Basic test:
```
$ truss -f -o truss.log -- sh -c '. /etc/rc.subr ; list_vars SHELL\*' && rg read\\\( truss.log | wc -l
SHELL
SHELLCHECK_OPTS
3401
$ truss -f -o truss.log -- sh -c '. ./rc.subr ; list_vars SHELL\*' && rg read\\\( truss.log | wc -l
SHELL
SHELLCHECK_OPTS
63
```