Bash 99 8 chars (Thanks @FireFly)

rot13 <$0rot13<$0

This mathmatch requirement:

• output same amount of character in the same shape as the code
• rot13

Practical

echo  >autorot13.sh 'rot13 <$0' 
chmod  +x autorot13.sh 
./autorot13.sh 
ebg13 <$0ebg13<$0