Crontab Scheduler Guide — Cron Expressions and Practical Examples

What Is cron?

cron is a system daemon in Linux/Unix that schedules and executes recurring tasks. It automatically handles periodic tasks like backups, log cleanup, report generation, and data synchronization without manual intervention.

Here’s a summary of cron-related terminology.

Term	Description
cron daemon (crond)	Scheduler process running in the background
crontab	Configuration file defining the list of cron jobs
cron job	An individual task registered in crontab
cron expression	5-field expression specifying execution time

Cron Expression Syntax

A cron expression specifies execution time using 5 fields.

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

Commonly used patterns:

# Run every minute
* * * * * /opt/scripts/healthcheck.sh

# Run at the top of every hour
0 * * * * /opt/scripts/hourly-report.sh

# Run daily at 3:00 AM
0 3 * * * /opt/scripts/daily-backup.sh

# Run every Monday at 9:00 AM
0 9 * * 1 /opt/scripts/weekly-report.sh

# Run at midnight on the 1st of every month
0 0 1 * * /opt/scripts/monthly-cleanup.sh

# Run at 8:00 AM on weekdays (Mon-Fri)
0 8 * * 1-5 /opt/scripts/workday-task.sh

# Run every 10 minutes
*/10 * * * * /opt/scripts/check-service.sh

# Run daily at 9:00 AM and 6:00 PM
0 9,18 * * * /opt/scripts/sync-data.sh

# Run daily at 2:00 AM from January to March
0 2 * 1-3 * /opt/scripts/quarterly-job.sh

Special character meanings:

Character	Meaning	Example
`*`	Every value	`* * * * *` (every minute)
`,`	Multiple values	`0,30 * * * *` (at 0 and 30 minutes)
`-`	Range	`1-5` (Mon-Fri)
`/`	Interval	`*/15` (every 15)

crontab Commands

Commands for editing and managing crontab files.

# Edit current user's crontab
crontab -e

# List current user's crontab
crontab -l
# 0 3 * * * /opt/scripts/daily-backup.sh
# */10 * * * * /opt/scripts/healthcheck.sh

# Check specific user's crontab (requires root)
sudo crontab -u deploy -l

# Delete entire crontab (caution!)
crontab -r

# Confirm before deleting
crontab -ri

How to set environment variables in crontab:

# Environment variable settings in crontab (edit via crontab -e)
# cron runs in a minimal environment, so PATH must be specified
SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin
MAILTO=admin@example.com

# Load environment variables file before execution
0 3 * * * source /opt/my-app/.env && /opt/scripts/backup.sh

# Or use absolute paths explicitly
0 3 * * * /usr/bin/env bash -c 'source /opt/my-app/.env && /opt/scripts/backup.sh'

Setting MAILTO sends the cron job’s output (stdout/stderr) via email. If email is unnecessary, redirect the output.

Checking Logs

How to verify whether cron jobs ran and check for errors.

# cron daemon log (Ubuntu/Debian)
sudo grep CRON /var/log/syslog
# Mar  5 03:00:01 server CRON[1234]: (deploy) CMD (/opt/scripts/daily-backup.sh)

# cron daemon log (CentOS/RHEL)
sudo grep CRON /var/log/cron

# Check only recent cron execution records
sudo grep CRON /var/log/syslog | tail -20

# Check cron execution records for a specific user
sudo grep "CRON.*deploy" /var/log/syslog

Patterns for saving cron job output to log files.

# Record both stdout and stderr to a log file
0 3 * * * /opt/scripts/backup.sh >> /var/log/my-backup.log 2>&1

# Create date-based log files
0 3 * * * /opt/scripts/backup.sh >> /var/log/backup-$(date +\%Y\%m\%d).log 2>&1

# Completely suppress output (not recommended — cannot trace errors)
0 3 * * * /opt/scripts/backup.sh > /dev/null 2>&1

In crontab, % is interpreted as a newline. You must escape it as \% when using % within commands.

