ascii to hex

echo -en 'hello\nyou\nthere' | sed -z "s/\(.\)/'\1\x0/g" | xargs -0 printf '%02x'

68656c6c6f0a796f750a7468657265

hex to ascii

echo '68656c6c6f0a796f750a7468657265' | sed 's/\(..\)/\\\\x\1/g' | xargs printf

hello
you
there