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Cron Job Scheduler: A Practical Guide

Covers crontab syntax, environment variable setup, and logging methods for automating recurring jobs.

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Contents
  1. What is Cron?
  2. Basic crontab Usage
  3. Cron Syntax
  4. Environment Variable Setup
  5. Logging
  6. The /etc/cron.d Approach (Recommended)
  7. Common Mistakes

What is Cron?

A time-based job scheduler. Use it to automate recurring tasks such as backups, log cleanup, and report generation.

Basic crontab Usage

Bash
crontab -e        # Edit for the current user
crontab -l        # List currently registered jobs
crontab -r        # Delete all (caution)
sudo crontab -u www-data -e

Cron Syntax

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

Common Patterns

Bash
# Run every minute
* * * * * /opt/scripts/check.sh

# Backup every day at 2 AM
0 2 * * * /opt/scripts/backup.sh

# Every Monday at 9 AM
0 9 * * 1 /opt/scripts/weekly_report.sh

# Midnight on the 1st of every month
0 0 1 * * /opt/scripts/monthly_clean.sh

# Every 5 minutes
*/5 * * * * /opt/scripts/heartbeat.sh

# Weekdays (Mon–Fri) 8 AM–6 PM, every 30 minutes
*/30 8-18 * * 1-5 /opt/scripts/sync.sh

Environment Variable Setup

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

0 2 * * * /opt/scripts/backup.sh

Logging

Bash
# Save stdout and stderr to a file
0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1

# Add a date timestamp
0 2 * * * echo "[$(date)] Backup started" >> /var/log/backup.log && /opt/scripts/backup.sh >> /var/log/backup.log 2>&1
Bash
# /etc/cron.d/myapp
SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin

# Run daily at 3 AM as the deploy user
0 3 * * * deploy /opt/myapp/scripts/cleanup.sh >> /var/log/myapp-cron.log 2>&1

Common Mistakes

MistakeFix
Command path not specifiedUse an absolute path (/usr/bin/python3)
Missing environment variablesAdd source /etc/environment at the top of the script
No execute permissionchmod +x script.sh
Hard to verify resultsAppend >> /var/log/cron-job.log 2>&1
#cron#crontab#linux
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