Synchronize history across bash sessions

sync-history.sh

# Synchronize history between bash sessions
#
# Make history from other terminals available to the current one. However,
# don't mix all histories together - make sure that *all* commands from the
# current session are on top of its history, so that pressing up arrow will
# give you most recent command from this session, not from any session.
#
# Since history is saved on each prompt, this additionally protects it from
# terminal crashes.


# keep unlimited shell history because it's very useful
export HISTFILESIZE=-1
export HISTSIZE=-1
shopt -s histappend   # don't overwrite history file after each session


# on every prompt, save new history to dedicated file and recreate full history
# by reading all files, always keeping history from current session on top.
update_history () {
  history -a ${HISTFILE}.$$
  history -c
  history -r  # load common history file
  # load histories of other sessions
  for f in `ls ${HISTFILE}.[0-9]* 2>/dev/null | grep -v "${HISTFILE}.$$\$"`; do
    history -r $f
  done
  history -r "${HISTFILE}.$$"  # load current session history
}
if [[ "$PROMPT_COMMAND" != *update_history* ]]; then
  export PROMPT_COMMAND="update_history; $PROMPT_COMMAND"
fi

# merge session history into main history file on bash exit
merge_session_history () {
  if [ -e ${HISTFILE}.$$ ]; then
    cat ${HISTFILE}.$$ >> $HISTFILE
    \rm ${HISTFILE}.$$
  fi
}
trap merge_session_history EXIT


# detect leftover files from crashed sessions and merge them back
active_shells=$(pgrep `ps -p $$ -o comm=`)
grep_pattern=`for pid in $active_shells; do echo -n "-e \.${pid}\$ "; done`
orphaned_files=`ls $HISTFILE.[0-9]* 2>/dev/null | grep -v $grep_pattern`

if [ -n "$orphaned_files" ]; then
  echo Merging orphaned history files:
  for f in $orphaned_files; do
    echo "  `basename $f`"
    cat $f >> $HISTFILE
    \rm $f
  done
  echo "done."
fi

@hoefkensj ambitious@nice, but could not make it work. In the end I just:
export HISTFILE=~/.bash/.bash_history

Is there a way to record only direct inputs? Now it seems it catches all inputs, history file is full of mc's outputs like:
cd "printf '%b' '\0057home"

Still trying to fix this to use with tmux. Now it catches all the commands sent from random apps, but later this breaks the input ending up with:
$ mc
$ ls -la
$ ls -latop
can't remove that ^ ls -la
upd: Ok, that part probably was caused by cyd01/KiTTY and fixed by changing termtype to tmux-256color

MC's internal commands has a leading space so it won't be saved in history, but it still gets saved. Fixed it by checking for leading space and exiting the function. But is there a better solution to rrecord only user inputs?

update_history () {
    lastCmd=$(fc -lnr | head -n1)
    # clean the spaces left by fc
    lastCmdReal=${lastCmd:2}
    # remove all leading spaces
    lastCmdRealClean=${lastCmd##+([[:space:]])}

    if [[ "$lastCmdReal" == " $lastCmdRealClean"  ]]; then
       return 0
    fi