How to write a bash script to set global environment variable?

Each and every shell has its own environment. There's no Universal environment that will magically appear in all console windows. An environment variable created in one shell cannot be accessed in another shell.

It's even more restrictive. If one shell spawns a subshell, that subshell has access to the parent's environment variables, but if that subshell creates an environment variable, it's not accessible in the parent shell.

If all of your shells need access to the same set of variables, you can create a startup file that will set them for you. This is done in BASH via the $HOME/.bash_profile file (or through $HOME/.profile if $HOME/.bash_profile doesn't exist) or through $HOME/.bashrc. Other shells have their own set of startup files. One is used for logins, and one is used for shells spawned without logins (and, as with bash, a third for non-interactive shells). See the manpage to learn exactly what startup scripts are used and what order they're executed).

You can try using shared memory, but I believe that only works while processes are running, so even if you figured out a way to set a piece of shared memory, it would go away as soon as that command is finished. (I've rarely used shared memory except for named pipes). Otherwise, there's really no way to set an environment variable in one shell and have another shell automatically pick it up. You can try using named pipes or writing that environment variable to a file for other shells to pick it up.

Imagine the problems that could happen if someone could change the environment of one shell without my knowledge.

Just run your shell script preceded by "." (dot space).

This causes the script to run the instructions in the original shell. Thus the variables still exist after the script finish

Ex:

cat setmyvar.sh
export myvar=exists

. ./setmyvar.sh

echo $myvar
exists

Actually I found an way to achieve this (which in my case was to use a bash script to set a number of security credentials)

I just call bash from inside the script and the spawned shell now has the export values

export API_USERNAME=abc
export API_PASSWORD=bbbb
bash

now calling the file using ~/.app-x-setup.sh will give me an interactive shell with those environment values setup