HISTFILE=$HOME/.bash_history
HISTSIZE=10000
HISTFILESIZE=20000
shopt -s histappend

__bash_shared_history() {
    history -a
    history -n
}

__bash_prompt_command_decl=$(declare -p PROMPT_COMMAND 2>/dev/null || printf "")

case "$__bash_prompt_command_decl" in
declare\ -a*)
    __bash_shared_history_found=0
    for __bash_prompt_command in "${PROMPT_COMMAND[@]}"; do
        [ "$__bash_prompt_command" = "__bash_shared_history" ] && __bash_shared_history_found=1
    done
    (( __bash_shared_history_found )) || PROMPT_COMMAND+=(__bash_shared_history)
    ;;
*)
    case ";${PROMPT_COMMAND:-};" in
        *";__bash_shared_history;"*) ;;
        *) PROMPT_COMMAND="__bash_shared_history${PROMPT_COMMAND:+; $PROMPT_COMMAND}" ;;
    esac
    ;;
esac

unset __bash_prompt_command_decl __bash_prompt_command __bash_shared_history_found