I'd like to add the Unicode skull and crossbones to my shell prompt (specifically the 'SKULL AND CROSSBONES' (U+2620)), but I can't figure out the magic incantation to make echo spit it, or any other, 4-digit Unicode character. Two-digit one's are easy. For example, echo -e "\x55", .
In addition to the answers below it should be noted that, obviously, your terminal needs to support Unicode for the output to be what you expect. gnome-terminal does a good job of this, but it isn't necessarily turned on by default. Go to Terminal-> Set Character Encoding and choose Unicode (UTF-8).
In UTF-8 it's actually 6 digit (or 3 byte).
$ echo -e "\xE2\x98\xA0" ☠
To check how it's encoded by your console, you can use hexdump.
echo -n ☠ | hexdump