commit d449f2338cdeae504d01e14c8a91e2318769fcca
Author: zlei9 <zlei9@126.com>
Date:   Sun Mar 29 09:43:23 2026 +0800

    Initial commit with translated description

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+---
+name: cron-scheduling
+description: "使用cron和systemd定时器安排和管理重复任务。在设置cron作业、编写systemd定时器单元、处理时区感知调度、监控失败作业、实现重试模式或调试计划任务未运行的原因时使用。"
+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=admin@example.com
+
+# Set shell explicitly
+SHELL=/bin/bash
+
+# Full crontab example
+PATH=/usr/local/bin:/usr/bin:/bin
+MAILTO=admin@example.com
+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" admin@example.com
+> <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" admin@example.com 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.
diff --git a/_meta.json b/_meta.json
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+{
+  "ownerId": "kn7f6g2r31qsb1ts8cf5x7rpk180fn9j",
+  "slug": "cron-scheduling",
+  "version": "1.0.0",
+  "publishedAt": 1770158609900
+}
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