What I want to see:
$ printf "\U1F600\n"|hexdump -v -e '1/1 " %03o"'|tr ' ' '\\'
\360\237\230\200\012
What I tried:
$ printf "\U1F600\n"|hexdump -v -e '1/1 "\\%03o"'
hexdump: bad conversion character %�
$ printf "\U1F600\n"|hexdump -v -e '1/1 "\%03o"'
hexdump: bad conversion character %%
$ printf "\U1F600\n"|hexdump -v -e '1/1 "\\\%03o"'
hexdump: bad conversion character %\
$ printf "\U1F600\n"|hexdump -v -e '1/1 "\\\\%03o"'
\\\\360\\\\237\\\\230\\\\200\\\\12