在这种情况下，它看起来像 ADD 命令将基本映像添加到文件系统。如果您运行 docker history --no-trunc docker / whalesay ，则完整命令为：

In this specific case, it looks like the ADD command added the base image to the file system. If you run docker history --no-trunc docker/whalesay, the full command is:

/bin/sh -c #(nop) ADD file:f4d7b4b3402b5c53f266bb7fdd7e728493d9a17f9ef20c8cb1b4759b6e66b70f in /

docker history 报告特定层为188MB。让我们更详细地看一下这些层：

docker history reports that particular layer is 188MB. Let's look at these layers in more detail:

$ docker save docker/whalesay -o whalesay.tar
$ tar tvf whalesay.tar

...
-rw-r--r-- 0/0       197181952 2015-05-25 22:04 cc88f763e297503d2407d6b462b2b390a6fd006b30f51c8efa03dd88fa801b89/layer.tar
...

看起来像一个不错的候选人！现在，您可以提取该层并从中提取文件。

Looks like a pretty good candidate! You can now extract that layer and pull files out of it.

$ tar xf whalesay.tar cc88f763e297503d2407d6b462b2b390a6fd006b30f51c8efa03dd88fa801b89/layer.tar
$ tar xf cc88f763e297503d2407d6b462b2b390a6fd006b30f51c8efa03dd88fa801b89/layer.tar etc/passwd

如果您要拉一个特定文件不在图层中，但是您不知道哪个图层，您可以执行此操作。首先，提取所有图层：

If you're looking to pull a particular file out of a layer, but you don't know which layer, you could do this. First, extract all the layers:

$ tar xf whalesay.tar

现在，您已经将所有图层作为单独的.tar文件。让我们找到一个文件：

Now you've got all the layers as individual .tar files. Let's find a file:

$ for layer in */layer.tar; do tar -tf $layer | grep docker.cow && echo $layer; done
usr/local/share/cows/docker.cow
0523c5a0c4588dde33d61d171c41c2dc5c829db359f4d56ab896ab1c185ed936/layer.tar
cowsay/cows/docker.cow
40e8ae7bb4e5b9eaac56f5be7aa614ed50f163020c87ba59e905e01ef0af0a4f/layer.tar
cowsay/cows/docker.cow
f9bc8676543761ff3033813257937aeb77e9bc84296eaf025e27fe01643927cf/layer.tar

最后，从文件中提取文件所需的图层：

Finally, extract the file from the layer you want:

$ tar xf 0523c5a0c4588dde33d61d171c41c2dc5c829db359f4d56ab896ab1c185ed936/layer.tar \
      usr/local/share/cows/docker.cow

这将提取具有相对于当前目录的完整路径的文件。

This will extract that file with the full path relative to the current directory.

$ cat usr/local/share/cows/docker.cow 
##
## Docker Cow
##
$the_cow = <<EOC;
    $thoughts
     $thoughts
      $thoughts     
                    ##        .            
              ## ## ##       ==            
           ## ## ## ##      ===            
       /""""""""""""""""\___/ ===        
  ~~~ {~~ ~~~~ ~~~ ~~~~ ~~ ~ /  ===- ~~~   
       \\______ o          __/            
        \\    \\        __/             
          \\____\\______/   
EOC