How can I modify an environment variable across all running shells?

I use Terminal.app and iTerm, both of which support running mul开发者_JAVA技巧tiple shells simultaneously via multiple tabs and multiple windows. I often use this feature, but as a result, if I want to change an environment variable setting, I generally have to run the same command once in every single tab and window I have open -- as well as any new tabs or windows that I open in the future. Is it possible for my shells to communicate with each other such that I can change an environment variable once, and have that change propagate to all my other currently running shells?

I know that I can statically set env variables in a startup file like .bashrc. I also know that I can have subshells inherit the environment of parent shells, either normally or via screen. Neither of those options address this question. This question is specifically about dynamically changing the environment of multiple currently-running shells simultaneously.

Ideally, I'd like to accomplish this without writing the contents of these variables to disk at any point. One of the reasons I want to be able to do this is so that I can set sensitive information in an env variable, such as hashed passwords, and refer to them later on in other shells. I would like to be able to set these variables once when I log in, and be able to refer to them in all my shells until I log out, or until the machine is restarted. (This is similar to how ssh-agent works, but as far as I'm aware, ssh-agent will only store SSH keys, not env variables.)

Is it possible to make shells communicate like this?

Right. Since each process has it's own copy of the environment variables, you can't magically change them all at once. If you bend your mind enough though, there are strange workarounds.

For instance, if you currently have a command you run to update each one, you can automate running that command. Check the bash man page for PROMPT_COMMAND, which can run a command each time the bash prompt is printed. Most shells have something similar.

As far as not putting a hashed password on disk because you are pulling it from an envvar instead of something like ssh-agent...that would be a whole 'nother topic.

Unless you write your own shell, you can't. ssh-agent works by having each SSH client contact it for the keys, but most common shells have no similar mechanism.