cron-scheduling

Cron & Scheduling

Cron Syntax

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

Common schedules

* * * * *           # Every minute
*/5 * * * *         # Every 5 minutes
0 * * * *           # Every hour at :00
30 2 * * *          # Every day at 2:30 AM
0 9 * * 1           # Every Monday at 9:00 AM
0 0 1 * *           # First day of every month at midnight
0 3 * * 0           # Every Sunday at 3 AM
@reboot             # Run once at startup
@daily              # 0 0 * * *
@hourly             # 0 * * * *

Crontab Management

crontab -e           # Edit crontab
crontab -l           # List crontab
sudo crontab -u www-data -e
crontab -l > crontab-backup-$(date +%Y%m%d).txt

Best practices

PATH=/usr/local/bin:/usr/bin:/bin
[email protected]

0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1
*/5 * * * * /opt/scripts/healthcheck.sh || /opt/scripts/alert.sh "Health check failed"
* * * * * flock -n /tmp/myjob.lock /opt/scripts/slow-job.sh

Systemd Timers

# /etc/systemd/system/backup.service
[Unit]
Description=Daily backup

[Service]
Type=oneshot
ExecStart=/opt/scripts/backup.sh
User=backup
StandardOutput=journal
StandardError=journal

# /etc/systemd/system/backup.timer
[Unit]
Description=Run backup daily at 2 AM

[Timer]
OnCalendar=*-*-* 02:00:00
Persistent=true
RandomizedDelaySec=300

[Install]
WantedBy=timers.target

sudo systemctl daemon-reload
sudo systemctl enable --now backup.timer
systemctl list-timers
systemctl list-timers --all
systemctl status backup.service
journalctl -u backup.service --since today
sudo systemctl start backup.service   # run manually
sudo systemctl disable --now backup.timer

OnCalendar syntax

OnCalendar=daily
OnCalendar=Mon *-*-* 09:00:00
OnCalendar=Mon..Fri *-*-* 08:00:00
OnCalendar=*-*-01 00:00:00       # Monthly
OnCalendar=0/6:00:00             # Every 6 hours

systemd-analyze calendar "Mon *-*-* 09:00:00"
systemd-analyze calendar --iterations=5 "Mon..Fri *-*-* 08:00:00"

Monitoring and Debugging

systemctl status cron
grep CRON /var/log/syslog
journalctl -u cron --since today
# Test with cron's minimal environment:
env -i HOME=$HOME SHELL=/bin/sh PATH=/usr/bin:/bin /opt/scripts/backup.sh

Job wrapper with logging

#!/bin/bash
set -euo pipefail
JOB_NAME="${1:?Usage: cron-wrapper.sh <job-name> <command> [args...]}"
shift
LOG_DIR="/var/log/cron-jobs"
mkdir -p "$LOG_DIR"
LOG_FILE="$LOG_DIR/$JOB_NAME.log"
log() { echo "[$(date -u '+%Y-%m-%dT%H:%M:%SZ')] $*" >> "$LOG_FILE"; }
log "START: $*"
START_TIME=$(date +%s)
if "$@" >> "$LOG_FILE" 2>&1; then
    log "SUCCESS ($(( $(date +%s) - START_TIME ))s)"
else
    EXIT_CODE=$?
    log "FAILED with exit code $EXIT_CODE"
    exit $EXIT_CODE
fi

Tips

• Always redirect output: >> /var/log/job.log 2>&1
• Test with env -i to simulate cron's minimal environment
• Use flock to prevent overlapping runs
• Make all scheduled jobs idempotent
• systemd-analyze calendar to verify timer schedules
• Never schedule critical jobs 1:00-3:00 AM if DST applies — use UTC
• Log start time, end time, exit code of every job
• Prefer systemd timers over cron for production services (journald logging, Persistent=true)