If you open a lot of terminal screens at once while working on Linux, you may have noticed that the commands you type in one terminal are not available in the other, and usually once you close all terminals, and open another again, you will see that only the commands written in one of them (The first one opened before I think) are available in history.

That is not good if you need that command written in the terminal that got not saved in the History file, so, how to solve that?

Linux history file united

You will prefer to have just one history file, with all commands you typed in any ever opened terminal window, to accomplish this just enter this command in your $HOME/.bashrc file.

shopt -s histappend

That is it.