How to Use ENTRYPOINT in a Dockerfile

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Use exec-form ENTRYPOINT for the executable that defines your container’s job, and use exec-form CMD for default arguments that users may replace. A typical image therefore looks like this:

FROM alpine:3.20
COPY entrypoint.sh /usr/local/bin/entrypoint.sh
RUN chmod +x /usr/local/bin/entrypoint.sh
ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
CMD ["--help"]

When no setup wrapper is needed, point ENTRYPOINT directly at the application:

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FROM python:3.13-slim
WORKDIR /app
COPY . .
ENTRYPOINT ["python", "-m", "myapp"]
CMD ["--config", "/etc/myapp/config.yml"]

This arrangement gives the image a stable executable while retaining a clean way to provide alternate arguments at runtime. It also normally makes the application process PID 1, which is important for receiving Docker’s stop signal and shutting down gracefully.

What ENTRYPOINT does

Docker’s ENTRYPOINT instruction configures a container to run as an executable. It expresses the image’s primary job: an image with an entrypoint behaves like the program named by that instruction rather than like a generic operating-system shell.

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Docker supports two syntaxes:

There must be at least one runnable CMD or ENTRYPOINT command in an image. In practice, an executable image commonly uses both instructions: ENTRYPOINT identifies the program, and CMD supplies its defaults.

Start with the executable role

Decide first what the container is supposed to be. If the image exists to run a Python module, make the interpreter and module the entrypoint. If the image needs initialization before launching the application, make an executable wrapper script the entrypoint and have that script hand control to the application.

Direct application entrypoint

For a Python module, split the executable and each argument into separate JSON elements:

ENTRYPOINT ["python", "-m", "myapp"]

Docker starts python directly, passes -m and myapp as arguments, and keeps the command’s argument boundaries unambiguous. This is different from placing the entire command in one element.

Wrapper-script entrypoint

A wrapper is useful when the image must perform setup such as preparing directories or validating environment state before starting the application:

FROM alpine:3.20
COPY entrypoint.sh /usr/local/bin/entrypoint.sh
RUN chmod +x /usr/local/bin/entrypoint.sh
ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
CMD ["--help"]

The RUN chmod +x line makes the copied script executable. Adapt the path and permission step to the selected base image, but keep the entrypoint in exec form unless shell interpretation is an intentional requirement.

How Docker combines ENTRYPOINT and CMD

Think of the final launch as two parts: a fixed executable portion and an argument portion. The exec-form ENTRYPOINT supplies the fixed portion. The exec-form CMD supplies argument defaults when the caller gives none. If the caller writes arguments after the image name, those runtime arguments replace the CMD array; they do not append to it.

For example:

ENTRYPOINT ["top", "-b"]
CMD ["-c"]

Running docker run image uses the entrypoint and its default argument, equivalent to top -b -c. Running docker run image -H keeps top -b but substitutes -H for -c. This is why options that users are expected to change belong in CMD, while the identity of the program belongs in ENTRYPOINT.

Do not expect a positional runtime argument to replace the executable. To replace the executable itself, use --entrypoint. Keeping these two override mechanisms separate makes an image predictable for both automated jobs and interactive troubleshooting.

Why an explicit CMD matters after adding ENTRYPOINT

If the base image supplied a CMD, adding an ENTRYPOINT in your Dockerfile can reset that inherited default. Define the desired CMD again in the current Dockerfile instead of assuming the parent image’s arguments will remain available.

Only the final ENTRYPOINT instruction has an effect. Multiple entrypoint lines do not create a sequence of programs; later instructions replace earlier ones. Keep one deliberate entrypoint in the final image and pair it with the command defaults you actually want.

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ENTRYPOINT and CMD: the intended pairing

The most useful mental model is that ENTRYPOINT is stable and CMD is replaceable. With an exec-form entrypoint, Docker appends arguments supplied after the image name to the entrypoint and uses them instead of the CMD defaults.

ENTRYPOINT ["top", "-b"]
CMD ["-c"]

Starting the image without extra arguments produces the equivalent of top -b -c. Supplying a different argument list replaces the default command arguments while preserving the entrypoint:

docker run --rm image -H

That invocation retains top -b and passes -H as the runtime argument. This pattern is appropriate when callers should be able to select configuration files, modes, or flags without changing the program the image runs.

Exec form versus shell form

The syntax choice changes argument handling, shell features, and process behavior.

