Bash – How to print apparently hidden environment variables

You can write the environment to a file with export -p and read it back periodically in the other instances with . (the include command). But as you note, if there are concurrent modifications, the result won't be pretty.

If you want modifications to propagate instantly, you'll have to use a wrapper around the export builtin that also propagates the changes to the other shells. On Linux, you can use the flock utility.

global_export () {
  {
    flock 0
    . /path/to/env
    [ $# -eq 0 ] || export "$@"
    export -p >/path/to/env
  } </path/to/env
}

Note that you'll need to call global_export whenever you assign a new value to an exported variable.

This is rather clumsy. It's likely that there's a better way to solve your problem. The most obvious way is to have whatever command is using those environment variable read a configuration files instead.

When I type set | wc the result is 9571 bytes long!

Assuming you got that number correct, it is in fact quite small, probably because you are using QNX. On a normal desktop system, it is much larger. Here's what I get on fedora 20:

> set | wc --bytes
133195

133 kB. I did not count the entries as many of them are sourced functions (git seems to install a lot of these), but I did skim them over and there doesn't seem to be anything untoward. A few kB at most is from custom stuff by me.

I am worried about overrunning some internal limit and causing odd behavior in the OS

I very much doubt this is possible because a lack of bounds checking there would indicate a very significant bug in the implementation -- preventing someone from just writing a long variable to inject data into memory has to have been a basic concern. Beyond that, as you say, 9kB is nothing memory wise. I would presume lookups on these are done with a hash table, so the number of entries would not impair performance.