I have a partition structure as follows:

I am trying to mount partition 2 or partition 3 dynamically depending on the file present in partition 4.

For example:- Partition 2 will be mounted if partition 4 consists of a file named two. Partition 3 will be mounted if partition 4 consists of a file named three.

Note:- This partition will never be mounted together i.e. if Partition 2 is mounted partition 3 will be not be mounted until partition 2 is unmounted. Thus I can use a common directory for both partitions.

As I have systemd available on my os I can write a startup script which can read from partition 4 and mount the appropriate partition at boot and write the partition record into /etc/fstab.

But according to my understanding fstab is critical file and if any failure happens or fstab get's corrupt it's going to stop the system from booting.

Question:

Now what I am trying to achieve is can I add an entry in fstab which will read dynamically partition 4 and add the entry for partition 2 or partition 3 depending on the file that exists in the partition 4.

I am a pretty newbie to Linux.

I think you do not need to modify fstab. You need to use some kind of automounter (see autofs), but I think there is no such tool available, so you need to implement something.

I don't think that fstab will be able to do so. Of course, you could always write your own script, to be executed at startup, to do that.

I saw autofs but it won't be helpful in my case. Thanks for the response.
Yes, that's my last option to write a custom script.

You may find that it is your first option.

Yes, I have started to think that this is only the option. Thank you for the response.

You don’t need fstab to mount a filesystem. You need the mount command.
Perhaps. You can use the device files to identify partitions, e.g. /dev/sdc3, but you can also use the filesystem’s UUID or its label, e.g. LABEL=mylabel. Your script could manipulate the labels of the two “dynamic” filesystems. But I think it’s much simpler to just mount one filesystem or the other, without playing around with fstab entries or filesystem labels.

You could even replace the mount command executable with a script that conditionally mounts one of the “dynamic” partitions, then calls the original mount command.

However fstab is just a list of filesystems to be mounted and has no mechanism for doing so conditionally.

I've never played with autofs dynamic mounting but it is possible. This is untested code.

In the auto.master file
In the autofs-auto-mapper.sh script
And change permissions to make the script executable.

You can USE the fstab to define how to mount - but include the option "noauto".

Now you can use a script to pick what to mount without having to specify the full list.

Without that, I prefer the "LABEL=volname" to mount filesystems post boot. It is easier than using the UUID for the device, and you can't depend on the /dev/sd... name.

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In addition to the above suggestion using the user option allows a regular user to mount a filesystem.

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