How to test an Internet connection with bash?

Question

How can an internet connection be tested without pinging some website? I mean, what if there is a connection but the site is down? Is there a check for a connection with the

Answer 1

If your goal is to actually check for Internet access, many of the existing answers to this question are flawed. A few things you should be aware of:

1. It's possible for your computer to be connected to a network without that network having internet access
2. It's possible for a server to be down without the entire internet being inaccessible
3. It's possible for a captive portal to return an HTTP response for an arbitrary URL even if you don't have internet access

With that in mind, I believe the best strategy is to contact several sites over an HTTPS connection and return true if any of those sites responds.

For example:

connected_to_internet() {
  test_urls="\
  https://www.google.com/ \
  https://www.microsoft.com/ \
  https://www.cloudflare.com/ \
  "

  processes="0"
  pids=""

  for test_url in $test_urls; do
    curl --silent --head "$test_url" > /dev/null &
    pids="$pids $!"
    processes=$(($processes + 1))
  done

  while [ $processes -gt 0 ]; do
    for pid in $pids; do
      if ! ps | grep "^[[:blank:]]*$pid[[:blank:]]" > /dev/null; then
        # Process no longer running
        processes=$(($processes - 1))
        pids=$(echo "$pids" | sed --regexp-extended "s/(^| )$pid($| )/ /g")

        if wait $pid; then
          # Success! We have a connection to at least one public site, so the
          # internet is up. Ignore other exit statuses.
          kill -TERM $pids > /dev/null 2>&1 || true
          wait $pids
          return 0
        fi
      fi
    done
    # wait -n $pids # Better than sleep, but not supported on all systems
    sleep 0.1
  done

  return 1
}

Usage:

if connected_to_internet; then
  echo "Connected to internet"
else
  echo "No internet connection"
fi

Some notes about this approach:

1. It is robust against all the false positives and negatives I outlined above
2. The requests all happen in parallel to maximize speed
3. It will return false if you technically have internet access but DNS is non-functional or your network settings are otherwise messed up, which I think is a reasonable thing to do in most cases