When shell behavior is genuinely required

Exec form does not expand variables, connect pipelines, perform redirection, or process command chaining. If the application really needs those shell features, invoke the shell explicitly rather than assuming Docker will provide it:

ENTRYPOINT ["/bin/sh", "-c", "exec myapp "$@"", "--"]

Here the shell is an explicit part of the entrypoint. The exec in the command string replaces the shell with myapp after parsing, preserving the desired process behavior while retaining shell syntax. Use this pattern only when that parsing is required; otherwise, call the application directly in exec form.

PID 1 and graceful shutdown

Docker sends a container’s stop signal to its main process. With an exec-form entrypoint, the application normally becomes PID 1, so it can receive the signal and perform its own graceful shutdown.

A shell-form entrypoint can leave the shell as the main process. In that arrangement, the shell may receive the signal while the application continues running or fails to perform its normal cleanup. This is a common reason an application appears not to stop correctly.

Docker’s build check named JSONArgsRecommended recommends JSON-array CMD and ENTRYPOINT for this signal-handling reason. Treat a warning from that check as a prompt to examine whether a shell is being inserted unnecessarily.

The required wrapper-script hand-off

If setup code requires a script, finish the script by replacing itself with the final command:

#!/bin/sh
set -eu
# optional setup here
exec "$@"

The shell script remains responsible for the optional setup, but exec "$@" replaces the script process with the application. The application then occupies PID 1 instead of remaining a child hidden behind the wrapper.

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Writing a reliable entrypoint script

1. Choose a base image that contains the interpreter you need. A script beginning with #!/bin/sh requires a compatible shell in the image.
2. Copy the script into a known executable path. For example, copy it to /usr/local/bin/entrypoint.sh.
3. Set execute permission during the build. Use a RUN chmod +x step or an equivalent file-permission method supported by the build.
4. Perform only the setup the image needs. Keep the wrapper’s responsibilities clear so that runtime arguments can still be passed to the application.
5. End with exec "$@". This hand-off makes the final application the container’s main process.
6. Declare the script with exec-form ENTRYPOINT. Keep default application flags in a separate exec-form CMD.

A wrapper should not silently discard arguments. If the image is started with a replacement argument list, those values should reach the final application through the script’s positional parameters.

Keep setup and application arguments separate

Initialization belongs before the hand-off; application options belong after it. Avoid baking mutable flags into the wrapper when they are intended to be changed by callers. Put those defaults in CMD so a command such as docker run --rm myimage --config /tmp/test.yml can replace them without editing the image.

Likewise, do not add a second entrypoint merely to perform another setup phase. Combine the required setup in one script, then use one final exec. Since only the last ENTRYPOINT instruction is effective, multiple instructions cannot form a chain.

Overriding commands at runtime

Replace CMD defaults

Put ordinary arguments after the image name when you want a different argument list but the same executable:

docker run --rm myimage --config /tmp/test.yml

With an exec-form ENTRYPOINT, this replaces the image’s CMD defaults and passes --config /tmp/test.yml to the configured entrypoint.

Replace ENTRYPOINT itself

Use --entrypoint when you need to replace the configured executable:

docker run --rm --entrypoint /bin/sh -it myimage

This is useful for diagnosing startup failures, checking whether files were copied to the expected locations, or launching a one-off maintenance command. It bypasses the image’s normal entrypoint rather than merely changing its arguments.

Choosing the right override

Common ENTRYPOINT mistakes and fixes

Putting a whole command in one JSON element

This instruction asks Docker to find one executable whose name contains spaces:

ENTRYPOINT ["python -m myapp"]

Split the executable and its arguments into separate elements:

ENTRYPOINT ["python", "-m", "myapp"]

Using single quotes in exec form

Exec form is JSON array syntax, and JSON requires double quotes. Use "python", not 'python', for each array element. If you need shell quoting rules, invoke the shell deliberately rather than mixing shell notation into a JSON array.

Expecting shell expansion from exec form

An instruction such as ENTRYPOINT ["myapp", "$CONFIG"] does not automatically expand $CONFIG; no shell is involved. Pass the value through the application’s own configuration mechanism or invoke a shell explicitly when expansion is part of the requirement.

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Using shell form when arguments must be overridable

Shell-form ENTRYPOINT ignores CMD and positional arguments supplied to docker run. If users need to replace default flags, change the entrypoint to exec form and place defaults in CMD.

Leaving out exec in a wrapper

A wrapper that ends after launching the application remains PID 1 and may absorb stop signals. Finish it with exec "$@" so the application replaces the wrapper process.

Relying on an inherited CMD

Adding an entrypoint can clear a base image’s command defaults. Define an explicit CMD in your Dockerfile whenever the image depends on particular default arguments.

Defining several ENTRYPOINT instructions

Only the last entrypoint is effective. Remove obsolete lines or ensure the final instruction is the one that describes the image’s actual executable role.

