Linux 定时任务：crontab、anacron 与systemd timer

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 复制代码

[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 复制代码

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 复制代码

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。