Scheduling¶

Running jobs on a schedule with cron, once with at, and with systemd timers, and why jobs that work in a terminal fail when scheduled.

Revision Card¶

Fact	Value
Crontab fields	minute, hour, day of month, month, day of week
Day fields in cron	Day of month or day of week; systemd requires both
`%` in a crontab	Newline; escape as `\%`
cron `PATH` on RHEL	`/usr/bin:/bin:/usr/sbin:/sbin`, no `/usr/local/bin`
cron `PATH` on Ubuntu	From `/etc/environment` through `pam_env`
`/etc/cron.d` lines	Need a user field; without it: `bad command` (RHEL), `bad username` (Ubuntu)
Dot in a file name	Ignored by Debian's `run-parts` and cron in `cron.daily` and `cron.d`
Periodic directories	Scripts must be executable
Output without an MTA	RHEL logs it as `CMDOUT`; Ubuntu discards it
Access	`cron.allow` wins; otherwise `cron.deny`; same for `at`
anacron	Catches up missed daily, weekly and monthly jobs
Timer pair	`name.timer` starts `name.service`; enable the timer
`Persistent=true`	Runs a missed calendar job after boot
Missing service	"Refusing to start, unit ... to trigger not loaded"

Task	Command
Edit or list a crontab	`crontab -e`, `crontab -l`, `sudo crontab -u alice -l`
See cron's environment	`* * * * * env > /tmp/cron-env.txt`
Cron logs	`journalctl -u crond` (RHEL), `journalctl -u cron` (Ubuntu)
Which periodic scripts run	`run-parts --test /etc/cron.daily` (Ubuntu)
One-off job	`at 23:00` (commands on standard input), `atq`, `atrm 4`
Check a calendar expression	`systemd-analyze calendar "Mon..Fri 09:00"`
Enable a timer	`sudo systemctl enable --now name.timer`
Next and last runs	`systemctl list-timers --all`
Transient timer	`sudo systemd-run --on-active=10min --unit=name cmd`

File	Covers	Track	Weight
cron and at	Syntax, user and system crontabs, anacron, the cron environment, `run-parts`, access control, `at`	Core	High
Systemd Timers	Timer and service pairs, calendar expressions, transient timers, timers vs cron	Core	Med