Scripting single-user mode

As I have written earlier, it was possible to add commands to /etc/rc.server, and they would be executed in a context very similar to single-user mode. However, with the the 10.5.7 upgrade, /etc/rc.server was moved to a later point in the boot sequence, to an environment more similar to ordinary multi-user mode. So not only is the context different, but this indicates how fragile the whole /etc/rc* vestige is in OS/X. A new method is required.

The best alternative I've come up with is to use actual single-user mode. It is possible to get into single-user mode from a script via this sequence (executed as root): « nvram boot-args=-s » ; reboot. At some point once single-user mode is entered, the command « nvram boot-args= » must be run in order to re-eneable multi-user mode.

There is a script that is executed by the shell, and that can be hooked for the purpose of scripting maintenance in single-user mode: /etc/profile, the shared, system-wide start-up file for all shells in the sh family. However, since this location can (and should) be used to customize the shell environment at the system level for all users, it should be changed as "invisibly" as possible.

I prefer to deal with these issues as follows: I'll put one line at the top of /etc/profile that contains some fast heuristics and slower deterministic tests for single-user mode which if passed result in a call to jidaemon (which is the script I want to run in single-user mode). The presence of this line at the top of /etc/profile is required. It can be checked by comparing [[ "$THELINE" == `head -1 < /etc/profile` ]]. The heuristics should all be based on the shell's internal environment, and should be as fast as possible, because /etc/profile is called every time the shell starts up. The heuristics are UID=0, HOME=""; if those are true, the deterministic tests are `sysctl -n kern.singleuser`=1 and -x /var/root/jidaemon. If those are true, run /var/root/jidaemon. Within jidaemon, all those tests are repeated, and some additional tests are run: nvram boot.args == *-s*, read-only root, -f /tmp/just.imagine and so on. Also, if -s is set in nvram boot.args, jidaemon must clear it while preserving any other flags. If any of these tests fail, then jidaemon returns to caller and the only result (beyond clearing the boot.args -s flag) is a slight delay--the shell will continue and an interactive single-user mode session will begin. If jidaemon runs normally, it will restart the system when complete.