How to Use Cron Jobs for Task Scheduling

What Is Cron?

Cron is a time-based job scheduler on Linux. It runs commands or scripts at specified intervals — every minute, hourly, daily, weekly, or on custom schedules.

Crontab Format

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

Managing Crontab

# Edit your crontab
crontab -e

# List current crontab
crontab -l

# Edit another user's crontab (as root)
crontab -u www-data -e

Common Examples

# Every day at 3:00 AM
0 3 * * * /usr/local/bin/backup.sh

# Every 5 minutes
*/5 * * * * /usr/local/bin/check-health.sh

# Every Monday at 9 AM
0 9 * * 1 /usr/local/bin/weekly-report.sh

# First day of every month at midnight
0 0 1 * * /usr/local/bin/monthly-cleanup.sh

# Every weekday (Mon-Fri) at 8 AM
0 8 * * 1-5 /usr/local/bin/daily-digest.sh

Shortcuts

@reboot    /usr/local/bin/startup.sh    # Run once at boot
@hourly    /usr/local/bin/hourly.sh     # 0 * * * *
@daily     /usr/local/bin/daily.sh      # 0 0 * * *
@weekly    /usr/local/bin/weekly.sh     # 0 0 * * 0
@monthly   /usr/local/bin/monthly.sh    # 0 0 1 * *

Logging Output

# Log output to a file
0 3 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

# Discard output (no email)
*/5 * * * * /usr/local/bin/check.sh > /dev/null 2>&1

Common Pitfalls

• Cron uses a minimal PATH — use full paths for commands
• Cron does not load your shell profile — set environment variables in the crontab
• Make scripts executable: chmod +x script.sh
• Use 2>&1 to capture both stdout and stderr