4chan image downloader

#! /usr/bin/env bash

## website:auto_update_delay:pictures_format
FMT_REF=(
  '4chan':5:'images\.4chan\.org/\w+/src/[0-9]+\.\w+'
  'imagefap':0:'fap\.to/images/full/[0-9]+/[0-9]+/\w+\.\w+'
)

function fatal {
        echo "$@" 1>&2
        exit 1
}

function display_source {
  curl "$@" 2>/dev/null
}

function download_file {
  wget -o /dev/null -O "$1" "$2"
}

function loop_urls {
        local URL="$1"
        local FOLDER="$2"
  local FMT="$3"

        echo "Building URLs index from ${URL}..."
        local U=( $(display_source "$URL" | egrep -o "$FMT" | sort -u | uniq) )
        local NU=${#U[@]}

        test "$NU" -lt 1 && fatal "Unable to fetch $URL (404?)"

        local n=0
        for u in "${U[@]}"; do
                local F=$(basename "$u")

                n=$((n + 1))
                echo -n "Downloading pic #${n}/${NU} (${F})..."

                test -f "${FOLDER}/${F}" && echo " Cached" && continue

    download_file "${FOLDER}/${F}" "http://${u}" \
                        || fatal "Unable to download pic (404?)"
                echo ' OK'
        done
}

function main {
  local URL="$1"
  local FOLDER="$2"

        echo "Creating folder $2..."
        mkdir -p "$FOLDER" 2>/dev/null || fatal "Unable to create folder $FOLDER"

  local FMT=''
  local DELAY='0'
  for i in "${FMT_REF[@]}"; do
    local k=$(echo "$i" | cut -d: -f1)
    local d=$(echo "$i" | cut -d: -f2)
    local v=$(echo "$i" | cut -d: -f3)

    echo "$URL" | grep -q "$k" \
        && FMT="$v" \
        && DELAY="$d" \
        && break
  done

  test -z "$FMT" && fatal "Unsupported website"
  echo "$DELAY" | egrep -q '[0-9]+' || fatal "Delay isn't a number"

  echo "Detected URL format: $FMT (delay: ${DELAY})"

  while true; do
    loop_urls "$URL" "$FOLDER" "$FMT"

    if [ "$DELAY" -gt 0 ]; then
      sleep "$DELAY"
    else
      break
    fi
  done
}

if [ $# -eq 2 ]; then
  main "$1" "$2"
else
  fatal "Usage: $0 <url> <folder>"
fi