Bash script to download xkcd comics

xkcd-dl

#!/usr/bin/env bash

# Print usage message.
usage() {
    local program_name
    program_name=${0##*/}
    cat <<EOF
Usage: $program_name [-option]
Options:
    -h          Print this message
    -a          Download all images
    -s [value]  Download from specific comic number
EOF
}

# Download image and JSON file.
download_xkcd_images() {
    TOTAL=$(curl https://xkcd.com/info.0.json | grep -Po '"num": [0-9]+' | grep -Po '[0-9]+')
    download_dir="$HOME/Downloads/xkcd"
    mkdir -p "$download_dir/JSON"

    for i in $(seq "${FROM:-1}" "$TOTAL"); do
        URL=https://xkcd.com/$i/info.0.json
        JSON=$(curl "$URL")
        IMG=$(echo "$JSON" | grep -Po '"img":\s"https:\/\/imgs.xkcd.com\/comics\/[\w\(\)-]+.(png|jpg|gif)"' | grep -Po 'https://imgs.xkcd.com\/comics\/[\w\(\)-]+.(png|jpg|gif)' | sed 's/\\//g')
        TYPE=$(echo "$IMG" | grep -o "...$")
        NUM=$(echo "$JSON" | grep -Po '"num": [0-9]+' | grep -Po '[0-9]+')
        TITLE=$(echo "$JSON" | grep -Po '"title": ".*?"' | grep -Po '"[A-Z0-9].*?"' | sed 's/"//g')

        echo "$JSON" > "$download_dir/xkcd $NUM: $TITLE.json"
        wget "$IMG" -O "$download_dir/xkcd $NUM: $TITLE.$TYPE"
    done

    # Verification, there should be 2 files for each comic (image, JSON).
    # Some don't have an image (404, 1446, 1608, 1663), so a small amount of errors being reported is ok.
    for i in $(seq "${FROM:-1}" "$TOTAL"); do
        if [[ $(ls "$download_dir" | grep "xkcd $i" | wc -l) -lt 2 ]]; then
            echo "Error at xkcd $i"
        fi
    done

    mv -f "$download_dir"/*.json "$download_dir/JSON"
}

main() {
    if [ -z "$1" ]; then
        usage
        exit
    fi

    while getopts has: option; do
        case "${option}" in
            h)
                usage
                exit 0
                ;;
            a)
                download_xkcd_images
                ;;
            s)
                FROM=${OPTARG}
                download_xkcd_images
                ;;
        esac
    done
}

main "$@"

Shortened URL for raw text of this script: https://git.io/xkcd-dl