How to Set Up Cron Jobs on Linux (crontab Guide)

Cron is the standard job scheduler on Unix-like systems. The cron daemon (crond) runs in the background and checks every minute for tasks to execute. Tasks are defined in crontab (cron table) files.

Every Linux distribution includes cron by default. It's the foundation that most other scheduling tools build on.

Edit your crontab:

crontab -e

This opens your user's crontab in the default editor. Add a line:

*/5 * * * * echo "Hello from cron" >> /tmp/cron.log 2>&1

Save and exit. Your job is now scheduled.

List your crontab:

crontab -l

Remove your crontab (all jobs):

crontab -r

A cron expression has 5 fields:

┌───────────── minute (0-59)
│ ┌───────────── hour (0-23)
│ │ ┌───────────── day of month (1-31)
│ │ │ ┌───────────── month (1-12)
│ │ │ │ ┌───────────── day of week (0-7, 0 and 7 = Sunday)
│ │ │ │ │
* * * * * command

Special characters:

• * — any value
• , — list: 1,3,5
• - — range: 1-5
• / — step: */5 (every 5)

Examples:

• */5 * * * * — Every 5 minutes
• 0 9 * * 1-5 — Weekdays at 9 AM
• 0 0 1,15 * * — 1st and 15th of each month at midnight

Most cron implementations support shorthand strings:

• @reboot — Run once at startup
• @hourly — Same as 0 * * * *
• @daily (or @midnight) — Same as 0 0 * * *
• @weekly — Same as 0 0 * * 0
• @monthly — Same as 0 0 1 * *
• @yearly (or @annually) — Same as 0 0 1 1 *

@reboot /opt/myapp/start.sh
@daily /opt/myapp/backup.sh

• * * * * * — Every minute
• */5 * * * * — Every 5 minutes
• */15 * * * * — Every 15 minutes
• 0 * * * * — Every hour
• 0 0 * * * — Daily at midnight
• 0 9 * * * — Daily at 9 AM
• 0 9 * * 1-5 — Weekdays at 9 AM
• 0 0 * * 0 — Every Sunday at midnight
• 0 0 1 * * — First day of every month
• 0 0 1 1 * — January 1st at midnight

User crontab (crontab -e):

• Per-user schedule
• Stored in /var/spool/cron/crontabs/
• Commands run as that user
• No user field in the entry

System crontab (/etc/crontab):

• System-wide schedule
• Includes a user field between the time and command
• Requires root to edit

# /etc/crontab format (note the user field)
*/5 * * * * root /opt/scripts/cleanup.sh
0 2 * * *  backup /opt/scripts/backup.sh

Cron directories:

• /etc/cron.d/ — Drop-in cron files (system crontab format)
• /etc/cron.hourly/ — Scripts run every hour
• /etc/cron.daily/ — Scripts run daily
• /etc/cron.weekly/ — Scripts run weekly
• /etc/cron.monthly/ — Scripts run monthly

Cron runs with a minimal environment — your .bashrc, .profile, and PATH are NOT loaded. This is the #1 cause of "works in terminal but not in cron" issues.

Set variables in your crontab:

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

*/5 * * * * /opt/myapp/task.sh

Or source your profile in the command:

*/5 * * * * . $HOME/.profile; /opt/myapp/task.sh

MAILTO: Set this to receive email notifications of cron output. Set MAILTO="" to suppress emails.

By default, cron emails any output to the user. For better logging:

Redirect to a log file:

*/5 * * * * /opt/myapp/task.sh >> /var/log/myapp-cron.log 2>&1

• >> appends stdout to the file
• 2>&1 redirects stderr to stdout

Discard output:

*/5 * * * * /opt/myapp/task.sh > /dev/null 2>&1

Log with timestamps:

*/5 * * * * /opt/myapp/task.sh 2>&1 | while read line; do echo "$(date '+\%Y-\%m-\%d \%H:\%M:\%S') $line"; done >> /var/log/cron.log

Check cron's own log:

grep CRON /var/log/syslog       # Ubuntu/Debian
journalctl -u cron              # systemd
grep cron /var/log/cron         # CentOS/RHEL

Checklist for debugging:

1. Is cron running?

systemctl status cron    # or crond on CentOS

2. Is the crontab installed?

crontab -l

3. Is the PATH correct?

Use full paths to all binaries: /usr/bin/python3 not just python3

4. Are permissions correct?

Scripts must be executable: chmod +x /opt/myapp/task.sh

5. Check cron logs:

grep CRON /var/log/syslog | tail -20

6. Check for errors by redirecting output:

*/5 * * * * /opt/myapp/task.sh >> /tmp/cron-debug.log 2>&1

7. Is the user allowed to use cron?

Check /etc/cron.allow and /etc/cron.deny.

Lock files to prevent overlapping:

*/5 * * * * flock -n /tmp/myapp.lock /opt/myapp/task.sh

flock -n exits immediately if the lock is held, preventing overlapping runs.

Timeouts:

0 2 * * * timeout 3600 /opt/myapp/backup.sh

Kills the job after 1 hour if it's still running.

Monitoring: Ping a health check service on success:

0 * * * * /opt/myapp/task.sh && curl -s https://hc-ping.com/uuid > /dev/null

Centralized logging: Use logger to send output to syslog:

*/5 * * * * /opt/myapp/task.sh 2>&1 | logger -t myapp-cron