Randomly shuffle the numbers 0-9, lowercase letters a-f, and uppercase letters A-F and output 16 characters with a space between each character.
shuf -e $(echo {0..9} {a,b,c,d,e,f} {A,B,C,D,E,F} | tr ' ' '\n') -n 16 | sed 's/\(.\)/\1 /g' | tr -d '\n'