bashrc snippet to preserve history across sessions since `HISTFILESIZE=-1` is unreliable.

.bashrc

export DATEFORMAT='%F/%R:%S';
export HISTCONTROL=ignoreboth
export HISTFILE=$HOME/.bash_history
export HISTFILESIZE=-1
export HISTSIZE=81920
export HISTTIMEFORMAT="$DATEFORMAT "
export HISTLOG=~/.bash_history.d
export PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:.:~/bin
export PROMPT_COMMAND='echo -en "\n\e[36m$?\e[0m; "; history -a; histlog "$(history 1)"'

# Place subsequent `histlog` command in ~/bin/histlog and it should take care of the rest.

histlog

#!/bin/bash
# @date: 2021-04-25
# @author: Markizano Draconus
# @description: I got sick and tired of my bash history dissapearing despite using
#  HISTFILESIZE=-1, which disables truncating $HISTFILE
#  This script created to export history commands to a sub-directory in my ~/.bash_history.d/
#  folder to avoid my commands from getting truncated from my $HISTFILE.
# @usage: export PROMPT_COMMAND='histlog $(history 1)'

# Acquire the history files and directory...
export HISTLOG=${HISTLOG:-~/.bash_history.d}
export HISTLOGFILE="$HISTLOG/`date +%Y-%m`.bash_history"

# The last history item will be presented as the first argument.
# Since history operations are not available from scripts.
HISTLASTITEM="$1"

# Make sure directory exists.
test -d "$HISTLOG" || {
  mkdir -p "$HISTLOG"
  # ... and is owned by me.
  test "$UID" -eq 0 && chown $SUDO_UID:$SUDO_GID "$HISTLOG"
}

# Strip the leading spaces, index number so we get $date: $command
histItem=`echo "$HISTLASTITEM" | perl -pe 's/^\s*\d+\s+//'`

# If I am just pressing [Enter] and no command was run,
test -e "$HISTLOGFILE" && {
  # Don't append the previous command again to the log.
  lastItem=`tail -n1 "$HISTLOGFILE"`
  test "$histItem" == "$lastItem" && exit 0
}

# Append the history command.
echo "$histItem" >> "$HISTLOGFILE"

# Make sure the history file is owned by me.
test "$UID" -eq 0 && chown $SUDO_UID:$SUDO_GID "$HISTLOGFILE"