Merge and keep bash history from different folders and processes

merge_bash_history.sh

#!/bin/bash
# BASH history merger by Agustín F. Pozuelo (c) 2023
# Merge history files from multiple sessions, removing duplicate commands

# TODO: Maintain order of commands in the most recent appearances

# BASH history merger.
# I use this to maintain history of my many BASH sessions
# Each session writes history to a separate file
# Running this from inside .bashrc, the 10 most recent sessions are merged together
# duplicate lines are removed, then they are all sorted by timestamp and place in a new history file
# The new file is used throughout the session then saved

# My history settings:
export HISTCONTROL=ignoredups:erasedups  # Avoid (only) consecutive dups
export HISTSIZE=20000  # Got 17846 as of January 2023
export HISTFILESIZE=20000
export HISTTIMEFORMAT="%F %T " 
export HISTFILE="$HOME/.history-$(date +%Y%m%d)-$(basename $(pwd))-$$"

echo "Biggest story files:"
ls -Shs ~/.*history* | head

function merge_histories {
    >&2 echo "Merging history files:"
    >&2 ls -lh "$@"
    for h in "$@"; do
        unset timestamp
#        N=0
        TS_PATTERN='^\#[0-9]{10}'
        cat $h | while read -r line; do
#            ((N+=1))
            if [[ $line =~ ^\#[0-9]{10} ]]; then
                if [[ -v timestamp ]]; then
#                    >&2 echo "NUL between '$timestamp' and '$line'..."
                    echo -en '\0'
                fi
                timestamp=$line
#                >&2 echo "Got TIMESTAMP=$timestamp ($h:$N)"
                echo -n "$timestamp "
            elif [[ -v timestamp ]]; then
#                >&2 echo "Got COMMAND LINE=$line ($h:$N)"
                echo "$line"
            else
                >&2 echo "No timestamps in file: $h"
                # TODO: use the file's timestamp? "#$(stat -c%Y $h)"
                break
            fi
        done | while read -rd $'\0' timestamp commands; do
#            >&2 echo -e "$h:TIMESTAMP=$timestamp\n$h:COMMANDS=$commands"
            echo -en "$commands $timestamp\0"
        done
    done |sort -zr|  # r-sort to keep last timestamp for same command
    while read -rd $'\0' l; do
        timestamp=${l##*#}
#        >&2 echo "GOT LINE: $l, timestamp=$timestamp"
        echo -en "$timestamp ${l%#*}\0"  # Replace timestamp in front and uniq by the rest
    done |uniq -zf1|sort -z|  # Unique and sort by timestamp
    while read -rd $'\0' timestamp rest; do
        echo -e "#$timestamp\n$rest"
#        >&2 echo "WROTE TIMESTAMP=#$timestamp REST=$rest"
    done
}

histories=$(ls -t1 ~/.*history*|head)  # 10 most recent
merge_histories $histories > $HISTFILE

ls -lh $HISTFILE
history -r $HISTFILE
echo "History lines: $(history|wc -l)"