bind mount over a directory, how to access underlying files?

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You can only mount into an empty directory. You cannot mix two directories with mount.

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Well that's bullshit, I can mount to directories with contents

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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.

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The results are the same no matter which order I do the mounts in.

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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/folderApple is always empty
/home/user/folderApple-original mounts ontop of /home/user/folderApple at boot
then /mnt/drive/folderBanana also mounts ontop of /home/user/folderApple when/if it becomes available (later in the order)

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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?

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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.

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In another subthread I came up with the below, is this what you mean? I haven't tried it yet.

/home/user/folderApple is always empty
/home/user/folderApple-original mounts ontop of /home/user/folderApple at boot
then /mnt/drive/folderBanana also mounts ontop of /home/user/folderApple when/if it becomes available (later in the order)

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Basically, yeah. Mount 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.)

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Check out section 4.1 here https://www.baeldung.com/linux/bind-mounts

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bind mount over a directory, how to access underlying files?