Simple bash script to add apt keys + repositories in a better way than using apt-key. It is intended to make it easy to convert instructions online that still use apt-key, to the newer mechanism.

add-repository

#!/bin/bash
# Simple script to add apt keys in a better way than using apt-key
# It is intended to make it easy to convert instructions online
# that still use apt-key, to the newer mechanism.
# Download from: https://gist.githubusercontent.com/Ghostbird/83eb5bcd2ffd4a6b6966137a2e1c4caf
# MIT LICENCE:
# Copyright © 2022 Gijsbert “Ghostbird” ter Horst
# Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
# The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
# THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

KEYRING_DIR='/usr/share/keyrings'
APT_DIR='/etc/apt/sources.list.d'

set -e

usage()
{
cat << EOF
Usage: $0 [-h] [-k KEYRING_DIR] [-a APT_DIR] KEY_URI APT_CONF NAME

  Add a repository to apt and add a its key to keyrings
  Make sure to escape positional arguments with spaces!

DEFAULTS:
  Keyrings are in ${KEYRING_DIR}
  Apt sources are in ${APT_DIR}

ARGUMENTS:
  KEY_URI   Uri where to HTTP GET the key
  APT_CONF  Configuration that should be in the apt sources file.
            Do not included a [signed-by], it will be generated.
  NAME      Basename to use for keyring and apt sources file

OPTIONS:
  -h        Show this message
  -k DIR    Alternative path to keyrings directory
  -a DIR    Alternative path to apt sources directory

ERROR CODES:
  1         Invalid or missing arguments
  2         Excess arguments. Probably an argument was not properly quoted.
  3         Execution would clobber existing file, investigate and resolve manually.
EOF
}

while getopts "hk:a:" OPTION
do
  case $OPTION in
    h)
      usage
      exit 1
      ;;
    k)
      KEYRING_DIR=$OPTARG
      ;;
    a)
      APT_DIR=$OPTARG
      ;;
    ?)
      usage
      exit 1
      ;;
  esac
done

KEY_URI=${@:$OPTIND:1}
APT_CONF=${@:$OPTIND+1:1}
NAME=${@:$OPTIND+2:1}
EXCESS=${@:$OPTIND+3:1}

if [ \( -z "${KEY_URI}" \) -o \( -z "${APT_CONF}" \) -o \( -z "${NAME}" \) ]
then
  usage
  1>&2 echo "ERROR: Missing required argument!"
  exit 1
fi

if [ -n "${EXCESS}" ]
then
  usage
  1>&2 echo "ERROR: Excess positional arguments! Please quote your arguments properly!"
  exit 2
fi

KEYRING_FILE="${KEYRING_DIR}/${NAME}.gpg"
APT_FILE="${APT_DIR}/${NAME}.list"

# Prevent clobbering
if [ -e "${KEYRING_FILE}" ]
then
  1>&2 echo "ERROR: File exists: ${KEYRING_FILE}"
  exit 3
fi

if [ -e "${APT_FILE}" ]
then
  1>&2 echo "ERROR: File exists: ${APT_FILE}"
  exit 3
fi

cd $(mktemp -d)
curl -fsS -o key.tmp "${KEY_URI}"

# Ensure dearmoured
if [ -n "$(file key.tmp | grep 'PGP public key block')" ]
then
  cat key.tmp | gpg --dearmour > key2.tmp
  mv key2.tmp key.tmp
fi

sudo install -D -o root -g root -m 644 key.tmp "${KEYRING_FILE}"
echo "${APT_CONF}" | sed 's/http:/https:/' | sed 's/^deb https/deb [] https/' | sed -E "s|^deb *\[|deb \[signed-by=${KEYRING_FILE}|" | sed -E "s|^deb (.+)|deb \1\ndeb-src \1|" | sudo tee "${APT_FILE}" > /dev/null

rm -r $(pwd)
cd - > /dev/null

Added reference to this URI, and MIT licence

Updated to automatically also add deb-src line.