Can I run fsck or e2fsck when Linux file system is mounted?

Question

I mean I'm already here right. If not, why?

Answer 1

No.

Do not run fsck on a live or mounted file system. fsck is used to check and optionally repair a Linux file systems. Running fsck on a mounted filesystem can usually result in disk and/or data corruption.

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Outdated method (SysVinit/upstart based, Ubuntu before version 15.04):

This will force a check on next boot:

sudo touch /forcefsck

So will this but it will also reboot the machine at the moment you hit enter:

shutdown -rF now

There are more ways (like telling the machine to go to init 1 and then umount the partition/disc your want to check) but these 2 are the easiest.

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Nowadays (systemd based systems, Ubuntu 15.04 and later) we can use

sudo tune2fs -c 1 /dev/sdX

replace sdX by your partitions device name. 1 is the amount of boots for it to do a new filesystemcheck (so 1 is every time. 10 would check every 10 boots. 0 or -1 disables it).

So after the boot do

sudo tune2fs -c 0 /dev/sdX

to disable it.

Example:

rinzwind@discworld:~$ sudo tune2fs -c 1 /dev/nvme0n1p3
tune2fs 1.47.0 (5-Feb-2023)
Setting maximal mount count to 1
rinzwind@discworld:~$ 

Using it on a device with the wrong filesystem errors out:

inzwind@discworld:~$ sudo tune2fs -c 1 /dev/nvme0n1p1
tune2fs 1.47.0 (5-Feb-2023)
tune2fs: Bad magic number in super-block while trying to open /dev/nvme0n1p1
/dev/nvme0n1p1 contains a vfat file system

(that is the /boot/EFI one ;) )