The root filesystem is mounted read-write and you need it read-only for a moment. You obviously cannot unmount it. Which command changes it in place?
LPIC-1 Exam 101-500, objective 104. Devices, Linux filesystems, filesystem hierarchy standard medium
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The options
Not correct umount -r /
Wrong as a deliberate tool. -r means: if the unmount fails, fall back to remounting read-only. You would be relying on a failure path, and umount of / will not succeed anyway.
Not correct mount -o ro /
Wrong. Without remount, mount treats this as a fresh mount request for the fstab entry covering /, which does not alter the options of the mount that already exists.
Not correct mount -a
Wrong. mount -a mounts everything in /etc/fstab that is not already mounted and not marked noauto. Existing mounts keep their current options.
Correct mount -o remount,ro /
Correct. The remount option tells mount to change the options of an existing mount rather than establish a new one, so / becomes read-only without being detached.
Why
`mount -o remount,<options> <target>` edits a live mount. It is how you flip a filesystem read-only before a check, or add noatime without a reboot. Adding `remount` alone re-applies the fstab options, which is why it is usually written with the options you actually want.
Where this comes from
- Cited
- LPI exam objective 104.3
- What it says
- Mount and unmount filesystems, including changing options with remount.
Practise this
Reading one question is not practice. The trainer will draw a set from objective 104 and space the ones you get wrong.