Check out section 4.1 here https://www.baeldung.com/linux/bind-mounts
this post was submitted on 28 Jan 2025
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Only way I know of is to unmount the binded folder. I don't think there is a way to have the filesystem show files from both at the same time.
The easy solution would be to have a third common mount point for the two that is switched if the external drive is connected or not.
In another subthread I came up with the below, is this what you mean? I haven't tried it yet.
/home/user/folderAppleis always empty/home/user/folderApple-originalmounts ontop of/home/user/folderAppleat boot- then
/mnt/drive/folderBananaalso mounts ontop of/home/user/folderApplewhen/if it becomes available (later in the order)
Basically, yeah. Bind the "local" path on boot and then have systemd triggers for when USB mounts and unmounts to swap them automatically.
(Personally I wouldn't do it like this though because it will become trouble with any open files or shell or whatever in a path that is replaced by a different mount.)
You need to
sudo mount --bind "/home/user/folderApple" "/home/user/folderApple-original"
before you
sudo mount --bind "/mnt/drive/folderBanana" "/home/user/folderApple".
The results are the same no matter which order I do the mounts in.
So... are you saying the contents of /home/user/folderApple-original really changes after you bindmount something over /home/user/folderApple?
This seems odd.
Do you have submounts under the original directory? Try with --rbind?
Sounds like you may want to use a union filesystem like overlayfs. I'm not sure if the specific behavior of overlayfs will work for you, but it's worth investigating.
Thank you for putting your use-case in your post, since otherwise I think this might be an XY problem.
EDIT: There's also mergefs and unionfs. I don't know what the features and drawbacks are for these three union filesystems. mergefs seems like it might be the most configurable, but it's also FUSE. unionfs and overlayfs are both in-kernel, so they'll perform better (which may not matter for your use-case). overlayfs is the one I'm most familiar with of those two, since it's used by most container runtimes.
~~You can only mount into an empty directory.~~ (Edit: Ok that is not really correct. You can mount into directories with content, but then the content will no longer be accessible from that point.) You cannot mix two directories with mount.
Edit: You could have two points using one Mountpoint at a time maybe. With a script you unmount the fallback, and if you connect the new drive you mount it with the script. And if you are done, use your unmount script to reverse it. Just an idea.
Some more Edit: In short you create an empty directory that is the Mountpoint, lets say "/home/user/Apple". Now your real local files are at "/home/user/folderApple". You mount folderApple to Apple. This is your fallback. Then if you connect the other drive, with your script you unmount that and mount your "/mnt/drive/Banana" to "/home/user/Apple", which is empty again after the unmount. And reverse it if you want to unplug.
Well that's bullshit, I can mount to directories with contents
Alright, I didn't know you can mount "over" a directory. But my point was, you cannot mix them, you do not mount into the directly. It just replaces it. Which also would make this directory no longer accessible, but he wants both accessible at the same time.
Yes
Ideally I'd like to avoid a script because my experience is they aren't very durable. I make mistakes and they are difficult to troubleshoot. So I am trying to just use the tools that are already available in the system.
But maybe there is something in the idea of using a second mount, like if
/home/user/folderAppleis always empty/home/user/folderApple-originalmounts ontop of/home/user/folderAppleat boot- then
/mnt/drive/folderBananaalso mounts ontop of/home/user/folderApplewhen/if it becomes available (later in the order)