How to create a read-only RAM disk on macOS from the command line?
Get the device number of the RAM disk: diskutil list (e.g. disk3)

Add some content to the RAM disk .
Unmount the volume: diskutil umount disk3
Mount the RAM disk read-only: diskutil mount readOnly /dev/disk3.


Apply the disk number according to your environment!


unmount/mount works for an HFS+ disk (in case of a RAM disk)/volume or an APFS volume only
unmountDisk/mountDisk works for HFS+ disks and APFS container schemes.


If you format the RAMDisk as APFS container and one APFS volume

diskutil partitionDisk $(hdiutil attach -nomount ram://1048576) 1 GPTFormat APFS 'RAMDisk' '100%'


you can either mount the APFS container scheme or the APFS volume:


Get the device number of the RAM disk's container scheme: diskutil list (e.g. disk4)
Unmount the volume: diskutil umountDisk disk4
Mount the RAM disk read-only: diskutil mountDisk readOnly /dev/disk4


or the RAM disk's APFS volume:


Unmount the volume: diskutil umount disk4s1  
Mount the RAM disk read-only: diskutil mount readOnly /dev/disk4s1  




As one-liner for an HFS+ volume with the unique name RAMDisk:

RD=$(diskutil list | awk '/RAMDisk/ { print $5 }'); diskutil umount $RD; diskutil mount readOnly $RD; exit


As one-liner for an APFS volume with the unique name RAMDisk:

RD=$(diskutil list | awk '/RAMDisk/ { print $7 }'); diskutil umount $RD; diskutil mount readOnly $RD; exit




Thanx to user3439894 for all his hints &amp; comments...