Saul note: Although this post idea, writings and its final formatting are my own work, I am disclosing having used generative AI (Claude 3.5) to aid with some of the text/code generation for it. The AI output has been used as a writing aid and it does not constitute the final or main form of the article.

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Command Syntax

The primary command to create a read-only mount is:

sudo mount --bind -o ro /folder/folder2 /location/location2

Steps:

1. Preparation
   • Ensure the destination directory exists:
   sudo mkdir -p /location/location2
2. Bind Mount with Read-Only Option
   • Use the mount --bind command
   • The -o ro flag specifically sets the mount as read-only
   • Replace /folder/folder2 with your source directory
   • Replace /location/location2 with your destination mount point

Permanent Mount (Optional)

To make this mount persistent across reboots, add an entry to /etc/fstab:

/folder/folder2 /location/location2 none bind,ro 0.

To verify:

To confirm the read-only status:

mount | grep /location/location2

Important Considerations

• Root privileges are required
• Destination directory must exist before mounting
• Read-only mounts prevent any write operations

Unmounting

To remove the mount:

sudo umount /location/location2


Read-Only mount permissions considerations:

The read-only mount is enforced at the filesystem level, which means:

• Permissions changes are blocked by the mount’s read-only attribute
• The chmod command cannot override the mount’s fundamental read-only state

Details

Mount Options Take Precedence

• The -o ro (read-only) flag creates an immutable mount point
• This flag overrides local filesystem permissions
• Even if you’re root, you cannot modify permissions on a read-only mount

How to Change Permissions

If you need to modify permissions:

1. Unmount the read-only volume first
2. Change permissions on the original source directory
3. Remount with desired configuration

Alternative Approaches

• Use mount -o remount,rw to temporarily make it writable
• Modify source directory permissions before binding
• Use more granular permission controls like ACLs

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How to get the complete and exact list of mounted filesystems, though?

You can see ALL virtual and physical mounted filesystems in /proc/mounts, but sometimes this output will not show actions performed using mount or unmount, which we just have. If you check inside /etc/mtab, you will see your manual mounts, though.

Although you could just use findmnt which will give you a way more human-readable format. Please see this source for more information on using findmnt and some –parsing –examples for an even cleaner output.