Shell history backup function

shell_history_backup.zsh

# shellcheck shell=bash
# shellcheck disable=SC1091

# source from .zshrc or similar:
# source "$DOTFILES/shell_history_backup.zsh" && _backup_shell_hist

function _backup_shell_hist(){
    if [ -n "$SHELL_HIST_BACKUP_LOC" ]
    then
        _histfile_base="$(basename "$HISTFILE")"

        # .zsh_history_2023-09-14
        _current="$_histfile_base"_"$(date -u +"%Y-%m-%d")"
        _fq_current="$SHELL_HIST_BACKUP_LOC/$_current"

        if [ -f "$_fq_current" ];
        then
            # no need to backup if we've backed up recently
            return 0
        fi

        mkdir -p "$SHELL_HIST_BACKUP_LOC"

        # list all history backups, sort descending by time modified, grab the first, which is the newest
        _fq_last="$(find "$SHELL_HIST_BACKUP_LOC" -name "$_histfile_base"_\* -print0 | xargs -0 ls -t | head -n  1)"
        _last_size=0

        # copy the live history file to today's backup
        if ! cp "$HISTFILE" "$_fq_current";
        then
            echo "*****************************************" >&2
            echo "!! WARNING: shell history backup failed!!" >&2
            echo "*****************************************" >&2
            return 1
        fi

        # filesize is a shell function defined elsewhere
        # it essentially does stat -f"%z" <filename>
        _current_size="$(filesize "$_fq_current")" || return

        # if there was at least one existing backup, compare its size to today's size
        if [ -n "$_fq_last" ];
        then
            _last_size="$(filesize "$_fq_last")" || return

            # shell history size should only grow
            # so if today's backup is smaller than the previous
            # then we have a problem
            if [ "$_current_size" -lt "$_last_size" ];
            then
                echo "**************************************************" >&2
                echo "!! WARNING: '$HISTFILE' appears to be truncated !!" >&2
                echo "**************************************************" >&2
            fi
        fi
    fi
}