I believe you'd most likely want to have a
/home partition shared between all your installations. That way, all per-user customizations will be shared, but all distro-specific system configuration is separate.
When you install your system for the first time, you usually can select custom partitioning, and there also choose existing partitions and specify a mount point for those. That would be the easiest approach. So for your first system you'd do custom partitioning, create new partitions as you want them, have one of them for this linux
/, one for
/home, and both of these newly formatted. For subsequent installs, you'd use one of the other partitions for
/ after formatting, and the existing file system for
/home without formatting.
If you want to start from an existing system, things become difficult and errors might have severe consequences. You'd have to
- shrink the existing file system
- shrink the containing partition
- create a new partition
- creating a file system on that
- boot into single user mode for subsequent steps
- rename the old home
- create a mountpoint for the new home
- create a
fstab entry for the new mount
- mount the new home
cp -a stuff from old to new home
- reboot again in multi-user mode
Some further aspects you should be aware of:
- Linux file ownership information is stored using numeric user and group identifiers. The files
/etc/group map those to names. You should ensure that the mappings for users are the same on both systems. There usually are a a large number of system accounts for various services, but only very few user accounts. And most distros these days might allocate ids in the same way. So it might work out automatically, but if files suddenly become inaccessible, this would be the likely cause.
- Make sure you don't start any linux while another one is hibernating. Hibernation will leave the filesystem in an inconsistent state, and even worse, the kernel will expect the system not to change during hibernation, and might cache things for this reason. So major data corruption could be caused by this access pattern.