robots.txt >
disallow

Disallow: /
disallow robots to read and search all file after the first "/"
often say every thing after your domain or
after your IP

Disallow:
It is empty after "Disallow:" then it means allow

---------- attention -----------
not all the r

Robots.txt shall be added at root, no other places
ex: http://----.com/robots.txt

User-Agent:*
Disallow: /
( disallow all robots to search all files)
------------- attention -----------

if you want allow all search engines
then suggest you :
way 1: To delete robots.txt because you do not need a robots.txt
way 2: To keep robots.txt BLANK so no errors fill in error log 
when bots come.
Way 3: no robots.txt but create a customized error page

Do not recommend you to use 
User-Agent:*
Disallow:
to allow all robots to read your files

Because not all robots follow the same rules
the better way to allow all robots to read your all sites
which is

Way 1: " to delete your robots.txt "
But without robots.txt will create errors when bots come.

Way 2: "to keep robots.txt with nothing in" 
so you can avoid errors when bots come.

Way 3: no robots.txt but create a customized error page

Big Tips: read robots line by line each time

http://---- .com / robots.txt
----------- disallow all (*) after (http://----.com/)
User-Agent:*
Disallow: / 
----------- disallow all (*) after  (http://----.com/folder/)
User-Agent:*
Disallow: /folder/

-------------------------------------
http://www.w3.org/TR/html4/appendix/notes.html#h-B.4.1.1
example:
Disallow: /help 
disallows both /help.html and /help/index.html, whereas

Disallow: /help/ 
would disallow /help/index.html but allow /help.html. 
------------------------------------------
User-agent: *
Disallow: /cgi-bin/
Disallow: /privatedir/
Disallow: /hotnews/not4u.htm
------------------------
Allowing Googlebot
block all bots but allow Googlebot

User-agent: *
Disallow: /

User-agent: Googlebot
Disallow:
------------------------
Block Googlebot entirely but allow ( Googlebot-Mobile ): 
crawls pages
PS: ( Allow: ) this syntax may only work for Google bot only

User-agent: Googlebot
Disallow: /

User-agent: Googlebot-Mobile
Allow: 

or

User-agent: Googlebot-Mobile
Disallow:
------------------------

User-Agent: Googlebot
Disallow: /folder1/
Allow: /folder1/myfile.html

------------------------
User-agent: Googlebot
Disallow: /folder
------------------------
User-agent: Googlebot
Disallow: /*.gif$ 
------------------------
User-agent: Googlebot
Disallow: /*?

------ more how to block googlebot ------

http://www.google.com/support/webmasters/bin/
answer.py?answer=40364&topic=8846

------ more control msnbot ------

http://search.msn.com/docs/siteowner.aspx?
t=SEARCH_WEBMASTER_REF_RestrictAccessToSite.htm

------ more yahoo bot info -----

http://help.yahoo.com/help/us/ysearch/slurp/

---------- some standard -------
http://www.robotstxt.org/wc/exclusion-admin.html

What to put into the robots.txt file
The "/robots.txt" file usually contains a record looking like this: 
User-agent: *
Disallow: /cgi-bin/
Disallow: /tmp/
Disallow: /~joe/

In this example, three directories are excluded. 

Note that you need a separate "Disallow" line for every URL 
prefix you want to exclude -- you cannot say 
"Disallow: /cgi-bin/ /tmp/". 
Also, you may not have blank lines in a record, 
as they are used to delimit multiple records. 

Note also that regular expression are not supported in either 
the User-agent or Disallow lines. The '*' in the User-agent 
field is a special value meaning "any robot". Specifically, 
you cannot have lines like "Disallow: /tmp/*" or "Disallow: *.gif". 

What you want to exclude depends on your server. Everything 
not explicitly disallowed is considered fair game to retrieve. 
Here follow some examples: 

To exclude all robots from the entire server
User-agent: *
Disallow: /

To allow all robots complete access
User-agent: *
Disallow:

Or create an empty "/robots.txt" file. 

To exclude all robots from part of the server
User-agent: *
Disallow: /cgi-bin/
Disallow: /tmp/
Disallow: /private/

To exclude a single robot
User-agent: BadBot
Disallow: /

To allow a single robot
User-agent: WebCrawler
Disallow:

User-agent: *
Disallow: /

To exclude all files except one
This is currently a bit awkward, as there is no "Allow" field. 
The easy way is to put all files to be disallowed into a separate 
directory, say "docs", and leave the one file in the level above 
this directory: 
User-agent: *
Disallow: /~joe/docs/

Alternatively you can explicitly disallow all disallowed pages: 
User-agent: *
Disallow: /~joe/private.html
Disallow: /~joe/foo.html
Disallow: /~joe/bar.html

User-Agent:*
Disallow: /

if you allow all to visit your all sites
better not to keep the robots.txt like this
User-Agent:*
Disallow:
sometimes it will confused with
User-Agent:*
Disallow: /
sometomes may cause strange results for
http:://----.com -vs- http:://----.com/