The contorted rules for reporting the state of the parent shell suggest
that maybe the entire approach needs re-thinking.
Several approaches come to mind.
1. Create a built-in “parent” command where “parent ulimit” or “parent
trap” answers based on the parent shell rather than the subshell itself.
(Obviously it would be the snapshot taken when the processes forked, rather
than the parent's *current* state, unless we want to get really fancy with
some kind of IPC. I suspect this would not play well with the current
"longjmp" implementation of subshells.)
2. Extend all the commands that report on the shell's internal state
analogously to printf: take a '-v VAR' argument to which they write what
would normally be their output, so there's no need for a subshell, and
therefore doubt about which shell they're reporting on.
3. Provide tracking variables so that it's not necessary to use commands at
all. e.g. "${BASH_ULIMIT[core]}" would report the maximum size of core dump
(and "${BASH_ULIMIT[hard-core]}" would report the maximum value that the
former can be set to), while "${BASH_TRAP[HUP]}" would match the command
string from trap -p HUP (and [[ -v BASH_TRAP[HUP] ]] would tell you whether
its unset or just empty).
4. For "jobs" only, always report the viewpoint of the nearest shell that
has functioning job control - the one that actually uses setpgrp(). "fg",
"bg", and any other command used with a job-spec should recognize that
they're in the "wrong shell" and complain rather than simply being silent.
I wonder how "job control enabled" should be reported in subshells. A
simple reading of POSIX suggests that $- must contain 'm' in subshells of a
session shell that has job control enabled, even though they don't actually
do job control themselves. Maybe there should be an 'M' in $- when job
control is actually active (in the top level session shell). What do others
think about this?
- Martin
