How to detect operating system using user agent

In computing, a user agent is software (a software agent) that is acting on behalf of a user. For example, an email reader is a mail user agent, and in the Session Initiation Protocol (SIP), the term user agent refers to both end points of a communications session.

In many cases, a user agent acts as a client in a network protocol used in communications within a client–server distributed computing system. In particular, the Hypertext Transfer Protocol (HTTP) identifies the client software originating the request, using a "User-Agent" header, even when the client is not operated by a user.

Standard User Agent browser

Most Web browsers use a User-Agent value as follows: Mozilla/[version] ([system and browser information]) [platform] ([platform details]) [extensions]. For example, Safari on the iPad has used the following:

Mozilla/5.0 (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us) AppleWebKit/531.21.10 
(KHTML, like Gecko)Mobile/7B405

The components of this string are as follows:

• Mozilla/5.0: Previously used to indicate compatibility with the Mozilla rendering engine
• (iPad; U; CPU OS 3_2_1 like Mac OS X; en-us): Details of the system in which the browser is running
• AppleWebKit/531.21.10: The platform the browser uses
• (KHTML, like Gecko): Browser platform details
• Mobile/7B405: This is used by the browser to indicate specific enhancements that are available directly in the browser or through third parties. An example of this is Microsoft Live Meeting which registers an extension so that the Live Meeting service knows if the software is already installed, which means it can provide a streamlined experience to joining meetings.

Notable exceptions include Opera, which is a web browser, but does not include Mozilla in its User-Agent string.

Window User Agent

Mozilla/5.0 (Windows NT 5.1; rv:5.0) Gecko/20100101 Firefox/5.0

• Windows NT 5.1; : Window XP operating system as platform
• Firefox/5.0: Mozilla Firefox version 5.0 as a browser

Mozilla/5.0 (Windows NT 6.0) AppleWebKit/535.2 (KHTML, like Gecko) Chrome/15.0.874.120 
Safari/535.2

• Windows NT 6.0: Window Vista operating system as platform
• Chrome/15.0.874.120: Google chrome version 15.0.874.120 as a browser

Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)

• Windows NT 6.1: Window 7 operating system as platform
• MSIE 10.0: Internet explorer version 10 as a browser
• WOW64;: A 32-bit window application is running on a 64-bit processor

Mozilla/5.0 (Windows; U; Windows NT 6.2; en-US) AppleWebKit/533.20.25 (KHTML, like Gecko)
Version/5.0.4 Safari/533.20.27

• Windows NT 6.2: Window 8 operating system as platform
• Version/5.0.4 Safari/533.20.27: Safari version 5.0.4 as a browser
• U;: Security value (N for no security, U for strong security, I for weak security)