A single report must be generated once, at 22:00 tonight, and never again. Which approach fits best?
LPIC-1 Exam 102-500, objective 107. Administrative tasks medium
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The options
Not correct Add a crontab entry `0 22 * * *` and remember to delete it tomorrow
Wrong as a design. cron schedules are recurring by nature, so this runs every night until a human remembers to remove it. Relying on that memory is exactly the failure mode that at exists to avoid.
Correct `at 22:00` and type the command at the at> prompt
Correct. at queues a command for one single execution at a given time, then discards the job. Times may be absolute (22:00, midnight, teatime) or relative (now + 2 hours).
Not correct Put the script in /etc/cron.daily
Wrong. Scripts under /etc/cron.daily are run once per day, every day, at a time chosen by the system (via run-parts or anacron). That is recurring, and you do not control the hour.
Not correct Add an entry to /etc/anacrontab
Wrong. anacron exists to run periodic jobs that were missed while a machine was switched off. Its schedule is a period in days, not a wall-clock time, and it is still recurring.
Why
at handles one-off jobs and needs the atd daemon running. `atq` lists the queue with job numbers, `atrm N` (equivalently `at -d N`) deletes a queued job, and `at -c N` shows the full environment and commands of a job. Access is controlled by /etc/at.allow and /etc/at.deny, the same way cron uses /etc/cron.allow and /etc/cron.deny.
Where this comes from
- Cited
- LPI exam objective 107.2
- What it says
- Use at to schedule a job for one-off execution at a specific time.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 107 and space the ones you get wrong.