Cool bind mount trick with systemd

Lounge EndeavourOS pub
1

The effect is to have folders on a partition bind mounted at various locations, but the full partition not mounted anywhere.

E.g. if /dev/sdb1 contains /f1 and /f2 and fstab looks like this:

# Mount to /mnt because it already exists and won't leave an extra empty folder after unmounted.
# But if you want it to remain, create a root folder, maybe hidden like ".bindmounts" and mount to that.
UUID=[uuid for /dev/sdb1] /mnt ext4 defaults 0 0

# Create root /myf1 and /myf1 first
/mnt/f1 /myf1 none bind,defaults 0 0
/mnt/f2 /myf2 none bind,defaults 0 0

So after that, if you reboot, ls /mnt would show f1 and f2. How to hide/remove that? Make a systemd service. E.g. I’ll call it unmount-mnt.service

[Unit]
Description=Unmount mnt
After=mnt.mount myf1.mount myf2.mnt

[Service]
Type=oneshot
ExecStart=umount /mnt

[Install]
WantedBy=local-fs.target

Put it in /lib/systemd/system then run

systemctl enable unmount-mnt.service

Reboot, and /mnt should be empty, and any content you have on sdb1 in folders f1 and f2 should now be visible under /myf1 and /myf2.

This works because systemd converts /etc/fstab to *.mount units. And this service waits until everything is mounted and then unmounts /mnt.

It’s my first shot at a systemd service, so I’m not sure if I did everything “right.”

QUESTION FOR MORE EXPERIENCED PEOPLE:

Will this cause any problems that you know of? I tested it in a VM and it works, but I didn’t test exhaustively to see if something down the road could drop the bind mounts, or something like that.

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IMHO keep it simple (KISS acronym) is the way… within fstab method and systemd.automount you can direct mount folders everywhere you need outside /mnt and share mountpoints by link it with ln [OPTION]... -s DIRECTORY TARGET...it a symbolink link.

According to fstab wiki, I set my mountpoint automagic with this fstab entry for my external ssd full of music files, for example:

LABEL=MUKKA525 /home/babiz/Musica vfat defaults,noauto,x-systemd.automount 0 0

I use LABEL mountpoint cause is very comfortable for me lol, after reboot you get drive mounted in folder ( everywhere you want according to your R/W permissions ) but a empty folder must be present on target mountpoint.
$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 489G 0 disk
└─sda1 8:1 0 489G 0 part /home/babiz/Musica
nvme0n1 259:0 0 476,9G 0 disk
├─nvme0n1p1 259:1 0 1000M 0 part /efi
├─nvme0n1p2 259:2 0 380,1G 0 part /
├─nvme0n1p3 259:3 0 16M 0 part
├─nvme0n1p4 259:4 0 78,9G 0 part
└─nvme0n1p5 259:5 0 16,9G 0 part [SWAP]

Next you can make many symlink ahead mountpoint and target directory you want from terminal commands:

$ mkdir ~/mounts is a example folder inside my home
$ ln -s ~/Musica/Musika/Abba/ mounts point my automount folder everywhere I want

$ ls -l mounts/
totale 0
lrwxrwxrwx 1 babiz babiz 31 11 feb 00.32 Abba → /home/babiz/Musica/Musika/Abba/
lrwxrwxrwx 1 babiz babiz 32 11 feb 00.15 AC-DC → /home/babiz/Musica/Musika/AC-DC/

For example, if you need to hide master mountpoint for all users, you can mount it under /root directory and adjust access for users by change symlink permission.

In short this is the way I learned many years ago when systemd isn’t here and system-V rulez lol.
Indeed fstab adjusted with systemd automount feature is super cool thing mate.
Acutally Arch Wiki for fstab is super cool filled with useful tips. :ghost: @discobot quote

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:left_speech_bubble: Do not be embarrassed by your mistakes. Nothing can teach us better than our understanding of them. This is one of the best ways of self-education. — Thomas Carlyle

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I know about symlinks, but there are cases where they don’t work exactly the same as a bind mount, e.g. in a chroot.

Yes, but the main reason I want to completely remove the master mount is, sometimes I’ll do something like “sudo find / -type f | [other processing commands]” and then have to filter out the master mount, every time I do it. Just gets tedious, and was looking for a cleaner way.

And partly, what I described was an experiment… “Can I do it.” If it’s useful, I’ll use it, if not I’ll do something else.

1 Like
5

Oh well now I understand, you aim to get more advanced stuffs, nice man.