Automation with Cron & Systemd Timers: Scheduling Your Scripts

# View your crontab
crontab -l

# Edit your crontab
crontab -e

# Remove your crontab
crontab -r

# Crontab syntax:
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of week (0 - 6) (Sunday=0 or 7)
# │ │ │ │ │
# * * * * * command_to_execute

# Examples:
# Run every minute
* * * * * /home/user/script.sh

# Run every hour at minute 0
0 * * * * /home/user/hourly_task.sh

# Run daily at 2:30 AM
30 2 * * * /home/user/daily_backup.sh

# Run weekly on Sunday at 3 AM
0 3 * * 0 /home/user/weekly_report.sh

# Run monthly on the 1st at midnight
0 0 1 * * /home/user/monthly_cleanup.sh

# Run every 15 minutes
*/15 * * * * /home/user/frequent_task.sh

# Run at 9 AM on weekdays
0 9 * * 1-5 /home/user/weekday_task.sh

# Run at 6 PM on weekends
0 18 * * 6,0 /home/user/weekend_task.sh

# Common cron patterns with real commands

# Backup database daily at 1 AM
0 1 * * * /usr/bin/mysqldump --all-databases > /backup/db_$(date +\%Y\%m\%d).sql

# Clear cache every hour
0 * * * * /usr/bin/find /tmp -type f -atime +1 -delete

# Check disk space and alert if low
*/30 * * * * /usr/bin/df -h / | /usr/bin/awk 'NR==2 {if ($5+0 > 90) system("echo 'Disk full' | mail -s Alert admin")}'

# Rotate logs daily
0 0 * * * /usr/sbin/logrotate /etc/logrotate.conf

# Update system weekly
0 2 * * 0 /usr/bin/apt update && /usr/bin/apt upgrade -y

# Check website uptime every 5 minutes
*/5 * * * * /usr/bin/curl -f https://example.com || /usr/bin/echo "Site down" | /usr/bin/mail -s "Alert" admin@example.com

# Sync files to backup server hourly
0 * * * * /usr/bin/rsync -avz /important/data/ backup@server:/backup/

# Cron runs with a minimal environment (not your shell)
# Common issues: PATH not set, environment variables missing

# ALWAYS use full paths in cron
# Bad:
0 * * * * myscript.sh

# Good:
0 * * * * /home/user/scripts/myscript.sh

# Set PATH at top of crontab
PATH=/usr/local/bin:/usr/bin:/bin

# Set working directory
0 * * * * cd /home/user/scripts && ./myscript.sh

# Set environment variables
MAILTO=admin@example.com
SHELL=/bin/bash
HOME=/home/user

# Source your profile if needed
0 * * * * source /home/user/.bashrc && /home/user/scripts/myscript.sh

# Redirect output for debugging
0 * * * * /home/user/script.sh > /tmp/script.log 2>&1

# Disable email output (add >/dev/null 2>&1)
0 * * * * /home/user/quiet_script.sh >/dev/null 2>&1

# Systemd timers are defined in two files:
# 1. Service file (what to run)
# 2. Timer file (when to run it)

# Service file: /etc/systemd/system/backup.service
sudo cat > /etc/systemd/system/backup.service << 'EOF'
[Unit]
Description=Daily backup job

[Service]
Type=oneshot
ExecStart=/home/user/scripts/backup.sh
User=root
Group=root
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=multi-user.target
EOF

# Timer file: /etc/systemd/system/backup.timer
sudo cat > /etc/systemd/system/backup.timer << 'EOF'
[Unit]
Description=Run backup daily at 2 AM
Requires=backup.service

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

[Install]
WantedBy=timers.target
EOF

# Enable and start timer
sudo systemctl daemon-reload
sudo systemctl enable backup.timer
sudo systemctl start backup.timer

# Check timer status
systemctl list-timers
systemctl status backup.timer

# Check service logs
journalctl -u backup.service

# Timer OnCalendar expressions (more readable than cron)

# Every minute
OnCalendar=*-*-* *:*:00

# Every hour at minute 0
OnCalendar=hourly
OnCalendar=*-*-* *:00:00

# Daily at 2:30 AM
OnCalendar=daily
OnCalendar=*-*-* 02:30:00

# Weekly on Monday at 3 AM
OnCalendar=weekly
OnCalendar=Mon *-*-* 03:00:00

# Monthly on the 1st at midnight
OnCalendar=monthly
OnCalendar=*-*-01 00:00:00

# Every 15 minutes
OnCalendar=*:0/15

# Weekdays at 9 AM
OnCalendar=Mon..Fri *-*-* 09:00:00

# First Sunday of each month
OnCalendar=Sun *-*-01..07 00:00:00

# Randomized delay (avoid thundering herd)
[Timer]
OnCalendar=daily
RandomizedDelaySec=30min

# Persistent (run missed timers after boot)
[Timer]
OnCalendar=daily
Persistent=true

# Accuracy (power saving)
[Timer]
OnCalendar=daily
AccuracySec=1h

#!/bin/bash
# automated_backup.sh - Complete backup with scheduling

# Backup script (save as /usr/local/bin/backup.sh)
BACKUP_DIR="/backup"
RETENTION_DAYS=30
LOG_FILE="/var/log/backup.log"

log() {
    echo "[$(date '+%Y-%m-%d %H:%M:%S')] $1" | tee -a "$LOG_FILE"
}

log "Starting backup..."

# Create backup
BACKUP_FILE="$BACKUP_DIR/backup_$(date +%Y%m%d_%H%M%S).tar.gz"
tar -czf "$BACKUP_FILE" /home/user/data 2>/dev/null

if [ $? -eq 0 ]; then
    SIZE=$(du -h "$BACKUP_FILE" | cut -f1)
    log "Backup successful: $BACKUP_FILE ($SIZE)"
    
    # Remove old backups
    find "$BACKUP_DIR" -name "backup_*.tar.gz" -mtime +$RETENTION_DAYS -delete
    log "Removed backups older than $RETENTION_DAYS days"
else
    log "ERROR: Backup failed!"
    exit 1
fi

log "Backup completed"

# Cron setup
# 0 2 * * * /usr/local/bin/backup.sh

# Systemd timer setup
# /etc/systemd/system/backup.service
# /etc/systemd/system/backup.timer

# Check cron logs (on Debian/Ubuntu)
grep CRON /var/log/syslog

# Check cron logs (on RHEL/CentOS)
grep CRON /var/log/cron

# Systemd timer logs
journalctl -u mytimer.timer
journalctl -u myservice.service

# List all systemd timers
systemctl list-timers --all

# Show next run times
systemctl list-timers | grep mytimer

# Get detailed timer info
systemctl show mytimer.timer

# Email alerts from cron
# Set MAILTO in crontab
MAILTO=admin@example.com
0 2 * * * /usr/local/bin/backup.sh

# Health check script
#!/bin/bash
# check_jobs.sh - Verify scheduled jobs ran

EXPECTED_TIME=$(date -d "yesterday 02:00:00" +%s)
ACTUAL_TIME=$(stat -c %Y /backup/latest_backup.tar.gz)

if [ $ACTUAL_TIME -lt $EXPECTED_TIME ]; then
    echo "Backup did not run last night!"
    exit 1
fi