Check at specified intervals whether an SSL certificate is valid

check_ssl_validity.sh

#!/bin/bash

URI=https://www.yahoo.com
INTERVAL_IN_SECONDS=0.5
RESULTS_PER_LINE=50

echo "Checking $URI at ${INTERVAL_IN_SECONDS}s intervals..."

check_uri () {

  if [ $COUNTER -le 0 ]; then
    TIME=$(date +"%T")
    printf "\n[${TIME}] "
    COUNTER=$RESULTS_PER_LINE
  fi

  # https://serverfault.com/a/749381
  CHECK_URI_RESULT=$(curl --insecure -vvI $URI 2>&1 | awk 'BEGIN { cert=0 } /^\* Server certificate:/ { cert=1 } /^\*/ { if (cert) print }')
  echo $CHECK_URI_RESULT | grep --quiet "SSL certificate verify ok"
  
  if [ $? -eq 0 ]; then
    printf ✓
  else
    printf ✗
  fi
  let "COUNTER-=1"

}

COUNTER=0
while true; do
check_uri
sleep $INTERVAL_IN_SECONDS
done