Puppet task to add a line into a file if not existing and not commented out (prefixed with "#")

etc-fileline-add.pp

# add STRING-XXXXXXX to FILE-YYYYYY
# if commented out, it gets re-added

exec { "[task name]":
    command => '/bin/echo "STRING-XXXXXXX" >> FILE-YYYYYY',
    unless  => '/bin/grep -E "^STRING-XXXXXXX$" FILE-YYYYYY'
}

for /etc/hosts: http://www.puppetcookbook.com/posts/adding-a-host-entry.html