What's changed: Initial version (topic 1.08, subtopics 1.08.1–1.08.3)
3.2Job Scheduling
Learn periodic and one-shot scheduling: cron (crontab syntax, /var/spool/cron/, /etc/crontab, /etc/cron.d/, cron.{hourly,daily,weekly,monthly}), one-time at (atq, atrm), catch-up runs with anacron (/etc/anacrontab), and access control (/etc/cron.allow, /etc/cron.deny, /etc/at.allow, /etc/at.deny).
Backups, log aggregation, certificate renewals—operations are fixed tasks at fixed times. Periodic = cron, one-shot = at, catch-up for powered-off periods = anacron: this three-way division is the backbone of the section.
3.2.1cron and crontab
- User jobs: edit with
crontab -e(stored under /var/spool/cron/—never edit directly);-llists,-rremoves all. - Syntax: minute hour day month weekday command (five fields).
30 2 * * 1 /opt/backup.sh= Mondays at 2:30;*/10= every 10 minutes;1-5= Mon–Fri. - System side: /etc/crontab (six fields—an extra user column), /etc/cron.d/ (package-provided), and dropping a script into cron.{hourly,daily,weekly,monthly}/ also schedules it.
3.2.2at, anacron, and access control
- at schedules one run (
echo "/opt/report.sh" | at 23:00,at now + 2 hours); list with atq, cancel with atrm job-number. - anacron manages daily-or-longer periods by interval rather than clock time and runs missed jobs after boot (for laptops and machines powered off at night). Config: /etc/anacrontab (period, delay, job id, command).
- Access control: cron.allow/cron.deny (at: at.allow/at.deny). If allow exists, only listed users may use it (deny is ignored); without allow, only deny-listed users are blocked.
Reading/writing the five crontab fields (min hour day month weekday) is guaranteed exam math. Also standard: /etc/crontab has the extra user column (six fields), one-shot = at, power-off catch-up = anacron, if an allow file exists, it decides everything. Being able to read interval forms like 0 */6 * * * (every six hours on the hour) seals it.
For crontab, chant "minute hour day month weekday" left to right—that is everything. "Daily at 3:15" = 15 3 * * *; "midnight on the 1st" = 0 0 1 * *; "weekdays 9:00" = 0 9 * * 1-5. Weekdays run 0 (Sun)–6 (Sat); note the subtlety that specifying both day and weekday means OR. cron's weakness: jobs never run if the machine was off at that moment—a 2:00 backup never fires on a machine shut down nightly. anacron compensates by interval: "if more than a day has passed since the last run, execute after boot (plus a delay)". Distribution cron.daily reliably runs daily precisely because anacron backs it. at is purely for one-shot bookings ("restart the service in 30 minutes", "switch over once at midnight"), managed with atq/atrm.
| Need | Use | Key point |
|---|---|---|
| Daily/weekly periodic runs | cron (crontab -e) | min hour day month weekday |
| One-shot booking | at (atq/atrm) | now + 2 hours, etc. |
| Catch up after power-off | anacron | Interval-based, /etc/anacrontab |
| Restrict who may use it | cron.allow / cron.deny | allow file wins if present |
Trap: misreading 30 2 * * 1 as "Jan 2:30" or "daily 2:30" is the classic—field five is the weekday, so Mondays 2:30. "A user in both cron.allow and cron.deny is denied" is wrong—with allow present, deny is ignored and the user is permitted. "anacron handles minute-level jobs" is wrong too (daily or longer).
3.2.3Section summary
- crontab = min hour day month weekday (/etc/crontab adds a user column); stored in /var/spool/cron/, cron.d, cron.daily, etc.
- at = one-shot (atq/atrm), anacron = daily+ with power-off catch-up; control = allow wins, else deny
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Quick check
(just a quick review)Q1. You want a backup script every Monday at 2:30 AM. Which crontab entry is correct?
Q2. On a machine shut down nightly, you want daily maintenance jobs to catch up after power-on. Which mechanism?
Q3. Both /etc/cron.allow and /etc/cron.deny exist; user suzuki appears only in cron.allow. Can suzuki use crontab?