如果这一步报如下错误：

root@serverdeb:/media/debs# gpg --clearsign -o InRelease Release
gpg: no default secret key: Unusable secret key
gpg: Release: clear-sign failed: Unusable secret key

说明需要生成gpg签名，步骤如下：

1. gpg --gen-key

对于生成签名，详细见如下：https://blog.csdn.net/qdujunjie/article/details/48291017

如果生成签名卡住(原因详见：https://www.cnblogs.com/styshoo/p/6415939.html)，执行如下命令：rng -r /dev/urandom    /// 前提是安装：apt-get install rng-tools

2. gpg -a --export pub_id | apt-key add -

root@server10:/media/debs# apt-get update
Get:1 file:/media/debs  InRelease [1912 B]
Get:1 file:/media/debs  InRelease [1912 B]
Err:1 file:/media/debs  InRelease                                              
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5789843F6B61B0B4
Hit:2 http://apt.postgresql.org/pub/repos/apt xenial-pgdg InRelease            
Reading package lists... Done

gpg -a --export 5789843F6B61B0B4 | apt-key add -     ///其中pub key可用gpg --list-keys查到

五 gpg -abs -o Release.gpg Release

最后执行apt-get update的输出如下 ：

root@server10:/media/debs# apt-get update
Get:1 file:/media/debs  InRelease [1912 B]
Get:1 file:/media/debs  InRelease [1912 B]
Get:2 file:/media/debs  Packages [74.1 kB]                                      
Hit:3 http://apt.postgresql.org/pub/repos/apt xenial-pgdg InRelease             
Reading package lists... Done

（1）清理apt的下载缓存区。

sudo rm -rf /var/cache/apt/archives/*

（2）下载所需要的组件

sudo apt-get -d install <包名>

（3）创建一个目录，将下载的包拷贝到该目录下

cp -r /var/cache/apt/archives  /yout-path

（4）修改目录权限

chmod 777 -R /your-path

（5）建立deb包的依赖关系

sudo touch /your-path/Packages.gz
    sudo dpkg-scanpackages /your-path/ /dev/null  | gzip > /your-path/Packages.gz

（6）将所有下载的文件和生成的gz文件拷贝到离线的ubuntu机器上，将/etc/apt/sources.list原有内容注释掉，新增：

（7）执行sudo apt-get update，之后就可以直接使用apt-get install 包名 来安装了
如果上面的步骤中报错说

·   Create a toplevel Release file, if it does not exist already. You can do this by running apt-ftparchive release (provided in apt-utils).

·   Sign it. You can do this by running gpg --clearsign -o InRelease Release and gpg -abs -o Release.gpg Release.

·   Publish the key fingerprint, so that your users will know what key they need to import in order to authenticate the files in the archive. It is best to ship your key in its
           own keyring package like Ubuntu does with ubuntu-keyring to be able to distribute updates and key transitions automatically later.

·   Provide instructions on how to add your archive and key. If your users can't acquire your key securely the chain of trust described above is broken. How you can help users
           add your key depends on your archive and target audience ranging from having your keyring package included in another archive users already have configured (like the default
           repositories of their distribution) to leveraging the web of trust.

Whenever the contents of the archive change (new packages are added or removed) the archive maintainer has to follow the first two steps outlined above.