How to Use Crontab -- Syntax, Examples & Common Errors

In this tutorial, you'll learn about How to Use Crontab. We cover key concepts, practical examples, and best practices.

The Problem

You set up a cron job with crontab -e but it never runs, or it runs but fails silently. The script works fine when you run it manually, but cron produces different results because of limited environment variables and missing paths.

Quick Fix

Step 1: Understand the five-field syntax

Crontab entries use five time fields followed by the command:

# * * * * * command
# | | | | |
# | | | | +-- Day of week (0-6, 0=Sunday)
# | | | +---- Month (1-12)
# | | +------ Day of month (1-31)
# | +-------- Hour (0-23)
# +---------- Minute (0-59)

Example -- run every day at 2:30 AM:

30 2 * * * /home/user/scripts/backup.sh

Step 2: Use absolute paths

Cron runs with a minimal PATH. Always use full paths:

# Wrong -- script may not be found
30 2 * * * backup.sh

# Right
30 2 * * * /home/user/scripts/backup.sh

Step 3: Redirect output to a log file

Capturing output helps debug failures:

30 2 * * * /home/user/scripts/backup.sh >> /home/user/logs/backup.log 2>&1

Step 4: Add PATH and environment variables

Set environment variables at the top of the crontab:

SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
MAILTO=admin@example.com

30 2 * * * /home/user/scripts/backup.sh

Step 5: Test commands with common crontab patterns

Run every minute (for testing):

* * * * * /home/user/scripts/test.sh

Run every hour:

0 * * * * /home/user/scripts/hourly.sh

Run every Sunday at midnight:

0 0 * * 0 /home/user/scripts/weekly.sh

Step 6: Check cron logs

View cron execution history:

sudo journalctl -u cron --no-pager -n 20

Or check syslog:

grep CRON /var/log/syslog | tail -10

Alternative Solutions

Use systemd timers instead of cron

Modern alternative with better logging and dependency control:

sudo systemctl edit --full my-timer.timer

Use special @reboot syntax

Run a command once at system boot:

@reboot /home/user/scripts/startup.sh

Common Mistakes to Avoid

Not using absolute paths in cron jobs. Cron runs with a minimal PATH. Always use full paths to executables and scripts.

Forgetting that % is special in crontab. The % character must be escaped with a backslash in cron commands. date +\%Y-\%m-\%d.

Not redirecting output to a log file. Without output redirection, cron errors go to the system mail, which may never be read.

Pro Tips

Use MAILTO to receive cron output. Set MAILTO=admin@example.com at the top of your crontab to receive job output via email.

Use flock to prevent overlapping jobs. Wrap your cron command with flock -n /tmp/myapp.lock to ensure only one instance runs at a time.

Use crontab guru for syntax validation. Visit crontab.guru to check your cron expressions before adding them to production systems.

Prevention

• Always use absolute paths for scripts and commands in cron jobs.
• Redirect stdout and stderr to a log file for troubleshooting.
• Test scripts manually with the same environment cron uses: env -i /bin/bash -c 'source /etc/profile && /path/to/script.sh'.