OS=Ubuntu-21.10
shell=Bash and more ksh93 for scripting

I've seen a few ways described to keep your shell history of all shells
and add it together best as can be hoped as you go.

I had something that seemed to be working for at least a few months.

It was plagiarized from the internet some time ago.
Probably was at least 1.5 yrs ago.

At some point every thing at the commented section shown below:

## PROMPT_COMMAND="$PROMPT_COMMAND;history -a" ; export PROMPT_COMMAND

and a few more I don't remember and are gone from my .bashrc file now.
(May have just been commented lines.)

And related history stuff in .bashrc:

HISTIGNORE='ls:cd:clear:fg:exit';export HISTIGNORE
HISTCONTROL='ignoreboth';export HISTCONTROL
HISTSIZE='30000';export HISTSIZE
HISTFILESIZE='300000';export HISTFILESIZE
HISTFILE=~/.bash_history;export HISTFILE

Seemed to be working for a while anyway.

What started happening was the PROMPT_COMMAND line and seems there were a
few more started showing up in every command I tried to run or when
ever a terminal was started.

I never figured out what was doing that, tried may different
combinations. Including stuff from google.

After tinkering for a while and not getting it right I just commented
it out and have just been using `shopt -s histappend'

Maybe I'm just daydreaming, but I wonder if there are tried and tested
ways to get the maximum possible history saved?