Shell script for logging CPU, memory, network and HDD usage

monitor.sh

#!/bin/bash

usage() {
    echo "Usage: $0 COMMAND"
    echo ""
    echo "Commands:"
    echo "  start <output-dir> <interval> [<cpu-mem-net-log> <hdd-log>]  Start the monitor"
    echo "  stop                                                         Stop the monitor"
    exit 1
}
[ "$#" -ge 1 ] || usage

PARSE_SAR='BEGIN { date="" } { if (NR == 1) date=$4; if (NF == 11 && $3 != "kbmemfree") printf("%s\t%s\t", $3, $4); if ($3 == "all") printf("%s\t%.2f\t", $9, (100.0-$9)); if ($3 == "eth0") printf("%s\t%s", $6, $7); if ($3 == "CPU") { if (NR > 3) printf("\n"); printf("%s %s %s\t", date, $1, $2); } }'

PARSE_IOSTAT='{ if (NF == 1) printf("%s\t", $1); if ($1 == "sda") printf("%s\t%s", $3, $4); if (NF == 0 && NR > 2) printf("\n"); }'

BASE_DIR=$(dirname $0)
COMMAND=$1
PID_FILE=$BASE_DIR/monitor.pid

if [ "$COMMAND" == "start" ]; then
    echo -ne "Starting monitor... "
    echo -ne "" > $PID_FILE
    
    OUTPUT_DIR=$2

    if [ -z "$3" ]; then
        INTERVAL=1
    else
        INTERVAL=$3
    fi
    
    if [ -z "$4" ]; then
        CPU_MEM_NET_LOG=$OUTPUT_DIR/cpu_mem_net.log
    else
        CPU_MEM_NET_LOG=$OUTPUT_DIR/$4
    fi
    
    if [ -z "$5" ]; then
        HDD_LOG=$OUTPUT_DIR/hdd.log
    else
        HDD_LOG=$OUTPUT_DIR/$5
    fi

    export S_TIME_FORMAT=ISO
    nohup sar -ur -n DEV $INTERVAL | awk -W interactive "$PARSE_SAR" > $CPU_MEM_NET_LOG 2>&1 &
    echo $! >> $PID_FILE
    
    nohup iostat -dmt $INTERVAL | awk -W interactive "$PARSE_IOSTAT" > $HDD_LOG 2>&1 &
    echo $! >> $PID_FILE
    
    echo -ne "DONE\n"
    exit 0
elif [ "$COMMAND" == "stop" ]; then
    echo -ne "Stopping monitor... "
    
    while read pid; do
        kill -9 $pid
    done <$PID_FILE
    
    echo -ne "" > $PID_FILE
    echo -ne "DONE\n"
elif [ "$COMMAND" == "info" ]; then
    echo "Log file: cpu_mem_net"
    echo "Columns:"
    echo "  timestamp    Reading date in format YYYY-MM-DD HH:mm:ss AM|PM"
    echo "  cpu_free     Percentage of free CPU"
    echo "  cpu_used     Percentage of used CPU"
    echo "  mem_free     Amount of free memory in KBytes"
    echo "  mem_used     Amount of used memory in KBytes"
    echo "  net_recv     Network download rate in KBytes/s"
    echo "  net_sent     Network upload rate in KBytes/s"
    echo ""
    echo "Log file: hdd"
    echo "Columns:"
    echo "  timestamp    Reading date in format YYYY-MM-DDTHH:mm:ssZZZZ"
    echo "  hdd_read     HDD read rate in MBytes/s"
    echo "  hdd_write    HDD write rate in MBytes/s"
else
    usage
fi

Hello,
I tried the script above and the result was the following:

./monitor.sh

Usage: ./monitor.sh COMMAND

Commands:
start [ ] Start the monitor
stop Stop the monitor

My question is what modification I need to do in order to get the desired result?

Thanks