Memory hook: Make it work now, at boot and after failure.
Must remember
systemd targets group units. get-default inspects the default boot target; set-default changes future boots; isolate transitions the running system and can stop unrelated services. Know multi-user, graphical, rescue and emergency behavior rather than treating every target as a simple runlevel synonym.
The bootloader loads a kernel and initial RAM filesystem before the real root filesystem is mounted. For authorised recovery on a disposable RHEL VM, practise interrupting boot, editing kernel arguments, entering the supported recovery environment, remounting the necessary root filesystem writable and changing the intended configuration. Password recovery may require a chroot and subsequent SELinux relabel. Firmware, bootloader passwords or encrypted storage can change the process: do not memorise one sequence as universal.
Use grubby to inspect or manage supported kernel arguments and keep boot configuration consistent with the system's bootloader layout. A working current boot is not proof that changed boot arguments will work next time.
at schedules a one-time job; inspect pending jobs with atq. cron repeats using minute, hour, day-of-month, month and day-of-week fields. User crontabs and system crontabs differ because system entries include the execution user. Jobs run with a limited environment; use absolute paths and handle output.
systemd timers activate service units using calendar or monotonic timing. systemctl list-timers verifies scheduling. Persistent calendar timers can catch a missed run when configured accordingly. Enable the timer, not merely the one-shot service.
chrony synchronises time; inspect chronyc sources and chronyc tracking. The daemon running is not proof of synchronisation. Correct time supports authentication, certificates and trustworthy logs.
Practical finish: verify changes, reboot the practice VM, then retest mounts, network access, services, SELinux and scheduled work. Persistent results are part of EX200 preparation.
Choose under exam pressure
| Requirement | Choice and reason |
|---|---|
| One-off future task | at; verify the queue and execution user. |
| Service-integrated scheduled task | A systemd timer activating a service. |
| Wrong time despite running daemon | Inspect selected time source, reachability and tracking status. |
Traps
- set-default does not immediately isolate the running system.
- An enabled timer with a broken service command still fails its job.
Active recall
1. Which command inspects the boot target?
systemctl get-default.
2. Does setting the default target change the current target immediately?
No; an explicit transition is a separate operation.
3. Cron or at for one run tomorrow?
at is designed for a one-time scheduled command.
4. Why use absolute paths in scheduled jobs?
The noninteractive environment and PATH may differ from your shell.
5. Why reboot before declaring a practice task finished?
It exposes temporary-only settings, invalid mounts and missing boot activation.