cat rot.txt | tr 'A-Za-z' 'N-ZA-Mn-za-m'

echo "hello world" | tr "$(echo -n {A..Z} {a..z} | tr -d ' ')" "$(echo -n {N..Z} {A..M} {n..z} {a..m} | tr -d ' ')"

uryyb jbeyq

function rot13() {
    cat | tr "$(echo -n {A..Z} {a..z} | tr -d ' ')" "$(echo -n {N..Z} {A..M} {n..z} {a..m} | tr -d ' ')"
}

➜ echo "hello world" | rot13
uryyb jbeyq