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
Machine-checked — no person has signed for it. This question was read against the source cited below by an automated pass, which found no contradiction. That is a weaker claim than it sounds: the same kind of process wrote the question, so it can confirm its own mistake.
Treat it as a good draft rather than as settled fact, and read the source below before you rely on it. It is not used in mock exams here — only questions a person has signed for are.
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
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 short set from objective 104 and space the ones you get wrong.
More questions on this objective
- A new disk /dev/sdb has no partition table at all. Which command writes an empty GPT partition table onto it without dropping you into an interactive editor? machine-checked
- You need the partition /dev/sdc1 on a USB stick to carry a FAT32 filesystem so that Windows machines can read and write it. Which command creates it? machine-checked
- You have just run `mkswap /dev/sdb2`. Which command makes the kernel start using that swap area immediately, without a reboot? machine-checked
- Why is it dangerous to run e2fsck against an ext4 filesystem that is currently mounted read-write? machine-checked
- On an ext4 filesystem at /dev/sda1 you want an automatic check to be forced after every 30 mounts. Which command sets that? machine-checked
- `df -h /var` reports the filesystem 100% full, but `du -sh /var` accounts for only about half that space. Nothing is hidden under a mount point. What is the most likely explanation? machine-checked
All questions on Devices, Linux filesystems, filesystem hierarchy standard