Cron Job Not Running? How to Diagnose and Fix It

When a cron job doesn’t run, the cause is almost always one of a handful of problems. Work through this checklist systematically.

Step 1: Verify the cron daemon is running

# Debian/Ubuntu
systemctl status cron

# CentOS/RHEL/Rocky
systemctl status crond

If it’s not running:

sudo systemctl start cron
sudo systemctl enable cron   # start on boot

Step 2: Check the cron log

Cron logs every execution. If your job isn’t even appearing in the log, the schedule or daemon is the problem.

# Ubuntu/Debian
grep CRON /var/log/syslog | tail -30

# CentOS/RHEL
grep CRON /var/log/cron | tail -30

# journald (systemd systems)
journalctl -u cron | tail -30
journalctl -u crond | tail -30

If cron ran the job, you’ll see lines like:

Apr 25 14:05:01 hostname CRON[12345]: (ubuntu) CMD (/home/ubuntu/script.sh)

If you see nothing, the expression is wrong or cron is not running.

Step 3: Validate your cron expression

Parse the expression carefully:

*/5 * * * * /script.sh
│   │ │ │ └── Day of week (0-7)
│   │ │ └──── Month (1-12)
│   │ └────── Day of month (1-31)
│   └──────── Hour (0-23)
└──────────── Minute (0-59)

Common mistakes:

• 0 */5 * * * — runs every 5 hours, not every 5 minutes
• 30 2 * * 7 — day-of-week 7 is Sunday in some implementations, check yours
• Missing space between fields
• Using % unescaped (see below)

Use crontab.io to validate the expression and see the next scheduled run times.

Step 4: The PATH problem

Cron runs with a minimal PATH: /usr/bin:/bin. Commands that work in your shell often fail in cron.

Test by running your command in a cron-like environment:

env -i HOME=/home/ubuntu PATH=/usr/bin:/bin /home/ubuntu/script.sh

Fix by using full paths in your crontab:

# Wrong
*/5 * * * * python3 /home/ubuntu/sync.py

# Right
*/5 * * * * /usr/bin/python3 /home/ubuntu/sync.py

Find the full path of any command:

which python3
# /usr/bin/python3

which node
# /usr/local/bin/node

Or set PATH at the top of your crontab:

PATH=/usr/local/bin:/usr/bin:/bin
HOME=/home/ubuntu

*/5 * * * * python3 /home/ubuntu/sync.py

Step 5: Script is not executable

ls -la /home/ubuntu/script.sh
# -rw-r--r-- ← no execute permission

Fix:

chmod +x /home/ubuntu/script.sh

Or call the interpreter explicitly instead of relying on the shebang:

*/5 * * * * /bin/bash /home/ubuntu/script.sh
*/5 * * * * /usr/bin/python3 /home/ubuntu/script.py

Step 6: Script errors hidden by output suppression

If you’ve redirected output to /dev/null, errors are swallowed:

*/5 * * * * /script.sh > /dev/null 2>&1   ← errors invisible

Temporarily log everything to diagnose:

*/5 * * * * /script.sh >> /tmp/cron-debug.log 2>&1

Then check:

cat /tmp/cron-debug.log

Step 7: Percent sign (%) in commands

In crontab, unescaped % is interpreted as a newline. This breaks commands that use %:

# Wrong: % in date breaks the command
0 0 * * * cp file.txt backup-$(date +%Y%m%d).txt

# Right: escape the %
0 0 * * * cp file.txt backup-$(date +\%Y\%m\%d).txt

Step 8: Relative paths

Cron’s working directory is not your home directory — it’s usually /. Relative paths don’t work:

# Wrong: relative path
*/5 * * * * ./script.sh
*/5 * * * * scripts/sync.py

# Right: absolute path
*/5 * * * * /home/ubuntu/script.sh
*/5 * * * * /home/ubuntu/scripts/sync.py

Step 9: Environment variables not loaded

.bashrc, .bash_profile, and .profile are not sourced by cron. If your script depends on env vars set there:

# Source profile explicitly in the script
#!/bin/bash
source /home/ubuntu/.profile
# or
source /etc/environment

# Or define env vars in the crontab
MY_API_KEY="secret"
*/5 * * * * /home/ubuntu/sync.py

Step 10: Permission to access files/directories

Even if the script runs, it may fail silently if it can’t read/write files:

# Test as the cron user
sudo -u ubuntu /home/ubuntu/script.sh

Check ownership and permissions on any files the script touches.

Quick diagnostic script

Save this as /tmp/cron-test.sh and add it to your crontab for 1 minute to verify cron can run jobs:

#!/bin/bash
echo "Cron ran at $(date)" >> /tmp/cron-test-output.log
echo "PATH=$PATH" >> /tmp/cron-test-output.log
echo "USER=$USER" >> /tmp/cron-test-output.log
echo "HOME=$HOME" >> /tmp/cron-test-output.log

chmod +x /tmp/cron-test.sh
crontab -e
# Add: * * * * * /tmp/cron-test.sh

Wait 1 minute, then check /tmp/cron-test-output.log. If it exists, cron is working.

Build valid cron expressions at crontab.io.