Matrix effect for linux terminal, working not in parallel

matrix.sh

#!/bin/bash

##########################
# Author: Roy Ros Cobo ###
##########################

CHARSET=(ｱ ｲ ｳ ｴ ｵ ｶ ｷ ｸ ｹ ｺ ｻ ｼ ｽ ｾ ｿ ﾀ ﾁ ﾂ ﾃ ﾄ ﾅ ﾆ ﾇ ﾈ ﾉ ﾊ ﾋ ﾌ ﾍ ﾎ ﾏ ﾐ ﾑ ﾒ ﾓ ﾔ ﾕ ﾖ ﾗ ﾘ ﾙ ﾚ ﾛ ﾜ ﾝ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9)
MAX_LINES=60

ROWS=$(tput lines)
COLS=$[$(tput cols) +1]
GREEN="\033[32m"; WHITE="\033[37m"; 
declare -a lines_row
declare -a lines_col
declare -a lines_len
declare -a lines_white
declare -a lines_prevChar

function rnd {

    if [ $# -eq 1 ]
        then echo $[$RANDOM%$1]
        else echo $[$RANDOM%$1+$2]
    fi
}
function print {
    #ROW, COL, CHAR, COLOR

    local CURSOR="\033[$1;$2H";
    echo -ne "$CURSOR$4$3";
}
function erase_char { 
    #ROW, COL

    print $1 $2 "\u0020"
}


function new_line {

    lines_row[$1]=0
    lines_col[$1]=$(rnd $COLS)
    lines_len[$1]=$(rnd $[ROWS-(ROWS/4)+1] 3)
    lines_white[$1]=$(rnd 3)
}
function actualize_line {

    ((lines_row[$1]++))
    local row=${lines_row[$1]}
    local col=${lines_col[$1]}
    local len=${lines_len[$1]}

    local new_char=${CHARSET[$(rnd ${#CHARSET[@]})]}
    local prev_char=${lines_prevChar[$1]}

    if [ ${lines_white[$1]} -eq 0 ]
    then
        if [ $row -le $[ROWS +1] ]; then print $[row -1] $col $prev_char $GREEN; fi
        if [ $row -le $ROWS ]; then print $row $col $new_char $WHITE; fi
    else 
        if [ $row -le $ROWS ]; then print $row $col $new_char $GREEN; fi
    fi
    if [ $row -gt $len ]; then erase_char $[row-len] $col; fi

    lines_prevChar[$1]=$new_char
}
function is_done_line {

    if [ $[${lines_row[$1]} - ${lines_len[$1]}] -eq $ROWS ]
        then echo 1
        else echo 0
    fi
}

function matrix {

    clear; setterm --cursor off; unclutter -idle 1 &
    
    for i in $(seq 0 $[MAX_LINES -1]); do new_line $i; done

    local n_line=0
    while true
    do 
        actualize_line $n_line
        if [ $(is_done_line $n_line) -eq 1 ]; then new_line $n_line; fi
   
        ((n_line++))
        if [ $n_line -eq $MAX_LINES ]; then n_line=0; fi
    done
}

matrix