Backup bash or zshell history shell to not lost any command

backup_history.sh

#!/bin/bash -       
#title           :backup_history.sh
#description     :This script will make a backup of bash history.
#author          :Edgard Leal <[email protected]>
#date            :20160221
#version         :0.3
#usage           :bash backup_history.sh
#notes           :
#bash_version    :4.1.5(1)-release
#==============================================================================

declare -r KEEP=200 
declare BASH_HIST

# you can use for zshell 
# ./backup_history.sh "~/.zsh_history"
#
if [ -z "$1" ]; then
    if [ -f "~/.zsh_history" ]; then
        BASH_HIST=~/.zsh_history
    else 
        BASH_HIST=~/.bash_history
    fi
else
    BASH_HIST="$1"
fi

declare -r BACKUP=$BASH_HIST.$(date +%y%m) 

declare -r current_size="$(wc -l $BASH_HIST | awk '{print $1}')"
echo $current_size

if [ -s "$BASH_HIST" -a "$BASH_HIST" -nt "$BACKUP" ]; then # history file is newer then backup 
    if [[ "$current_size" -gt "3000" ]]; then # there is already a backup 
        mv -f $BASH_HIST $BACKUP 
        tail -n$KEEP $BACKUP > $BASH_HIST 
        history -r 
    else # create new backup, leave last few commands and reinitialize 
        echo "Bash history is to short to create a bakup"
    fi 
fi