Dockerfile with CMD

In the Dockerfile for the previous example, you used the ENTRYPOINT instruction to specify the process to execute in the container. This time you’ll use the CMD instruction instead and then explore the differences between the two.

• Dockerfile.using_cmd

FROM debian
COPY get-time.sh /usr/local/bin
RUN chmod +x /usr/local/bin/get-time.sh
CMD [ "/usr/local/bin/get-time.sh" ]

Build a Docker image to run this.

docker build -f Dockerfile.using_cmd -t get-time .

Since Docker looks for Dockerfile by default and you’re going to try using a few different Dockerfiles, you need to use the -f option to be explicit about which one to use for the build.

Now print the time using a container.

docker run -it --rm --name timectr get-time

• Output

Fetch UTC(NIST) time every 5 seconds...
21-09-02 19:28:45
21-09-02 19:28:51

We used a new option for the docker run command: -it. This is actually a combined option using two short flags, -i and -t. The first allows us to pass input to the container over stdin; the second attaches a pseudo-terminal, which means for one thing it can respond to the SIGINT signal when you press Ctrl-C on your keyboard to terminate the process.

The container appears to behave the same way as when we used ENTRYPOINT before. However, one difference is that CMD acts more like a default. You can easily override the default command, as shown below.

docker run -it --rm --name timectr get-time echo hello

• Output

hello

The get-time.sh script is no longer executed; instead, the command echo with the argument hello was executed in the container. Since the base image for the get-time image was debian, the standard echo command exists in the path when the container is launched.

Because the argument that you provide in the docker run command replaces the default command, you can’t just provide the frequency option like this (since there is no 2 command).

# this won't work!
docker run -it --rm --name timectr get-time 2

Instead, you need to supply the entire command to override the default one:

docker run -it --rm --name timectr get-time get-time.sh 2

• Output

Fetch UTC(NIST) time every 2 seconds...
21-09-02 19:29:42
21-09-02 19:29:44
^C

Since /usr/local/bin is in the path for the debian image, it wasn’t necessary to fully qualify the path for get-time.sh. The fully-qualified path was used in the CMD instruction is often a good practice for clarity.

Let’s revisit using ENTRYPOINT next.