AWS Generic Metric Image Download

#!/bin/bash

DEFAULT_START_TIME="-PT1H"
DEFAULT_END_TIME="P0D"
DEFAULT_DOWNLOAD_JSON="download.json"

download() 
{
    local imageJson="${1}"
    local startTime="${2}"
    local endTime="${3}"
    local yAxis="${4}"
    local tempFile=$(mktemp)
    local imageFile="image.png"
    local imagePrefix="${5}"

    imageJson=$(echo "$imageJson" | jq ".start = \"$startTime\"")
    imageJson=$(echo "$imageJson" | jq ".end = \"$endTime\"")
    if [ "$yAxis" != "" ]; then
        imageJson=$(echo "$imageJson" | jq ".yAxis.left.max = $yAxis")
        echo "use $yAxis max for y-axis"
    fi

    imageFile=$(echo "$imageJson" | jq -r ".title" | tr [A-Z] [a-z] | tr " " "_")
    if [ "$imagePrefix" != "" ]; then
        imageFile="$imagePrefix""_$imageFile.png"
    else
        imageFile="$imageFile.png"
    fi

    echo "start download $imageFile ..."
    echo "$imageJson" > "$tempFile"
    aws cloudwatch get-metric-widget-image --metric-widget "file://$tempFile" --output-format png | jq -r '.MetricWidgetImage' | base64 --decode >| "$imageFile"
    rm "$tempFile"
}

usage()
{
    echo "usage: download.sh [-f file.json] [-s starttime] [-e endtime] [-y yaxismax] [-p image_file_prefix]"
}

startTime="$DEFAULT_START_TIME"
endTime="$DEFAULT_END_TIME"
downloadJson="$DEFAULT_DOWNLOAD_JSON"
yAxis=""
imagePrefix=""

while [ "$1" != "" ]; do
    case $1 in
        -f | --file )           shift
                                downloadJson=$1
                                ;;
        -s | --starttime )      shift
                                startTime=$1
                                ;;
        -e | --endtime )        shift
                                endTime=$1
                                ;;
        -y | --ymax )           shift
                                yAxis=($(echo $1 | tr "," " "))
                                ;;
        -p | --prefix )         shift
                                imagePrefix=$1
                                ;;
        * )                     usage
                                exit 1
    esac
    shift
done

i=0
jq -c '.[]' "$downloadJson" | while read imageJson; do
    download "$imageJson" "$startTime" "$endTime" "${yAxis[$i]}" "$imagePrefix"
    ((i++))
done