2024-01-21

# probably there already 
mkdir -pv /compat/linux

(In my case I actually wanted the files stored elsewhere, and it 
 worked fine to symlink:  
    mkdir /compat
    mkdir /local/linux
    ln -s /local/linux /compat/linux
 then continue, still treating /compat/linux as though it were a 
 simple directory .. it may sound obvious, but I wondered whether the 
 linux service mounting proc,dev, et.c. would object .. which it didn't.
 Later, because I prefer /compat/linux rather than /local/linux in the 
 paths made by annoying programs (matlab?) that like to resolve the symlinks
 manually or to call realpath, I did a bind mount instead of a symlink:
	mkdir /compat/linux
	mount -t nullfs /local/linux /compat/linux 
 which also worked well.
)

# copy the files from a working linux system that's able to run the 
# program we want, and the program itself; in this case we take them 
# from two different computers, and we don't have to change any 
# licensing setup in matlab because it uses a licence-server so its 
# config in the installation is just the licence-server name and port
# 
cd /compat/linux
ssh root@computerwithRHEL92 tar -c --one-file-system / | tar -xpf - 
ssh root@computerwithmatlab tar -c /opt/matlab/2023b | tar -xpf -

# load kernel modules and mount special compat filesystems
/etc/rc.d/linux start 
# modules include {linux,linux_comman,linux64,linsysfs,linprocfs}.ko 
# mounts are  /compat/linux/{proc,sys,dev,dev/fs,dev/shm} 

# make the interpreter for matlab's startup script be the one from the 
# linux system (then, it seems, it will further use other commands from 
# there, avoiding incompatibility of command options etc between the 
# linux and freebsd, or confusion of matlab in determining the 'platform';
# presumably it's when a binary is run that the system notices it needs 
# to be using the linux emulation 
# line1: change '#!/bin/sh' to '#!/compat/linux/bin/sh' 
vi opt/matlab/2023b/bin/matlab  

# make sure the starting script can be seen in both systems by the 
# same path: it will be called from /opt/matlab/2023b/bin/matlab 
# but when the interpreter runs it will apparently see /compat/linux 
# as being its root directory 
ln -s /compat/linux/opt /opt

# we're told it's nice to bind-mount /tmp , supposedly for X stuff, 
# although it seemed to be fine without this
mount -t nullfs /tmp /compat/linux/tmp 

# if wanting our home to be seen directly despite the /compat/linux
# looking like the root, then do a bind-type mount 
mkdir home/mydir
mount -t nullfs /home/mydir /compat/linux/home/mydir

# also, we should put an entry for our user-account into /compat/linux/etc/passwd 
# use of course the same login,uid,gid as in the system's (FreeBSD) /etc/passwd
# this was found necessary in order to prevent a directory named '?' being created 
# in our pwd when running matlab, containing ?/.java/fonts 
# it was reasoned that ? was its (java/matlab/fontconfig) way of saying "I tried 
# looking up your homedir rather than simply trying $HOME, and I couldn't find it,
# so I'm replacing $HOME with ? 
# with a suitable entry in the linux system's etc/passwd, it's always ~/.java/fonts
# that gets the fontconfig files