This is importing certificate in Ubuntu 18.04 & 16.04

import_ca_cert_ubuntu.sh

#!/bin/bash

# Reference: https://askubuntu.com/questions/73287/how-do-i-install-a-root-certificate

sudo_prefix=""

if [[ ${USER} != 'root' ]]; then
  sudo_prefix="sudo "
fi;

cert_dir=$1
cert_file_path=$2

if [[ ${cert_dir} == '' ]]; then
  echo "Please specify certificate directory name!"
  exit 1;
fi;

if [[ ${cert_file_path} == '' ]]; then
  echo "Please specify full certificate file path!"
  exit 1;
fi;

read -p "Do your ${cert_file_path} is PEM file?[Y/n] " ans_pem

if [[ ${ans_pem} == 'Y' || ${ans_pem} == '' ]]; then
  openssl x509 -in ${cert_file_path} -inform PEM -out "${cert_file_path}.crt"
  cert_file_path="${cert_file_path}.crt"
fi;

${sudo_prefix}mkdir "/usr/share/ca-certificates/${cert_dir}"
${sudo_prefix}cp ${cert_file_name} "/usr/share/ca-certificates/${cert_dir}/${cert_file_name}"
${sudo_prefix}update-ca-certificates

Refresh Ubuntu GPG Key Error and the reference is as follows:

Example: R-Studio, the GPG Key error is invalid.

• https://askubuntu.com/questions/650032/gpg-errorthe-following-signatures-were-invalid-keyexpired
• https://cran.rstudio.com/bin/linux/ubuntu/xenial/InRelease