Skill: gitgoodordietrying/cron-scheduling (version 1.0.0)

--- name: cron-scheduling description: Schedule and manage recurring tasks with cron and systemd timers. Use when setting up cron jobs, writing systemd timer units, handling timezone-aware scheduling, monitoring failed jobs, implementing retry patterns, or debugging why a scheduled task didn't run. metadata: {"clawdbot":{"emoji":"⏰","requires":{"anyBins":["crontab","systemctl","at"]},"os":["linux","darwin"]}} --- # Cron & Scheduling Schedule and manage recurring tasks. Covers cron syntax, crontab management, systemd timers, one-off scheduling, timezone handling, monitoring, and common failure patterns. ## When to Use - Running scripts on a schedule (backups, reports, cleanup) - Setting up systemd timers (modern cron alternative) - Debugging why a scheduled job didn't run - Handling timezones in scheduled tasks - Monitoring and alerting on job failures - Running one-off delayed commands ## Cron Syntax ### The five fields ``` ┌───────── minute (0-59) │ ┌─────── hour (0-23) │ │ ┌───── day of month (1-31) │ │ │ ┌─── month (1-12 or JAN-DEC) │ │ │ │ ┌─ day of week (0-7, 0 and 7 = Sunday, or SUN-SAT) │ │ │ │ │ * * * * * command ``` ### Common schedules ```bash # Every minute * * * * * /path/to/script.sh # Every 5 minutes */5 * * * * /path/to/script.sh # Every hour at :00 0 * * * * /path/to/script.sh # Every day at 2:30 AM 30 2 * * * /path/to/script.sh # Every Monday at 9:00 AM 0 9 * * 1 /path/to/script.sh # Every weekday at 8:00 AM 0 8 * * 1-5 /path/to/script.sh # First day of every month at midnight 0 0 1 * * /path/to/script.sh # Every 15 minutes during business hours (Mon-Fri 9-17) */15 9-17 * * 1-5 /path/to/script.sh # Twice a day (9 AM and 5 PM) 0 9,17 * * * /path/to/script.sh # Every quarter (Jan, Apr, Jul, Oct) on the 1st at midnight 0 0 1 1,4,7,10 * /path/to/script.sh # Every Sunday at 3 AM 0 3 * * 0 /path/to/script.sh ``` ### Special strings (shorthand) ```bash @reboot /path/to/script.sh # Run once at startup @yearly /path/to/script.sh # 0 0 1 1 * @monthly /path/to/script.sh # 0 0 1 * * @weekly /path/to/script.sh # 0 0 * * 0 @daily /path/to/script.sh # 0 0 * * * @hourly /path/to/script.sh # 0 * * * * ``` ## Crontab Management ```bash # Edit current user's crontab crontab -e # List current crontab crontab -l # Edit another user's crontab (root) sudo crontab -u www-data -e # Remove all cron jobs (be careful!) crontab -r # Install crontab from file crontab mycrontab.txt # Backup crontab crontab -l > crontab-backup-$(date +%Y%m%d).txt ``` ### Crontab best practices ```bash # Set PATH explicitly (cron has minimal PATH) PATH=/usr/local/bin:/usr/bin:/bin # Set MAILTO for error notifications MAILTO=[REDACTED_SECRET] # Set shell explicitly SHELL=/bin/bash # Full crontab example PATH=/usr/local/bin:/usr/bin:/bin MAILTO=[REDACTED_SECRET] SHELL=/bin/bash # Backups 0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1 # Cleanup old logs 0 3 * * 0 find /var/log/myapp -name "*.log" -mtime +30 -delete # Health check */5 * * * * /opt/scripts/healthcheck.sh || /opt/scripts/alert.sh "Health check failed" ``` ## Systemd Timers ### Create a timer (modern cron replacement) ```ini # /etc/systemd/system/backup.service [Unit] Description=Daily backup [Service] Type=oneshot ExecStart=/opt/scripts/backup.sh User=backup StandardOutput=journal StandardError=journal ``` ```ini # /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 ``` ```bash # Enable and start the timer sudo systemctl daemon-reload sudo systemctl enable --now backup.timer # Check timer status systemctl list-timers systemctl list-timers --all # Check last run systemctl status backup.service journalctl -u backup.service --since today # Run manually (for testing) sudo systemctl start backup.service # Disable timer sudo systemctl disable --now backup.timer ``` ### OnCalendar syntax ```ini # Systemd calendar expressions # Daily at midnight OnCalendar=daily # or: OnCalendar=*-*-* 00:00:00 # Every Monday at 9 AM OnCalendar=Mon *-*-* 09:00:00 # Every 15 minutes OnCalendar=*:0/15 # Weekdays at 8 AM OnCalendar=Mon..Fri *-*-* 08:00:00 # First of every month OnCalendar=*-*-01 00:00:00 # Every 6 hours OnCalendar=0/6:00:00 # Specific dates OnCalendar=2026-02-03 12:00:00 # Test calendar expressions systemd-analyze calendar "Mon *-*-* 09:00:00" systemd-analyze calendar "*:0/15" systemd-analyze calendar --iterations=5 "Mon..Fri *-*-* 08:00:00" ``` ### Advantages over cron ``` Systemd timers vs cron: + Logs in journald (journalctl -u service-name) + Persistent: catches up on missed runs after reboot + RandomizedDelaySec: prevents thundering herd + Dependencies: can depend on network, mounts, etc. + Resource limits: CPUQuota, MemoryMax, etc. + No lost-email problem (MAILTO often misconfigured) - More files to create (service + timer) - More verbose configuration ``` ## One-Off Scheduling ### at (run once at a specific time) ```bash # Schedule a command echo "/opt/scripts/deploy.sh" | at 2:00 AM tomorrow echo "reboot" | at now + 30 minutes echo "/opt/scripts/report.sh" | at 5:00 PM Friday # Interactive (type commands, Ctrl+D to finish) at 10:00 AM > /opt/scripts/task.sh > echo "Done" | mail -s "Task complete" [REDACTED_SECRET] > <Ctrl+D> # List pending jobs atq # View job details at -c <job-number> # Remove a job atrm <job-number> ``` ### sleep-based (simplest) ```bash # Run something after a delay (sleep 3600 && /opt/scripts/task.sh) & # With nohup (survives logout) nohup bash -c "sleep 7200 && /opt/scripts/task.sh" & ``` ## Timezone Handling ```bash # Cron runs in the system timezone by default # Check system timezone timedatectl date +%Z # Set timezone for a specific cron job # Method 1: TZ variable in crontab TZ=America/New_York 0 9 * * * /opt/scripts/report.sh # Method 2: In the script itself #!/bin/bash export TZ=UTC # All date operations now use UTC # Method 3: Wrapper TZ=Europe/London date '+%Y-%m-%d %H:%M:%S' # List available timezones timedatectl list-timezones timedatectl list-timezones | grep America ``` ### DST pitfalls ``` Problem: A job scheduled for 2:30 AM may run twice or not at all during DST transitions. "Spring forward": 2:30 AM doesn't exist (clock jumps 2:00 → 3:00) "Fall back": 2:30 AM happens twice Mitigation: 1. Schedule critical jobs outside 1:00-3:00 AM 2. Use UTC for the schedule: TZ=UTC in crontab 3. Make jobs idempotent (safe to run twice) 4. Systemd timers handle DST correctly ``` ## Monitoring and Debugging ### Why didn't my cron job run? ```bash # 1. Check cron daemon is running systemctl status cron # Debian/Ubuntu systemctl status crond # CentOS/RHEL # 2. Check cron logs grep CRON /var/log/syslog # Debian/Ubuntu grep CRON /var/log/cron # CentOS/RHEL journalctl -u cron --since today # systemd # 3. Check crontab actually exists crontab -l # 4. Test