CMD vs Entrypoint

In this guide, we’ll explore how Docker uses the CMD and ENTRYPOINT instructions to define the default process of a container. You’ll learn how to override or extend these defaults at runtime and bake permanent changes into your images.

Why Containers Exit Immediately

When you run a container without specifying a command, Docker launches the default process defined in the image’s Dockerfile. If that process ends, the container exits:

docker run ubuntu
docker ps              # no running containers
docker ps -a           # shows the new container in exited state

Unlike virtual machines, containers are lightweight and designed to run a single task—such as a web server, database, or script. When that main process completes or fails, the container stops.

Examining Official Images

Popular Docker images set up their primary service using CMD or ENTRYPOINT. Let’s look at two examples:

Nginx Dockerfile Excerpt

# Install Nginx.
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

# Define mountable directories.
VOLUME ["/etc/nginx/sites-enabled", "/etc/nginx/certs"]

# Define working directory.
WORKDIR /etc/nginx

# Define default command.
CMD ["nginx"]

MySQL Dockerfile Excerpt

# Install server and dependencies
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

# Define entrypoint script.
ENTRYPOINT ["/entrypoint.sh"]

# Healthcheck.
HEALTHCHECK CMD /healthcheck.sh

EXPOSE 3306 33060

# Default command to start the server.
CMD ["mysqld"]

The Default Ubuntu Container Runs Bash

The official Ubuntu image uses bash as its default command. Without an interactive TTY, Bash exits immediately:

FROM ubuntu:14.04

RUN \
    sed -i 's/# \(.*multiverse$\)/\1/g' /etc/apt/sources.list && \
    apt-get update && \
    apt-get -y upgrade && \
    apt-get install -y build-essential software-properties-common \
                       byobu curl git htop man unzip vim wget && \
    rm -rf /var/lib/apt/lists/*

ADD root/.bashrc /root/.bashrc
ADD root/.gitconfig /root/.gitconfig
ADD root/.scripts /root/.scripts

ENV HOME /root
WORKDIR /root

CMD ["bash"]

Running docker run ubuntu without -t gives Bash no TTY, so it exits immediately—causing the container to stop.

Overriding CMD at Runtime

You can override the default CMD by appending your own command in docker run:

docker run ubuntu sleep 5

This runs sleep 5, keeps the container alive for 5 seconds, then exits.

Making the Change Permanent with CMD

To bake a default command into your image, declare a new CMD in your Dockerfile:

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

Build and run:

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

Shell Form vs Exec Form

CMD can use shell form:

CMD sleep 5

or exec form (JSON array):

CMD ["sleep", "5"]

With exec form, Docker does not invoke a shell, and the first element must be the executable.

ENTRYPOINT vs CMD

Use ENTRYPOINT to fix the executable but allow arguments to vary:

FROM ubuntu
ENTRYPOINT ["sleep"]

Then:

docker run ubuntu-sleeper 10    # runs: sleep 10

With only CMD, any arguments passed to docker run replace the entire command line.

Combining ENTRYPOINT and CMD

To specify both a fixed executable and default arguments:

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

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

Quick Comparison

Overriding ENTRYPOINT

You can also override ENTRYPOINT at runtime:

docker run --entrypoint sleep2.0 ubuntu-sleeper 10

This runs sleep2.0 10 instead of the original sleep.

Links and References

• Dockerfile reference
• docker run reference
• Docker Official Images