that the crawlers look for when crawling your website. This file usually contains information on how to crawl your website. Some webmasters who purposely do not want their sites indexed can also prevent crawling by using the robots.txt file.

###  **Conclusion**

So Crawlers are small software bots that can be used to browse a lot of websites and help the search engine to get the most relevant data from the web.


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via: http://www.theitstuff.com/web-crawler-web-crawlers-work

作者：[Rishabh Kandari][a]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)

本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出

[a]:http://www.theitstuff.com/author/reevkandari
[1]:http://www.theitstuff.com/wp-content/uploads/2017/12/crawler.jpeg
[2]:http://www.theitstuff.com/wp-content/uploads/2017/12/sengine.png
