How can I find out what is contained in a saved Docker image tar file? - json

Question

Let's say that I have saved one or more Docker images to a tar file, e.g.
docker save foo:1.2.0 bar:1.4.5 bar:latest > images.tar
Looking at the tar file, I can see that in addition to the individual layer directories, there is a manifest.json file that contains some meta information about the archives contents, including a RepoTags array for each image:
[{
"Config": "...",
"Layers": [...],
"RepoTags": [
"foo:1.2.0"
]
},
{
"Config": "...",
"Layers": [...],
"RepoTags": [
"bar:latest",
"bar:1.4.5"
]
}]
Is there an easy way to extract that info from the tar file, e.g. through a Docker command - or do I have to extract the manifest.json file, run it through a tool like jq and then collect the tag info myself?
The purpose is to find out what is contained in the archive before importing/loading it on a different machine. I imagine that there must be some way to find out what's in the archive...

Answer 1

Since I have not received any answers so far, I'll try to answer myself using what I have tried, and it's working for me.
Using a combination of tar and jq, I came up with this command:
tar -xzOf images.tar.gz manifest.json | jq '[.[] | .RepoTags] | add'
This will extract the manifest.json file to stdout, pipe it into in the jq command, and jq combines the various RepoTags arrays into a single array:
[
"foo:1.2.0",
"bar:1.4.5",
"bar:latest"
]
The result is easy to read and works for me. Only downside is that it requires an installation of jq. I would love to have something that works without dependencies.
Still looking for a better answer, don't hesitate to post an answer if you have something that's easier to use!