如何在 CLI 中使用 ctr 在 containerd 中运行 docker 图像？

Question

Am exploring on how to use containerd in place of dockerd. This is for learning only and as a cli tool rather than with any pipelines or automation.

So far, documentation in regards to using containerd in cli (via ctr ) is very limited. Even the official docs are using Go lang to utilize containerd directly.

What I have learnt is ctr command plays the role of docker command to control containerd . I have thus far created a docker image and exported it to .tar format. Now using ctr i import hello.tar I have imported it as an image.

Now ctr i ls gives me the following output:

REF                                     TYPE                                       DIGEST                                                                  SIZE      PLATFORMS   LABELS
docker.io/library/hello-java-app:latest application/vnd.oci.image.manifest.v1+json sha256:ef4acfd85c856ea020328959ff3cac23f37fa639b7edfb1691619d9bfe1e06c7 628.7 MiB linux/amd64 -

Trying to run a container asks me for the image id:

root@slave-node:~/images/sample# ctr run
ctr: image ref must be provided
root@slave-node:~/images/sample# ctr run docker.io/library/hello-java-app:latest
ctr: container id must be provided

I am not sure on where to get the image id from. Are there any docs related to ctr or containerd that could be helpful for a beginner?

Just running the image as a container would be sufficient for me.

Answer 1

The ctr run command creates a container and executes it

ctr run <imageName> <uniqueValue>

eg, ctr run docker.io/library/hello-java-app:latest v1 --rm

This executes my basic docker java image with a print statement:

~~~~
HelloWorld from Java Application running in Docker.
~~~~

Steps followed:

1 - A java file:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("~~~~\nHelloWorld from Java Application running in Docker.\n~~~~");
    }
}

2 - An image:

FROM java:8
COPY HelloWorld.java .
RUN javac HelloWorld.java
CMD ["java", "HelloWorld"]

3 - Build image and export as .tar

• docker build -t hello-java-app .
• docker save -o ~/images/sample/hello-java-app.tar hello-java-app

4 - Import image (.tar) into containerd:

• ctr i import hello-java-app.tar

unpacking docker.io/library/hello-java-app:latest (sha256:ef4acfd85c856ea020328959ff3cac23f37fa639b7edfb1691619d9bfe1e06c7)...done

• ctr i ls

REF                                     TYPE                                       DIGEST                                                                  SIZE      PLATFORMS   LABELS
docker.io/library/hello-java-app:latest application/vnd.oci.image.manifest.v1+json sha256:ef4acfd85c856ea020328959ff3cac23f37fa639b7edfb1691619d9bfe1e06c7 628.7 MiB linux/amd64 -

5 - Run the image:

• ctr run docker.io/library/hello-java-app:latest v1 --rm

~~~~
HelloWorld from Java Application running in Docker.
~~~~

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I am still unsure of the use of creating a container. The run command creates a container and executes it once. ctr c create just creates a container which can then be listed with ctr c ls but I am not able to utilize them in any meaningful way. Can anyone clarify its purpose?

---

PS: Without the --rm flag, a new unique value is needed to be entered for every run as the old container is retained and we get an error: ctr: snapshot "v1": already exists