Ubuntu cron Basics: Why It's Not Running and How to Fix It

cron Basics - Scheduled Task Troubleshooting
Reading time: ~10 min Level: Intermediate

What You'll Learn

  • How to check and edit crontab
  • What to do when "cron job was set but not running"
  • How to investigate cron logs and distinguish typical failure patterns

Quick Summary

When cron isn't working, check these in order:

  1. Check crontab: crontab -l
  2. View logs: grep CRON /var/log/syslog
  3. Verify path: Use absolute paths in crontab
  4. Check permissions: Executable bit set?
  5. Test manually: Run the command as root or user

Table of Contents

  1. What is cron?
  2. How to Check/Edit crontab
  3. Schedule Syntax
  4. Checking Logs When Not Running
  5. Top 5 Mistakes
  6. Troubleshooting Steps

Prerequisites

  • OS: Ubuntu
  • sudo access

1. What is cron?

cron is a daemon that executes scheduled tasks. Combined with crontab, you can automate:

  • Periodic log rotation
  • Backups
  • Sending metrics
  • Batch processing

2. How to Check/Edit crontab

2-1. View current user's crontab

$ crontab -l

2-2. Edit crontab

$ crontab -e

2-3. View another user's crontab (requires sudo)

$ sudo crontab -u username -l

2-4. View root's crontab

$ sudo crontab -l

3. Schedule Syntax

* * * * * command
│ │ │ │ │
│ │ │ │ └─ Day of week (0-7, 0 and 7 = Sunday)
│ │ │ └─── Month (1-12)
│ │ └───── Day (1-31)
│ └─────── Hour (0-23)
└───────── Minute (0-59)

Examples:

  • 0 3 * * * - Every day at 3:00 AM
  • */5 * * * * - Every 5 minutes
  • 0 0 * * 0 - Every Sunday at midnight

4. Checking Logs When Not Running

4-1. Check cron logs

$ grep CRON /var/log/syslog | tail -n 50

4-2. Real-time log tracking

$ tail -f /var/log/syslog | grep CRON

4-3. Using journalctl

$ sudo journalctl -u cron -n 100

If logs show "(CRON) CMD..." but nothing happens, the command itself is failing. Run it manually to check.

5. Top 5 Mistakes

Mistake 1: Not using absolute paths

cron runs with minimal environment. Commands like python or node might not be found.

Fix: Use full paths like /usr/bin/python3

Mistake 2: Scripts not executable

$ chmod +x /path/to/script.sh

Mistake 3: Environment variables not set

Variables like PATH, HOME may not be available in cron.

Fix: Define them at the top of crontab or in the script.

Mistake 4: Editing wrong user's crontab

Using crontab -e as user vs sudo crontab -e gives different crontabs.

Mistake 5: Typo in schedule syntax

A space or number in the wrong place can prevent execution entirely.

6. Troubleshooting Steps

Step 1: Verify crontab exists

$ crontab -l
$ sudo crontab -l

Step 2: Check cron service

$ sudo systemctl status cron

Step 3: Test command manually

$ /full/path/to/script.sh

Step 4: Add output redirection for debugging

* * * * * /path/to/script.sh >> /tmp/cron.log 2>&1

Summary (Copy-Paste Template)

# Check crontab
crontab -l
sudo crontab -l

# Edit crontab
crontab -e

# Check cron service
sudo systemctl status cron

# View logs
grep CRON /var/log/syslog | tail -n 50
sudo journalctl -u cron -n 100

Related Articles

Test Environment

Commands in this article were tested on Ubuntu 24.04 LTS / bash 5.2.

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