install/59552: sysinst RFE: run /bin/sh with new system disk still mounted

[sr%genyosha.net@localhost]

>Number:         59552
>Category:       install
>Synopsis:       sysinst RFE: run /bin/sh with new system disk still mounted
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Jul 24 23:40:00 +0000 2025
>Originator:     Steve Rikli
>Release:        10.1
>Organization:
>Environment:
>Description:
Some OS installers allow dropping into a shell to perform tasks on the installed target system disk before finally exiting the installer and rebooting.

E.g. updating a config file, adding ssh keys, etc.

An earlier thread on this topic:
  https://mail-index.netbsd.org/netbsd-users/2024/08/12/msg031638.html

My comments which started the thread:
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After the system disk is partitioned and sets are installed, plus any "Configure the additional items as needed" (network, Timezone, etc.), sysinst has a couple options to run a shell, i.e. the "Utility menu" with "Run /bin/sh", or simply "Exit Install System" before "Reboot", which drops to a root shell prompt.

This is all good, but by that point sysinst has already unmounted the newly-installed sysdisk and removed disk wedges etc. Leaving just the installation media mounted.
----

As discussed in the emails there are some workarounds to get this functionality, but not as cleanly as an option provided by sysinst directly.
>How-To-Repeat:
use sysinst for new OS install, check the Utilities and other sub-menus for /bin/sh
>Fix: