Copying files from host to container and vice versa

TLDR; You can use base64 encoding/decoding to copy files

Here is a simple trick to deal with the menace of getting files from the host to container or vice versa. This can even work with VMs running inside your host. We usually use ssh to connect to VMs or containers. When working with the former ssh itself has solutions (like scp) to do the same thing. But it can still be done using the technique I am about to explain.

So in most popular docker images (linux based) there will always be a base64 utility program. (Or it can be installed via any package manager utility like apt or yum). The same utility program exists on several other popular operating systems out-of-the-box. And even if it not there, (especially on Windows), there are other applications that provide the same utility program. For e.g. git bash command line.

The steps are simple. Say you have a file called hello.txt with some text.

1. Run the base64 utility to encode the file.
$ base64 hello.txt
2. Copy the output to your clipboard.
3. In your target container instance, in an exec session, you need to do the following:
$ echo <base64_text> | base64 --decode - > hello.txt

The caveats

1. The container image should be linux based, and, have a base64 utility installed. If not installed there must be a script-only solution...will share when I find it.
2. In this example, I based the base64 binary based on what I experienced. Some binaries may have a different set of arguments. Please consult the help docs

3. You need to be able to write on the container file system.

NB: #3 is the usual bummer. But if you are able to write on the container file system, why not just use exec sessions⁉️

So why does it work?

I do not have a solid answer. You can youtube about it to understand how the algo works. All I know is that the encoding process is based on 64 ascii characters. All the 64 characters can be typed using a simple english keyboard (US layout).

Incidentally base64 is also a very popular with data transmission over the internet. Because it is mostly ascii based. I learned this the hard way when I explored SMTP. protocol and servers

So any digital sequence of bytes can be encoded, transferred and, then reformed using base64. That is the underlying principle of why this works.

I conclude this post with an interesting exercise. Suppose there is a file with content inside it. This file also has some set of file permissions. How do you get a file across (to a container or elsewhere) with the same permission set?