the command manually (with cron's environment) env -i HOME=$HOME SHELL=/bin/sh PATH=/usr/bin:/bin /opt/scripts/backup.sh # If it fails here but works normally → PATH or env issue # 5. Check permissions ls -la /opt/scripts/backup.sh # Must be executable ls -la /var/spool/cron/ # Crontab file permissions # 6. Check for syntax errors in crontab # cron silently ignores lines with errors # 7. Check if output is being discarded # By default, cron emails output. If no MTA, output is lost. # Always redirect: >> /var/log/myjob.log 2>&1 ``` ### Job wrapper with logging and alerting ```bash #!/bin/bash # cron-wrapper.sh — Run a command with logging, timing, and error alerting # Usage: cron-wrapper.sh <job-name> <command> [args...] set -euo pipefail JOB_NAME="${1:?Usage: cron-wrapper.sh <job-name> <command> [args...]}" shift COMMAND=("$@") 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: ${COMMAND[*]}" START_TIME=$(date +%s) if "${COMMAND[@]}" >> "$LOG_FILE" 2>&1; then ELAPSED=$(( $(date +%s) - START_TIME )) log "SUCCESS (${ELAPSED}s)" else EXIT_CODE=$? ELAPSED=$(( $(date +%s) - START_TIME )) log "FAILED with exit code $EXIT_CODE (${ELAPSED}s)" # Alert (customize as needed) echo "Cron job '$JOB_NAME' failed with exit $EXIT_CODE" | \ mail -s "CRON FAIL: $JOB_NAME" [REDACTED_SECRET] 2>/dev/null || true exit $EXIT_CODE fi ``` ```bash # Use in crontab: 0 2 * * * /opt/scripts/cron-wrapper.sh daily-backup /opt/scripts/backup.sh */5 * * * * /opt/scripts/cron-wrapper.sh health-check /opt/scripts/healthcheck.sh ``` ### Lock to prevent overlap ```bash # Prevent concurrent runs (job takes longer than interval) # Method 1: flock * * * * * flock -n /tmp/myjob.lock /opt/scripts/slow-job.sh # Method 2: In the script LOCKFILE="/tmp/myjob.lock" exec 200>"$LOCKFILE" flock -n 200 || { echo "Already running"; exit 0; } # ... do work ... ``` ## Idempotent Job Patterns ```bash # Idempotent backup (only creates if newer than last backup) #!/bin/bash BACKUP_DIR="/backups/$(date +%Y%m%d)" [[ -d "$BACKUP_DIR" ]] && { echo "Backup already exists"; exit 0; } mkdir -p "$BACKUP_DIR" pg_dump mydb > "$BACKUP_DIR/mydb.sql" # Idempotent cleanup (safe to run multiple times) find /tmp/uploads -mtime +7 -type f -delete 2>/dev/null || true # Idempotent sync (rsync only transfers changes) rsync -az /data/ backup-server:/backups/data/ ``` ## Tips - Always redirect output in cron jobs: `>> /var/log/job.log 2>&1`. Without this, output goes to mail (if configured) or is silently lost. - Test cron jobs by running them with `env -i` to simulate cron's minimal environment. Most failures are caused by missing `PATH` or environment variables. - Use `flock` to prevent overlapping runs when a job might take longer than its schedule interval. - Make all scheduled jobs idempotent. If a job runs twice (DST, manual trigger, crash recovery), it should produce the same result. - `systemd-analyze calendar` is invaluable for verifying timer schedules before deploying. - Never schedule critical jobs between 1:00 AM and 3:00 AM if DST applies. Use UTC schedules instead. - Log the start time, end time, and exit code of every cron job. Without this, debugging failures after the fact is guesswork. - Prefer systemd timers over cron for production services: you get journald logging, missed-run catchup (`Persistent=true`), and resource limits for free.


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