apt-get工具的进度条是通过curl库实现的。当apt-get下载软件包时，它会使用curl库，该库是一个用于从URL中获取数据的命令行工具和库。

apt-get使用curl库的easy interface和一些选项来显示进度条。具体来说，apt-get使用CURLOPT_PROGRESSFUNCTION选项来指定一个回调函数，它会周期性地调用回调函数以更新进度条。回调函数将显示下载进度的百分比和下载速度。

下面是一个使用curl的简单的示例代码，它可以显示进度条：

#include 
#include 
  
static int progress_func(void* ptr, double TotalToDownload, double NowDownloaded, double TotalToUpload, double NowUploaded)
{
    // calculate percentage
    double percent = NowDownloaded / TotalToDownload;
  
    // display percentage and speed
    printf("Progress: %3.0f%%", percent*100);
    printf(", %0.2f KiB/s\n", NowDownloaded / 1024.0);
  
    return 0;
}
  
int main(int argc, char* argv[])
{
    CURL* curl;
    CURLcode res;
  
    // initialize curl
    res = curl_global_init(CURL_GLOBAL_ALL);
    if (res != CURLE_OK) {
        fprintf(stderr, "curl_global_init() failed: %s\n", curl_easy_strerror(res));
        return 1;
    }
  
    // create curl handle
    curl = curl_easy_init();
    if (curl == NULL) {
        fprintf(stderr, "curl_easy_init() failed\n");
        curl_global_cleanup();
        return 1;
    }
  
    // set URL
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
  
    // set progress callback function
    curl_easy_setopt(curl, CURLOPT_PROGRESSFUNCTION, progress_func);
  
    // enable progress meter
    curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 0L);
  
    // perform transfer
    res = curl_easy_perform(curl);
  
    // check for errors