Can history files be unified in bash?

"There are two things you need to do:

Insert the command shopt -s histappend in your .bashrc. This will append to the history file instead of overwriting it.
Also in your .bashrc, insert PROMPT_COMMAND=""$PROMPT_COMMAND;history -a; history -n"" and the history file will be re-written and re-read each time bash shows the prompt.

EDIT: Thanks to e-t172 for the history -n trick"

"None of the solution here worked for me, but what did work was just using:
cat ~/.bash_history | grep something

instead of
history | grep something

Kudos to this SO answer."