what is a robots.txt file code example

Example 1: seo disallow

User-agent: *
Disallow: /

Example 2: robots txt

# Group 1
User-agent: Googlebot
Disallow: /nogooglebot/

# Group 2
User-agent: *
Allow: /

Sitemap: http://www.example.com/sitemap.xml

Example 3: robots txt no index entire site

User-agent: * 
Disallow: /