Commands and Arguments in Docker Recap

In this guide, we’ll dive into how Docker handles commands, arguments, and entrypoints. You’ll learn to:

Understand why docker run ubuntu exits immediately
See how Docker images set a default process with CMD
Override the default command at runtime
Bake custom commands into your own image
Differentiate between CMD and ENTRYPOINT
Combine CMD and ENTRYPOINT for flexible defaults
Replace an entrypoint on the fly

1. Why `docker run ubuntu` Exits Immediately

First, run the following:

docker run ubuntu
docker ps
docker ps -a

You’ll observe:

docker run ubuntu starts a container, then it exits right away.
docker ps shows no active containers.
docker ps -a lists your Ubuntu container with an Exited status.

Containers are designed to run a single process. When that process ends, the container stops.

Warning

Without an interactive shell or long-running process, the default /bin/bash has no TTY and quits immediately—so does the container.

2. How Images Define Default Commands (`CMD`)

Docker images declare a default executable in their Dockerfile using CMD. For example:

# Nginx official image
CMD ["nginx"]

# MySQL official image
ENTRYPOINT ["/entrypoint.sh"]
CMD ["mysqld"]

A combined snippet illustrating both setups:

# ───── Nginx Setup ─────
RUN add-apt-repository -y ppa:nginx/stable \
  && apt-get update \
  && apt-get install -y nginx \
  && rm -rf /var/lib/apt/lists/* \
  && echo "\ndaemon off;" >> /etc/nginx/nginx.conf \
  && chown -R www-data:www-data /var/lib/nginx

VOLUME ["/etc/nginx/sites-enabled", "/etc/nginx/certs"]
WORKDIR /etc/nginx
CMD ["nginx"]

# ───── MySQL Setup ─────
RUN rpmkeys --import https://repo.mysql.com/RPM-GPG-KEY-mysql \
  && yum install -y $MYSQL_SERVER_PACKAGE_URL $MYSQL_SHELL_PACKAGE_URL libpwquality \
  && yum clean all \
  && mkdir /docker-entrypoint-initdb.d

VOLUME /var/lib/mysql
COPY docker-entrypoint.sh /entrypoint.sh
COPY healthcheck.sh /healthcheck.sh
ENTRYPOINT ["/entrypoint.sh"]
HEALTHCHECK CMD /healthcheck.sh
EXPOSE 3306 33060
CMD ["mysqld"]

Note

Use the JSON array form for CMD and ENTRYPOINT to avoid shell string parsing.

3. Overriding the Default Command at Runtime

Append a new command to docker run to replace the CMD entirely:

docker run ubuntu sleep 5

Here, the container runs sleep 5 instead of Bash, pauses for 5 seconds, then exits.

4. Baking Your Custom Command into a New Image

To make the override permanent, author a custom Dockerfile:

FROM ubuntu
# Shell form
CMD sleep 5

# Or JSON form
# CMD ["sleep", "5"]

Build and run:

docker build -t ubuntu-sleeper .
docker run ubuntu-sleeper   # sleeps for 5 seconds

5. ENTRYPOINT vs. CMD

Instruction	Purpose	Runtime Override
CMD	Sets a default command and arguments	Fully replaceable
ENTRYPOINT	Configures the primary executable	Appends runtime args

CMD: default command, easily swapped by arguments you supply.
ENTRYPOINT: fixed executable; any extra args in docker run are appended.

6. Combining `ENTRYPOINT` with Default `CMD` Arguments

Define both to set defaults that users can override:

FROM ubuntu
ENTRYPOINT ["sleep"]
CMD ["5"]

docker run ubuntu-sleeper → runs sleep 5
docker run ubuntu-sleeper 10 → runs sleep 10

7. Replacing the Entrypoint at Runtime

Use --entrypoint to swap out the image’s entrypoint completely:

docker run --entrypoint sleep ubuntu-sleeper 2.0 10
# Executes: sleep 2.0 10

Links and References

Watch Video

Watch video content

1. Why docker run ubuntu Exits Immediately

2. How Images Define Default Commands (CMD)