bash脚本中条件替换错误

Question

我正在使用此代码下载和解压缩bash脚本文件中的数据

    for dump in $dumps; do
        echo "Downloading $dump into $download_folder"
        wget -c -P "$download_folder" "$geonames_general_data_repo/$dump"
        if [ ${dump: -4} == ".zip" ]; then
            echo "Unzipping $dump into $data_dir"
            unzip "$download_folder/$dump" -d $data_dir
        else
            if [ ${dump: -4} == ".txt" ]; then
                mv "$download_folder/$dump" $data_dir
            fi
        fi
    done

但我收到此错误：

importer.sh: Bad substitution 

此行：如果[$ {dump：-4} ==“ .zip”]

我正在使用具有最新更新的Ubunto 18.04