Monitoring Cron Jobs

Health Checks is perfectly suited for monitoring cron jobs. All you have to do is update your cron job command to send an HTTP request to Health Checks after completing the job.

Let's look at an example:

$ crontab -l
# m h dom mon dow command
  8 6 * * * /home/user/backup.sh

The above job runs /home/user/backup.sh every day at 6:08. The backup script is presumably a headless, background process. Even if it works correctly currently, it can start silently failing in the future without anyone noticing.

You can set up Health Checks to notify you whenever the backup script does not run on time, or it does not complete successfully. Here are the steps to do that.

1. If you have not already, sign up for a free Health Checks account.

2. In your Health Checks account, add a new check.

3. Give the check a meaningful name. Good naming will become increasingly important as you add more checks to your account.

4. Edit the check's schedule:

   • change its type from "Simple" to "Cron"
   • enter 8 6 * * * in the cron expression field
   • set the timezone to match your machine's timezone

5. Take note of your check's unique ping URL.

Finally, edit your cron job definition and append a curl or wget call after the command:

$ crontab -e
# m h dom mon dow command
  8 6 * * * /home/user/backup.sh && curl -fsS --retry 5 -o /dev/null https://healthcheck.ddmnet.com/ping/your-uuid-here

Now, each time your cron job runs, it will send an HTTP request to the ping URL. Since Health Checks knows your cron job's schedule, it can calculate the dates and times when the job should run. As soon as your cron job doesn't report at an expected time, Health Checks will send you a notification.

This monitoring technique takes care of various failure scenarios that could potentially go unnoticed otherwise:

• The whole machine goes down (power outage, janitor stumbles on wires, VPS provider problems, etc.)
• the cron daemon is not running or has an invalid configuration
• cron does start your task, but the task exits with a non-zero exit code

Curl Options

The extra options in the above example tell curl to retry failed HTTP requests, and silence output unless there is an error. Feel free to adjust the curl options to suit your needs.

&&

Run curl only if /home/user/backup.sh exits with an exit code 0.

-f, --fail

Makes curl treat non-200 responses as errors.

-s, --silent

Silent or quiet mode. Hides the progress meter, but also hides error messages.

-S, --show-error

Re-enables error messages when -s is used.

--retry <num>

If a transient error is returned when curl tries to perform a transfer, it will retry this number of times before giving up. Setting the number to 0 makes curl do no retries (which is the default). Transient error is a timeout or an HTTP 5xx response code.

-o /dev/null

Redirect curl's stdout to /dev/null (error messages still go to stderr).

Looking up Your Machine's Time Zone

If your cron job consistently pings Health Checks an hour early or an hour late, the likely cause is a timezone mismatch: your machine may be using a timezone different from what you have configured on Health Checks.

On modern GNU/Linux systems, you can look up the time zone using the timedatectl status command and looking for "Time zone" in its output:

$ timedatectl status

               Local time: C  2020-01-23 12:35:50 EET
           Universal time: C  2020-01-23 10:35:50 UTC
                 RTC time: C  2020-01-23 10:35:50
                Time zone: Europe/Riga (EET, +0200)
System clock synchronized: yes
              NTP service: active
          RTC in local TZ: no

Viewing Cron Logs Using journalctl

On a systemd-based system, you can use the journalctl utility to see system logs, including logs from the cron daemon.

To see live logs:

journalctl -f

To see the logs from e.g. the last hour, and only from the cron daemon:

journalctl --since "1 hour ago" -t CRON