Changing to another Linux system without rebooting
 

Is it possible to "boot" to another system, say from Slackware to Ubuntu, without going
through a complete system reboot?

I seem to remember something about this from 17 or 18 years ago, but it's too vague to
recall.

As far as I know, no.

kexec would let you do that.

No, kexec allows you to re-boot a different kernel.

The OP is potentially talking of changing from (for example) a SYSV init to a systemd based system. That would be interesting.

You could always skip the need to boot into another system and just run bedrock linux where you could do more than one distro at once as if it were just one system.

http://bedrocklinux.org/

Or you could run one of the systems in a virtual machine, if you have enough RAM.

why not chroot, boot distro one and chroot into distro two?

Probably right - especially in the time-frame mentioned.
Took the OP too literally.

Maybe he's thinking of dosexec or dosboot or something that I used back in the early 90s to boot a linux kernel from a dos command prompt?

Thanks for the replies. They confirmed what I thought:
chroot or virtual machine are the only ways.

But isn't intel doing something like that currently with its prototype android phone - the same kernel powers both android and debian. Here.

It still reloading the entire OS kernel space and userland for each kernel tool set. Android is based on Linux, but technically, it's not the same userland and each system handles the kernel and drivers differently. All this does is use a hypervisor to unload the userland and then reload the kernel without actually powering down the phone to reload the entire firmware image.

At the risk of repeating myself, there is still bedrock...

It does sound interesting. However, a quick perusal also seems like it might be more trouble than it's worth, and feel a little schizophrenic - you're never sure what distribution you're in... Also seems maybe not yet ready for prime-time?

Have you used it? Anyone else? Comments?

I was about to ask the same.
I looked at it some time back, but don't recall ever getting to the stage of actually installing it. It appealed to me in that I always have several (stand-alone) disparate systems. But there is potentially a lot of setup - and scope for confusion. But it happens I have a laptop now available that may serve as an appropriate sacrificial lamb .... :p