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Default Value for an ARG in a Dockerfile as an Empty String

In Dockerfiles, the ARG instruction lets us define variables that can be passed during the image build. However, setting an argument’s default value to an empty string (e.g., ARG MY_ARG=””) can lead to issues. Docker treats empty strings passed during the build (e.g., docker build –build-arg MY_ARG=””) as if the argument is unset, making the argument appear missing within the Dockerfile.

To avoid this, we use a non-empty default value like ARG MY_ARG=”default_value”. Alternatively, we can check for the argument’s existence in a script within the Dockerfile:

Dockerfile

RUN if [ -z "$MY_ARG" ]; then \
MY_ARG="default_value"; \
fi

Benefits

Setting a default value for an ARG in a Dockerfile to an empty string offers several benefits:

1. Allows us to specify the value during the build process if needed, providing flexibility for different build environments or configurations.

2. Keeps the Dockerfile simple and clean, especially when a default value is not required but the option to override is needed.

3. Ensures that the build process doesn’t fail due to a missing argument, as it defaults to an empty string if not specified.

4. Facilitates easy customization and testing without modifying the Dockerfile, simply by passing different arguments during the build.

5. Maintains consistent behavior by explicitly defining the argument, even if the default value is empty, making the build process predictable.

An Example

Here’s a basic example:

Dockerfile

# Set default value for ARG to an empty string
ARG MY_VAR=

# Build command
docker build -t my_image .

# Override default value
docker build --build-arg MY_VAR=new_value -t my_image .

# Explicitly set to an empty string
docker build --build-arg MY_VAR= -t my_image .

This way, MY_VAR is either set to the provided value or defaults to an empty string if not specified.

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Conclusion

This article explains about the default value for an ARG in a Dockerfile as an empty string.