DevOps Training Material

Docker RUN vs CMD vs ENTRYPOINT

RUN happens at build time. RUN executes command(s) in a new layer and creates a new image. E.g., it is often used for installing software packages.
CMD happens at run time. CMD sets default command and/or parameters, which can be overwritten from command line when docker container runs.
ENTRYPOINT configures a container that will run as an executable.

RUN apt-get install python3
CMD echo "Hello world"
ENTRYPOINT echo "Hello world"

Are you building dependencies into your image? Use RUN.
Are you launching / running your Dockerized process? Use CMD.

Docker COPY vs ADD

COPY and ADD are both Dockerfile instructions that serve similar purposes. They let you copy files from a specific location into a Docker image.
COPY takes in a src and destination. It only lets you copy in a local file or directory from your host (the machine building the Docker image) into the Docker image itself.
ADD lets you do that too, but it also supports 2 other sources. First, you can use a URL instead of a local file / directory. Secondly, you can extract a tar file from the source directly into the destination.

Chain Your Docker RUN Instructions to Shrink Your Images

RUN wget -O myfile.tar.gz http://example.com/myfile.tar.gz \
    && tar -xvf myfile.tar.gz -C /usr/src/myapp \
    && rm myfile.tar.gz

This is much better. Docker will only create 1 layer, and you still performed all 3 tasks. Shrinkage! You’ll even get faster build times too because each layer adds a bit of build time.

Difference between an Array and String Based CMD

The official terms for this are exec form and shell form commands. Both do nearly the same thing, but there’s an important difference.
Using [] is considered “exec form” and the plain string command is considered “shell form”
Exec Form runs your CMD’s binary as is, along with any arguments you optionally pass in.

CMD ["gunicorn", "-c", "python:config.gunicorn", "hello.app:create_app()"]

Shell Form runs your CMD’s binary through a shell which has the added benefit of using any shell functionality you want (such as using pipes and &&, etc.).

CMD gunicorn -c "python:config.gunicorn" "hello.app:create_app()"

Which One Should You Use?

Shell form sounds better in theory, but it can mess with signal processing. It also means the shell process ends up being PID 1 instead of whatever binary you’re running in your CMD.

Showing what PID 1 is using both methods:
# The output of `ps` when you use exec form:
PID   USER     TIME   COMMAND
  1   root     0:00   {gunicorn} /usr/local/bin/python /usr/local/bin/gunicorn
 15   root     0:02   {gunicorn} /usr/local/bin/python /usr/local/bin/gunicorn
188   root     0:00   ps

# The output of `ps` when you use shell form:
PID   USER     TIME   COMMAND
  1   root     0:00   /bin/sh -c gunicorn -c python:config.gunicorn hello.app:create_app()
  6   root     0:00   {gunicorn} /usr/local/bin/python /usr/local/bin/gunicorn
 16   root     0:00   {gunicorn} /usr/local/bin/python /usr/local/bin/gunicorn
 29   root     0:00   ps

Docker (and I) both recommend that you use exec form whenever possible, which really is most of the time. If you need to do complicated shell scripting when a container starts, you should probably use an ENTRYPOINT script.

Output All of Your Container’s ENV Variables

docker container run --rm alpine:3.7 env

# The above command produces this output:
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=157baf151103
HOME=/root

Show Total Disk Space Used by Docker

Docker has a system sub-command that has a few useful commands. One of them is docker system df which reports back disk space usage stats of your Docker installation.