I got your rot13() right here ...

gistfile1.sh

rot13 () 
{ 
    if [ $# = 0 ]; then
        tr "[a-m][n-z][A-M][N-Z]" "[n-z][a-m][N-Z][A-M]";
    else
        tr "[a-m][n-z][A-M][N-Z]" "[n-z][a-m][N-Z][A-M]" < $1;
    fi
}