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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

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

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

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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.

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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

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5. ENTRYPOINT vs. CMD

InstructionPurposeRuntime Override
CMDSets a default command and argumentsFully replaceable
ENTRYPOINTConfigures the primary executableAppends runtime args
  • CMD: default command, easily swapped by arguments you supply.
  • ENTRYPOINT: fixed executable; any extra args in docker run are appended.

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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

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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

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