Practical Automation Examples

Database Backup Script

#!/bin/bash
# /opt/scripts/backup-db.sh
# PostgreSQL daily backup script

# Environment variables
BACKUP_DIR="/opt/backups/db"
DB_NAME="myapp_production"
DATE=$(date +%Y%m%d_%H%M%S)
RETENTION_DAYS=7

# Create backup directory
mkdir -p "${BACKUP_DIR}"

# Backup with pg_dump (gzip compressed)
pg_dump "${DB_NAME}" | gzip > "${BACKUP_DIR}/${DB_NAME}_${DATE}.sql.gz"

# Check if backup succeeded
if [ $? -eq 0 ]; then
    echo "[$(date)] Backup successful: ${DB_NAME}_${DATE}.sql.gz"
    # Delete old backups (older than 7 days)
    find "${BACKUP_DIR}" -name "*.sql.gz" -mtime +${RETENTION_DAYS} -delete
    echo "[$(date)] Cleaned up backups older than ${RETENTION_DAYS} days"
else
    echo "[$(date)] Backup failed: ${DB_NAME}" >&2
    exit 1
fi

Log Cleanup Script

#!/bin/bash
# /opt/scripts/cleanup-logs.sh
# Script to clean up old log files

LOG_DIRS=(
    "/var/log/my-app"
    "/var/log/nginx"
    "/opt/my-app/logs"
)

RETENTION_DAYS=30

for dir in "${LOG_DIRS[@]}"; do
    if [ -d "${dir}" ]; then
        # Delete log files older than specified days
        deleted=$(find "${dir}" -name "*.log" -mtime +${RETENTION_DAYS} -delete -print | wc -l)
        # Also delete compressed logs
        deleted_gz=$(find "${dir}" -name "*.log.gz" -mtime +${RETENTION_DAYS} -delete -print | wc -l)
        echo "[$(date)] ${dir}: ${deleted} logs, ${deleted_gz} compressed logs deleted"
    fi
done

# Report disk usage
echo "[$(date)] Disk usage:"
df -h / | tail -1

crontab Registration

# Register via crontab -e
# Daily DB backup at 3:00 AM
0 3 * * * /opt/scripts/backup-db.sh >> /var/log/backup-db.log 2>&1
# Weekly log cleanup at 4:00 AM on Sundays
0 4 * * 0 /opt/scripts/cleanup-logs.sh >> /var/log/cleanup.log 2>&1
# Service health check every 5 minutes
*/5 * * * * curl -sf http://localhost:3000/health > /dev/null || systemctl restart my-app

Debugging cron Jobs

Things to check when cron jobs don’t work as expected.

# 1. Verify the script has execute permission
ls -l /opt/scripts/backup-db.sh
chmod +x /opt/scripts/backup-db.sh

# 2. Run the script manually to check for errors
/opt/scripts/backup-db.sh

# 3. Test in cron environment (simulate minimal environment)
env -i SHELL=/bin/bash PATH=/usr/bin:/bin HOME=/home/deploy /opt/scripts/backup-db.sh

# 4. Verify the cron daemon is running
systemctl status cron
# ● cron.service - Regular background program processing daemon
#      Active: active (running)

# 5. Validate cron expression (use crontab.guru)
# https://crontab.guru/#0_3_*_*_*

Practical Tips

Use absolute paths: cron does not load .bashrc/.profile, so PATH is limited. Use absolute paths (/usr/bin/python3) for all commands in scripts, or declare PATH at the top of the script.
Always redirect output: If you don’t redirect cron job output to a log file (>> /var/log/xxx.log 2>&1), output accumulates as emails or is lost entirely.
Shorthand expressions: Shortcuts like @reboot (on boot), @daily (midnight daily), @hourly (top of every hour), @weekly (Sunday midnight), and @monthly (1st at midnight) are also supported.
Prevent concurrent execution: For long-running tasks, use flock to prevent duplicate runs. Example: 0 * * * * flock -n /tmp/mylock.lock /opt/scripts/long-task.sh.
Compare with systemd timers: For new projects, consider systemd timers as well. They offer better log management than cron and support Persistent=true to catch up on missed runs. However, for simple recurring tasks, cron is easier to configure.