temporary activation

[duskmoss]

One of my favorite features of freebsd's bectl and loader is that you can activate a boot environment for a single boot.

It makes many workflows that safely test new system configuration/upgrades. Here's an example:

1. create new boot env
2. activate it
3. mess with system
4. temporary activate the currently booted boot env
5. reboot

6a) if it boots properlyeverything looks good, activate this boot env

6b) if it doesn't boot, or it boots wrong, reboot and the system boots how it was before step 1