定时任务「漏跑、跑两次、时区错乱」，常见于 cron 与 systemd timer 混用、休眠机器、UTC/本地时混淆。本文对照三类机制，给出选择与排障路径。
源码锚点
| 路径 / 手册 | 作用 |
|---|---|
man crontab |
用户任务表 |
man cron / /etc/cron.* |
系统 cron 目录 |
man anacron |
非 7×24 主机补跑 |
man systemd.timer |
timer 单元 |
man systemd.time |
日历与单调时间表达式 |
timer 示例：
ini
[Unit]
Description=Run backup daily
[Timer]
OnCalendar=*-*-* 02:30:00
Persistent=true
[Install]
WantedBy=timers.target配套 backup.service 写实际 ExecStart=。
调用链
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timer
anacron
时间到
机制
crond 读 crontab
systemd 触发 service
开机后补跑窗口
shell 跑命令
邮件/日志/自己重定向
重点知识
crontab 要点
bash
crontab -e
# m h dom mon dow command
grep CRON /var/log/syslog 2>/dev/null | tail
journalctl -u cron -b | tail默认环境变量极少（常只有 SHELL/PATH 子集）。脚本应用绝对路径，输出重定向到日志。
systemd timer 优点
- 依赖、资源限制、沙箱可复用 service 能力。
systemctl list-timers一眼看下次触发。Persistent=true利于休眠后补跑。
bash
systemctl list-timers --all
systemctl status backup.timer backup.service何时用谁
| 场景 | 更合适 |
|---|---|
| 用户级简单脚本 | crontab |
| 要可靠依赖/日志/权限 | systemd timer |
| 桌面/笔记本非全天开机 | anacron 或 Persistent timer |
常见坑
- 夏令时/时区：
timedatectl先确认。 - cron 里
%有特殊含义，需转义。 - 同时装 timer 与 crontab 跑同一任务 → 双份执行。