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Search Engine Robots or Web Crawlers

 Search Engine Robots or Web Crawlers

Most of the common users or visitors use different available search engines to search out the piece of information they required. But how this information is provided by search engines? Where from they have collected these information? Basically most of these search engines maintain their own database of information. These database includes the sites available in the  webworld which ultimately maintain the detail web pages information for each available sites. Basically search engine do some background work by using robots to collect information and maintain the database. They make catalog of gathered information and then present it publicly or at-times for private use.

In this article we will discuss about those entities which loiter in the global internet environment or we will  about web crawlers which move around in netspace. We will learn 

·	What it’s all about and what purpose they serve ?
·	Pros and cons of using these entities.
·	How we can keep our pages away from crawlers ?
·	Differences between the common crawlers and robots.

In the following portion we will divide the whole research work under the following two sections :

I.	Search Engine Spider : Robots.txt.
II.	Search Engine Robots : Meta-tags Explained.

I. Search Engine Spider : Robots.txt

What is robots.txt file ?

A web robot is a program or search engine software that visits sites regularly and automatically and crawl through the web’s hypertext structure by fetching a document, and recursively retrieving all the documents which are referenced. Sometimes site owners do not want all their site pages to be crawled by the web robots. For this reason they can exclude few of their pages being crawled by the robots by using some standard agents. So most of the robots abide by the ‘Robots Exclusion Standard’, a set of constraints to restricts robots behavior. 
‘Robot Exclusion Standard’ is a protocol used by the site administrator to control the movement of the robots. When search engine robots come to a site it will search for a file named robots.txt in the root domain of the site ( This is a plain text file which implements ‘Robots Exclusion Protocols’ by allowing or disallowing specific files within the directories of files. Site administrator can disallow access to cgi, temporary or private directories by specifying robot user agent names.

The format of the robot.txt file is very simple. It consists of two field : user-agent and one or more disallow field.

What is User-agent ?

This is the technical name for an programming concepts in the world wide networking environment and used to mention the specific search engine robot within the robots.txt file. 
For example :

User-agent: googlebot

We can also use the wildcard character “*” to specify all robots :
User-agent: * 

Means all the robots are allowed to come to visit.

What is Disallow ? 

In the robot.txt file second field is known as the disallow: These lines guide the robots, to which file should be crawled or which should not be. For example to prevent downloading email.htm the syntax will be:

Disallow: email.htm

Prevent crawling through directories the syntax will be:

Disallow: /cgi-bin/

White Space and Comments :

Using # at the beginning of any line in the robots.txt file will be considered as comments only and using # at the beginning of the robots.txt like the following example entail us which url to be crawled.

# robots.txt for

Entry Details for robots.txt :

1)	User-agent: *

The asterisk (*) in the User-agent field is denoting “all robots” are invited. As nothing is disallowed so all robots are free to crawl through.

2)	User-agent: *
	Disallow: /cgi-bin/
	Disallow: /temp/
	Disallow: /private/

All robots are allowed to crawl through the all files except the cgi-bin, temp and private file.

3)	User-agent: dangerbot
	Disallow: /
Dangerbot is not allowed to crawl through any of the directories. “/” stands for all directories.

4)	User-agent: dangerbot
 	Disallow: /
	User-agent: *
	Disallow: /temp/
The blank line indicates starting of new User-agent records. Except dangerbot all the other bots are allowed to crawl through  all the directories except “temp” directories.

5)	User-agent: dangerbot
	Disallow: /links/listing.html

	User-agent: *
	Disallow: /email.html/

Dangerbot is not allowed for the listing page of links directory otherwise all the robots are allowed for all directories except downloading email.html page.

6)	User-agent: abcbot
	Disallow: /*.gif$ 

To remove all files from a specific file type (e.g. .gif ) we will use the above robots.txt entry.

7)	User-agent: abcbot
	Disallow: /*?

To restrict web crawler from crawling dynamic pages we will use the above robots.txt entry.

Note : Disallow field may contain “*” to follow any series of characters and may end with “$” to indicate the end of the name.

Eg : Within the image files to exclude all gif files but allowing others from google crawling
	User-agent: Googlebot-Image
	Disallow: /*.gif$

Disadvantages of robots.txt : 

Problem with Disallow field:

Disallow: /css/  /cgi-bin/  /images/
Different spider will read the above field in different way. Some will ignore the spaces and will read /css//cgi-bin//images/ and may only consider either /images/ or /css/ ignoring the others.

The correct syntax should be :
Disallow: /css/
Disallow: /cgi-bin/
Disallow: /images/

All Files listing:

Specifying each and every file name within a directory is most commonly used mistake
Disallow: /ab/cdef.html
Disallow: /ab/ghij.html
Disallow: /ab/klmn.html
Disallow: /op/qrst.html
Disallow: /op/uvwx.html

Above portion can be written as:
Disallow: /ab/
Disallow: /op/

A trailing slash means a lot that is a directory is offlimits.



Though fields are not case sensitive but the datas like directories, filenames are case sensitive.

Conflicting syntax:

User-agent: *
Disallow: /
User-agent: Redbot

What will happen ? Redbot is allowed to crawl everything but will this permission override the disallow field or disallow will override the allow permission. 

II. Search Engine Robots: Meta-tag Explained:  

What is robot meta tag ?

Besides robots.txt search engine is also having another tools to crawl through web pages. This is the META tag which tells web spider to index a page and follow links on it, which may be more helpful in some cases, as it can be used on page-by-page basis. It is also helpful incase you don’t have the requisite permission to access the servers root directory to control robots.txt file. 
We used to place this tag within the header portion of html. 

Format of the Robots Meta tag :

In the HTML document it is placed in the HEAD section.
META NAME=”robots” CONTENT=”index,follow”
META NAME=”description” CONTENT=”Welcome to…….”

Robots Meta Tag options :

There are four options that can be used in the CONTENT portion of the Meta Robots. These are index, noindex, follow, nofollow.

This tag allowing search engine robots to index a specific page and can follow all the link residing on it. If site admin doesn’t want any pages to be indexed or any link to be followed then they can replace “ index,follow” with “ noindex,nofollow”. 
According to the requirements, site admin can use the robots in the following different options :

META NAME=”robots” CONTENT=”index,follow”> Index this page, follow links from this page.
META NAME=”robots” CONTENT =”noindex,follow”> Don’t index this page but follow link from this page.
META NAME=”robots” CONTENT =”index,nofollow”> Index this page but don’t follow links from this page
META NAME=”robots” CONTENT =”noindex,nofollow”> Don’t index this page, don’t follow links from this page.

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