Not convinced by this (like the idea though), for two reasons:

• The user agent may not/won't specify the distribution - e.g.

   Mozilla/5.0 (X11; Linux; rv:30.0) Gecko/20100101 Firefox/30.0
  

  though Windows can be guessed pretty easily:

   Windows / IE 11: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
  

• The question could be asked from another computer - e.g. say if the OP can't boot Ubuntu and need to ask it from a Windows machine. Also, people may use another machine more often (I 80% of the time use Fedora (I shall ignore the booing noises) the rest of the time Ubuntu, Windows, other OSs...)

Edit: here are a few user agents off different Linux machines, have guess where I got them from:

• Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:30.0) Gecko/20100101 Firefox/30.0

  Ubuntu, what a suprise...

• Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.153 Safari/537.3

  Ubuntu, Google Chrome

• Mozilla/5.0 (X11; Linux x86_64; rv:30.0) Gecko/20100101 Firefox/30.0

  oddly enough, Ubuntu

^{This of course all depends on whether just the User Agent is being used, or some other thing which I don't know about...}

Not convinced by this (like the idea though), for two reasons:

• The user agent may not/won't specify the distribution - e.g.

   Mozilla/5.0 (X11; Linux; rv:30.0) Gecko/20100101 Firefox/30.0
  

  though Windows can be guessed pretty easily:

   Windows / IE 11: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
  

• The question could be asked from another computer - e.g. if the OP can't boot Ubuntu and need to ask it from a Windows machine. 
  Also, people may use another machine more often to browse the site (I 80% of the time use Fedora (I shall ignore the booing noises) the rest of the time Ubuntu, Windows, other OSs...) - there is also the mobile browser which could be using a Mobile OS... (Though is is possible to use Ubuntu Touch)

Edit: here are a few user agents off different Linux machines, have a guess where I got them from:

• Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:30.0) Gecko/20100101 Firefox/30.0

  Ubuntu, what a suprise...

• Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.153 Safari/537.3

  Ubuntu, Google Chrome

• Mozilla/5.0 (X11; Linux x86_64; rv:30.0) Gecko/20100101 Firefox/30.0

  oddly enough, Ubuntu

• Windows / IE 10: Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0)

  Puzzled? There are things that can change user-agents... You need to use these with things like Pipelight.

^{This of course all depends on whether just the User Agent is being used, or some other thing which I don't know about...}

Not convinced by this (like the idea though), for two reasons:

• The user agent may not/won't specify the distribution - e.g.

   Mozilla/5.0 (X11; Linux; rv:30.0) Gecko/20100101 Firefox/30.0
  

  though Windows can be guessed pretty easily:

   Windows / IE 11: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
  

• The question could be asked from another computer - e.g. say if the OP can't boot Ubuntu and need to ask it from a Windows machine. Also, people may use another machine more often (I 80% of the time use Fedora (I shall ignore the booing noises) the rest of the time Ubuntu, Windows, other OSs...)

Not convinced by this (like the idea though), for two reasons:

• The user agent may not/won't specify the distribution - e.g.

   Mozilla/5.0 (X11; Linux; rv:30.0) Gecko/20100101 Firefox/30.0
  

  though Windows can be guessed pretty easily:

   Windows / IE 11: Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
  

• The question could be asked from another computer - e.g. say if the OP can't boot Ubuntu and need to ask it from a Windows machine. Also, people may use another machine more often (I 80% of the time use Fedora (I shall ignore the booing noises) the rest of the time Ubuntu, Windows, other OSs...)

Edit: here are a few user agents off different Linux machines, have guess where I got them from:

• Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:30.0) Gecko/20100101 Firefox/30.0

  Ubuntu, what a suprise...

• Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/35.0.1916.153 Safari/537.3

  Ubuntu, Google Chrome

• Mozilla/5.0 (X11; Linux x86_64; rv:30.0) Gecko/20100101 Firefox/30.0

  oddly enough, Ubuntu

^{This of course all depends on whether just the User Agent is being used, or some other thing which I don't know about...}