What is the difference between CMD and ENTRYPOINT in a Dockerfile?

Docker containers are built using Dockerfiles which contain step-by-step instructions to define the container environment. Among the various Dockerfile instructions, CMD and ENTRYPOINT are two critical commands that define what processes run inside containers. While they appear similar, they have distinct behaviors and use cases.

Before exploring their differences, it's important to understand the two forms of writing instructions in Dockerfiles:

Shell Form vs Executable Form

Shell Form

In shell form, the command is processed by a shell (/bin/sh -c). This allows shell features like variable expansion and command substitution.

INSTRUCTION command param1 param2

Executable Form

In executable form, the command runs directly without shell processing. This is the preferred form for CMD and ENTRYPOINT as it avoids signal handling issues.

INSTRUCTION ["executable", "param1", "param2"]

CMD Instruction

The CMD instruction sets default commands and parameters for the container. It executes only when no command is specified during docker run.

Key Characteristics:

• Provides default command for container startup

• Can be overridden by CLI arguments in docker run

• Only the last CMD instruction in a Dockerfile takes effect

CMD has three forms:

# Executable form (preferred)
CMD ["executable", "param1", "param2"]

# Shell form
CMD command param1 param2

# As default parameters for ENTRYPOINT
CMD ["param1", "param2"]

Example

FROM ubuntu:20.04
CMD ["echo", "Hello from CMD"]

$ docker run myimage
Hello from CMD

$ docker run myimage echo "Override CMD"
Override CMD

ENTRYPOINT Instruction

The ENTRYPOINT instruction sets the main command that will always execute when the container starts. Unlike CMD, it cannot be overridden by CLI arguments.

Key Characteristics:

• Always executes, cannot be overridden by CLI arguments

• CLI arguments are appended as parameters

• Can work with CMD to provide default parameters

ENTRYPOINT has two forms:

# Executable form (preferred)
ENTRYPOINT ["executable", "param1", "param2"]

# Shell form
ENTRYPOINT command param1 param2

Example

FROM ubuntu:20.04
ENTRYPOINT ["echo", "Hello from ENTRYPOINT"]

$ docker run myimage
Hello from ENTRYPOINT

$ docker run myimage "additional text"
Hello from ENTRYPOINT additional text

Comparison

CMD + ENTRYPOINT Combined

When both are used together, ENTRYPOINT defines the main command and CMD provides default parameters:

FROM ubuntu:20.04
ENTRYPOINT ["echo"]
CMD ["Hello World"]

$ docker run myimage
Hello World

$ docker run myimage "Custom Message"
Custom Message

Common Use Cases

Use CMD when:

• You want flexible container behavior

• Different commands may be needed at runtime

• Building general-purpose base images

Use ENTRYPOINT when:

• Container runs a specific application

• Consistent behavior is required

• CLI arguments should be treated as parameters

Conclusion

CMD provides default commands that can be overridden, making containers flexible. ENTRYPOINT ensures consistent execution of the main application, treating CLI arguments as parameters. Choose CMD for flexibility and ENTRYPOINT for consistency, or combine both for optimal control over container behavior.