Schedule Jobs with Cron and Check That They Ran

Cron runs commands at set times: a nightly backup, a cleanup, a report. It is built into every Linux server. The most common problem is a job that looks right but never runs, so this guide also shows how to check.

Before you start, connect to your VPS with SSH.

1. How a schedule is written

A cron line has five time fields, then the command:

minute  hour  day-of-month  month  day-of-week  command

Examples:

30 2 * * * at 02:30 every day
*/15 * * * * every 15 minutes
0 9 * * 1 at 09:00 every Monday
0 0 1 * * at midnight on the first day of each month

The server's time zone decides what "02:30" means. Check it with timedatectl. Most VPS servers use UTC; India is UTC plus 5 hours 30 minutes.

2. Add a job

crontab -e

Choose an editor if asked, then add a line at the bottom:

30 2 * * * /usr/local/bin/cleanup.sh >> /var/log/cleanup.log 2>&1

The >> /var/log/cleanup.log 2>&1 part saves the output and the errors to a file. Without it you will never see why a job failed. Save and exit.

3. Make the script safe to run

sudo chmod 700 /usr/local/bin/cleanup.sh

Start the script with #!/bin/bash. Always write full paths to programs and files, because cron does not use your usual search path. For example, write /usr/bin/node and not just node.

4. Check that it ran

crontab -l
grep CRON /var/log/syslog | tail -n 10
tail -n 20 /var/log/cleanup.log

If you see your command in the syslog but nothing happens, run the exact command by hand and read the error.

5. Common reasons a job does not run

The script is not executable, or the path is wrong.
A program is not found because the path was not full.
The percent sign % has a special meaning in cron; write \% in dates, for example date +\%F.
The job uses a password or environment variable that exists only in your own login. Put the values inside the script, in a file that only that user can read.
The server was off or rebooting at the scheduled time.

Good to know

Run a job as the user it belongs to. Use sudo crontab -e only for jobs that need root.
For a job that must also run after a missed time, use a systemd timer with Persistent=true instead of cron.

Run it on your own VPS

Cloud VPS plans: An always-on server for your scheduled jobs. Prices are in rupees, you can pay by UPI, and you get a GST invoice.