A shell script replacement for the (deprecated) `apt-key adv ...` that's even easier to use.

apt-install-key

#!/bin/bash

set -e

# Need to be root for this
[ $(/usr/bin/id -ur) -eq 0 ] || exec /usr/bin/sudo -u root -g root -- "$(realpath "$0")" "$@"

KEYS_ROOT=/etc/apt/trusted.gpg.d
KEYSERVERS=( hkp://keys.gnupg.net hkp://pgp.mit.edu hkp://keyserver.ubuntu.com hkp://keyserver.ubuntu.com:80 )

function die() {
  echo "$@" 1>&2
  echo 1>&2
  echo "Usage:" 1>&2
  echo -e "\t$0 KEY_ID [KEYSERVERS...]" 1>&2
  [ $? -eq 0 ] || exit 1
}

[ $# -ge 1 ] || die "Required arguments missing."

KEY_ID="$1"
shift 1

GNUPGHOME="$(mktemp -d "${TEMPDIR:-/tmp}/apt-import-key.XXXX")"
export GNUPGHOME
trap "rm -Rf '${GNUPGHOME}'" ERR EXIT INT
cd "${GNUPGHOME}"
umask 0133

KEYSERVERS=( "$@" "${KEYSERVERS[@]}" )
for server in "${KEYSERVERS[@]}"; do
  echo "keyserver ${server}"
done > "${GNUPGHOME}/dirmngr.conf"

echo
echo "Starting dirmngr"
eval "$(dirmngr --daemon)"
export DIRMNGR_PID="$(sed -r 's/^[^:]+:([[:digit:]]+):.*$/\1/' <<<"${DIRMNGR_INFO}")"
sleep 0.1
trap "dirmngr --shutdown" ERR EXIT INT
kill -n0 "${DIRMNGR_PID}" || die "Dirmngr failed to load."

echo "Attempting to retrieve key" 
gpg --verbose --recv-keys "${KEY_ID}" ||\
  die "Could not retrieve key ${KEY_ID} from any configured keyservers."

KEY_OWNER="$(gpg --with-colons -k "${KEY_ID}" | grep -E '^uid:' | cut -d ':' -f 10)" || \
  die "Could not parse owner of key '${KEY_ID}'."

KEY_OWNER_NAME="$(sed -r 's/\s*<[^>]+>//g;s/[[:cntrl:]]+//g' <<<"${KEY_OWNER}")"
KEY_OWNER_NAME="${KEY_OWNER_NAME// /_}"
KEYFILE_PATH="${KEYS_ROOT}/${KEY_OWNER_NAME}-${KEY_ID}.asc"
KEYFILE_PATH_REAL="$(realpath -sLq "${KEYFILE_PATH}")"

[ -n "${KEYFILE_PATH_REAL}" ] || die "Path to ${KEYFILE_PATH_REAL} does not exist."

if [ -e "${KEYFILE_PATH_REAL}" ] && [ ! -f "${KEYFILE_PATH_REAL}" ]; then
  die "A non-file already exists at ${KEYFILE_PATH_REAL}. Aborting."
fi

if [ -e "${KEYFILE_PATH_REAL}" ] && [ -s "${KEYFILE_PATH_REAL}" ]; then
  rm -f "${KEYFILE_PATH_REAL}"
  echo "NOTE: Auto-deleted blank file at ${KEYFILE_PATH_REAL}." 2>&1
fi

if [ -f "${KEYFILE_PATH_REAL}" ]; then
  echo "Keyfile at already exists at ${KEYFILE_PATH_REAL}. You will be prompted for if you'd like to overwrite." 1>&2
fi

if gpg --armor --export "${KEY_ID}" > "${KEY_ID}.asc" && [ -s "${KEY_ID}.asc" ]; then
  chmod a+r "${KEY_ID}.asc" 
else
  die "Failed to export key"
fi

echo "Saving key to ${KEYFILE_PATH_REAL}.."
cp -vi "${KEY_ID}.asc" "${KEYFILE_PATH_REAL}" || \
  die "Installing key aborted"

find "${KEYFILE_PATH_REAL}" -ls
echo "Done!"

exit 0