Choosing a pattern for your image

Use a direct exec-form entrypoint when the image has one clear application and needs no initialization. This is the smallest and easiest-to-debug design: Docker starts the application directly, callers can replace its default arguments, and the application normally receives the stop signal as PID 1.

Use an exec-form script entrypoint when setup is genuinely required, such as preparing directories or validating runtime state. The script adds an implementation requirement—the interpreter must exist, the file must be executable, and the final line must hand off with exec—but it preserves the same runtime override model when it forwards "$@".

Use an explicit shell entrypoint only when pipes, redirection, expansion, or command chaining are part of the container’s intended behavior. This choice makes shell parsing visible, but it gives up the convenient CMD-and-positional-argument replacement behavior and can complicate signal delivery. A shell dependency should therefore be a conscious design decision rather than a shortcut for writing a command on one line.

A practical design checklist

• Is the image’s primary executable clearly identified?
• Is ENTRYPOINT written in JSON-array exec form?
• Are the executable and every argument separate array elements?
• Are default, user-replaceable flags in an exec-form CMD?
• Does the design intentionally require shell pipes, redirection, expansion, or chaining?
• If a shell is required, is it invoked explicitly?
• If a wrapper script is used, is it executable in the image?
• Does the wrapper end with exec "$@"?
• Has the Dockerfile restored an explicit CMD after adding ENTRYPOINT?
• Can you use ordinary runtime arguments to change defaults and --entrypoint to debug the executable itself?

Putting the patterns together

For a reusable application image, the clearest pattern is:

ENTRYPOINT ["application"]
CMD ["--default-flag"]

For an application that needs setup:

ENTRYPOINT ["/usr/local/bin/entrypoint.sh"]
CMD ["--default-flag"]

For a command that intentionally depends on shell parsing, make that dependency visible:

ENTRYPOINT ["/bin/sh", "-c", "exec myapp "$@"", "--"]

The first two patterns provide predictable argument replacement and direct signal delivery. The third trades some of that simplicity for explicit shell behavior, so it should be reserved for cases where shell syntax is actually needed.

Final guidance

Think of ENTRYPOINT as the image’s executable identity, not as a general-purpose place to write an entire shell command. Use exec form by default, keep replaceable defaults in CMD, and use positional docker run arguments when changing those defaults. When setup requires a script, make the script executable and end it with an exec hand-off. Reserve shell form for deliberate shell parsing, and use --entrypoint when debugging or replacing the executable itself.

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Frequently Asked Questions

What does ENTRYPOINT do in a Dockerfile?

It configures the executable a container runs. In exec form, Docker starts the named program directly, making it the image’s stable executable role.

What is the difference between ENTRYPOINT and CMD?

Use ENTRYPOINT for the executable identity and CMD for default arguments. Runtime arguments after the image name replace CMD defaults while retaining an exec-form ENTRYPOINT.

Should I use exec form or shell form?

Use exec form by default. It gives predictable argument boundaries and normally makes the application PID 1. Choose shell form only when shell parsing such as pipelines, redirection, chaining, or expansion is intentional.

Why does docker stop not stop my application cleanly?

A shell-form entrypoint or a wrapper without exec can remain PID 1 and receive Docker’s stop signal instead of the application. Use exec-form ENTRYPOINT, or end a required wrapper with exec "$@".

How do I override an ENTRYPOINT?

Pass --entrypoint to docker run, for example docker run --rm --entrypoint /bin/sh -it myimage. This replaces the executable rather than merely changing its arguments.

How do I change only the default arguments?

Put ordinary arguments after the image name, such as docker run --rm myimage --config /tmp/test.yml. With exec-form ENTRYPOINT, these replace CMD defaults.

Why does ENTRYPOINT ["python -m myapp"] fail?

The JSON array contains one executable name with spaces. Docker does not split it through a shell. Write ENTRYPOINT ["python", "-m", "myapp"] instead.

Why did my base image’s CMD disappear?

Defining ENTRYPOINT can reset an inherited CMD. Add the desired CMD explicitly in the current Dockerfile, and remember that only the last ENTRYPOINT instruction is effective.

Can exec-form ENTRYPOINT expand environment variables?

No. Exec form does not invoke a shell, so values such as $CONFIG are not expanded automatically. Use the application’s configuration handling or invoke a shell explicitly when expansion is required.

The Bottom Line

Use exec-form ENTRYPOINT for the program your image is meant to run, pair it with exec-form CMD for replaceable defaults, and use --entrypoint only when replacing the executable itself. If a wrapper is necessary, make it executable and finish with exec "$@" so the real application remains PID 1. Reserve shell form for deliberate shell parsing.

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