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

This script sounds very interesting. But what am I supposed to do with the script? What do I have to configure how to use this?
Thanks

@dkadioglu save and source it in your shell configuration file (usually ~/.bashrc on Linux, ~/.bash_profile on Mac) - like this:

source sync-history.sh

Let me know if you need more detailed instructions.

Thank you very much, works so far. One thing though: It seems, that the deletion of entries (history -d offset) is not possible anymore. At least for me, whichever item I delete, it still appears in the history. I tried to find a solution for that, without success so far. Do you have any idea?