Check a website and alert when it is down

1 · Prepare

Put these pieces in place

• Pick a public web address that anyone can open without signing in. A page behind a login always looks down to the script.
• Use an account that can send email, because the alert goes to the address of the account that owns the script.
• Create a standalone Apps Script project at script.google.com, or use a project you already have, and keep both functions below in it.

2 · Review before running

What the script can access and change

3 · Copy

Paste this into the Apps Script editor

Create or open a script file, replace the example values marked above, then save the project.

const URL_TO_CHECK = 'https://example.com'; // The page the script asks for every hour. Replace it with your own address.
const STATUS_PROPERTY = 'SITE_STATUS'; // The script property that remembers a failure, so the same outage is not reported again and again.
const ALERT_SUBJECT = 'Website check failed'; // The subject of the email sent when the check fails.

function createWebsiteCheckTrigger() {
  const alreadyInstalled = ScriptApp.getProjectTriggers().some((trigger) => {
    return trigger.getHandlerFunction() === 'checkWebsite';
  });
  if (alreadyInstalled) return;

  ScriptApp.newTrigger('checkWebsite')
    .timeBased()
    .everyHours(1)
    .create();
}

function checkWebsite() {
  const properties = PropertiesService.getScriptProperties();
  const lastStatus = properties.getProperty(STATUS_PROPERTY);
  const owner = Session.getEffectiveUser().getEmail();

  let statusCode = 0;
  try {
    statusCode = UrlFetchApp.fetch(URL_TO_CHECK, { muteHttpExceptions: true }).getResponseCode();
  } catch (error) {
    statusCode = 0;
  }

  const isUp = statusCode >= 200 && statusCode < 400;
  Logger.log(URL_TO_CHECK + ' answered with status code ' + statusCode);

  if (!isUp && lastStatus !== 'down') {
    GmailApp.sendEmail(owner, ALERT_SUBJECT, URL_TO_CHECK + ' answered with status code ' + statusCode + '. A zero means no answer arrived at all.');
    properties.setProperty(STATUS_PROPERTY, 'down');
    return;
  }

  if (isUp && lastStatus === 'down') {
    GmailApp.sendEmail(owner, 'Website is back up', URL_TO_CHECK + ' answered with status code ' + statusCode + '.');
    properties.deleteProperty(STATUS_PROPERTY);
  }
}

4 · Run

Follow these steps in order

1. Test against a working address

   Paste both functions over the starter code and save. Choose checkWebsite in the function dropdown above the code, then click Run. Approve access when Google asks. The Execution log shows the status code. A status code is the short number a web server sends back to say how the request went. Here, anything from 200 to 399 counts as up. A healthy site sends no email.

2. Test the alert on purpose

   Change URL_TO_CHECK to an address that fails, such as https://example.com/page-that-is-not-there, save, and run checkWebsite again. One email with the subject Website check failed arrives. Run it a second time and no further email arrives, because the script has already saved the failure.

3. Watch it recover

   Put the working address back, save, and run checkWebsite once more. An email with the subject Website is back up arrives, and the saved property is deleted. Open Project Settings, the gear icon, and scroll to Script Properties to see that SITE_STATUS is gone.

4. Install the hourly trigger

   Choose createWebsiteCheckTrigger and click Run once. Running it again does not add a second trigger, because it stops when a trigger for checkWebsite already exists. Click Triggers, the clock icon in the left sidebar, to see the hour timer.

5 · Confirm

How to tell it worked

The Executions list shows a run each hour with a status code in the log, and your inbox holds at most two emails per outage, one when it starts and one when it ends.

If the result is different

Troubleshooting

The email says status code 0.

Check URL_TO_CHECK for a typo, and make sure it starts with https and opens in a browser. A zero means the request did not finish, so no code came back.

An alert arrives every hour for the same outage.

Open Project Settings, scroll to Script Properties, and check that SITE_STATUS is saved with the value down. Repeated alerts also happen when a site keeps switching between answering and failing, because each recovery clears the property.

The site is clearly down but no email arrives.

Delete SITE_STATUS under Project Settings, Script Properties, to let the next failing check alert you again. The script sends no second alert while the property holds the value down.

A page that opens in your browser reports status code 401 or 403.

Check a public page instead. Those codes mean the server wants a sign-in, and the script is not signed in to that site.