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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.

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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.
A container only runs as long as its main process is alive. Defining a long-running service or shell will keep it running.

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Examining Official Images

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

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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"]

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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"]

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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.

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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.

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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

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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.

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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.
ENTRYPOINT locks in your executable. Combine it with CMD to set default parameters.

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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
Always use the JSON array form for both ENTRYPOINT and CMD when combining them. This ensures proper argument handling.

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Quick Comparison

InstructionPurposeOverride Behavior
CMDDefault command or parametersReplaced by arguments on docker run
ENTRYPOINTFixed executable for the imageOverridable with --entrypoint flag

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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.

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