Before a maintenance window you must take a running systemd server down to the rescue state immediately, stopping the services that are not part of it, while leaving the machine's normal boot behaviour unchanged. Which command does that?
LPIC-1 Exam 101-500, objective 101. System architecture hard
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How these questions are written — where each question comes from, what the verification ledger records, and what happens when one is found wrong.
The options
Correct systemctl isolate rescue.target
Correct. isolate starts the named unit and its dependencies and stops everything else, which is the exact analogue of changing runlevel; it touches nothing that persists.
Not correct systemctl start rescue.target
Wrong, and the closest trap. start activates rescue.target and its dependencies but stops nothing, so the web server, the database and the display manager all keep running alongside it.
Not correct systemctl set-default rescue.target
Wrong direction. set-default changes only what the machine boots into next time and has no effect on the running system.
Not correct systemctl mask graphical.target
Wrong. mask links a unit to /dev/null so it cannot be started at all, deliberately or as a dependency. It stops nothing that is already running and would obstruct the next normal boot.
Why
isolate is the operation with no everyday equivalent: it makes the named unit the whole of the active set, so units outside its dependency tree are stopped. Only units whose file sets AllowIsolate=yes may be used this way, which in practice means targets, and it is why `systemctl isolate sshd.service` is refused. Distinguish the three verbs by their scope in time and space: start adds, isolate replaces, set-default schedules for the next boot.
Where this comes from
- Cited
- manual page systemctl(1)
Practise this
Reading one question is not practice. The trainer will draw a short set from objective 101 and space the ones you get wrong.
More questions on this objective
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