# dd if=/dev/urandom count=1 2> /dev/null | tr -d -c '[:alnum:]'Base 32 (that reduces ambiguities and improves readability when printed)
# dd if=/dev/urandom count=1 2> /dev/null | tr -d -c '[A-Z2-7]'All printable characters:
# dd if=/dev/urandom count=1 2> /dev/null | tr -d -c '[:print:]'