Learn what to do if Dockerfile ARG Default Value is not working. Our Docker Support team is here to help you with your questions and concerns.

Why Your Dockerfile ARG Default Value is Not Working

When working with Dockerfiles, especially with build arguments (ARG), it’s easy to run into issues.

In fact, many of our customers have had issues with the ARG default value not working error.

Our Experts have compiled this guide to help you through this error and solutions.

Multi-Stage Builds

In Dockerfiles that use multiple `FROM` stages, arguments defined in earlier stages are not visible to later stages by default. This happens because each stage creates a new execution context.

Fix: So, redefine the argument where it’s needed in each stage. If the argument is required throughout the build, define it in the first `FROM` stage and ensure it’s redefined in subsequent stages.

For example:

# Stage 1
FROM base AS stage1
ARG ARG_NAME
# Use $ARG_NAME
# Stage 2
FROM base AS stage2
ARG ARG_NAME
# Use $ARG_NAME again


Empty Strings as Defaults

Setting an empty string (`””`) as the default value for an argument can be problematic. If an empty string is passed during the build (`docker build -t myimage –build-arg ARG_NAME=”” .`), Docker treats it as an unset argument.

Fix: So, use a non-empty default value if we want to apply it when the argument is not explicitly set.

For example:

ARG ARG_NAME=default_value
RUN if [ -z "$ARG_NAME" ]; then \
ARG_NAME="default_value"; \
fi
# Use $ARG_NAME here


Shell Variable Expansion

When using arguments within shell commands, issues with variable expansion can occur if the variable is not properly quoted.

Fix: In this case, we have to make sure we are using double quotes (`”`) around the variable name to prevent unintended word splitting or interpretation by the shell.

ARG ARG_NAME
RUN echo "The value of the argument is: $ARG_NAME"


Variable Scoping

Arguments in a Dockerfile are environment variables within the build context.

Fix: So, make sure to access the variable within the intended scope (e.g., within the same `RUN` instruction or subsequent instructions).

Handling Default Values in Docker ARG

Using a default value for an `ARG` in a Dockerfile does not guarantee that the value will be used if no value is provided during the build process. Docker treats the variable as undefined if no value is provided.

For example:

# Set default value for ARG
ARG MY_VAR=default_value
# Use the ARG in a command
RUN echo "My variable's value is: $MY_VAR"


If we build this Dockerfile without providing a value for `MY_VAR`, Docker will consider it undefined, and it won’t use the default value specified in the Dockerfile.

Fix: We have to make sure a default value is used by explicitly setting the `ARG` value again before we use it.

For example:

# Set default value for ARG
ARG MY_VAR=default_value
# Set ARG value to default if not provided
ARG MY_VAR=${MY_VAR}
# Use the ARG in a command
RUN echo "My variable's value is: $MY_VAR"


With this setup, if we don’t provide a value for `MY_VAR` during the build, Docker will use the default value specified in the Dockerfile.

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Conclusion

In brief, our Support Experts demonstrated how to fix Command Failed with Error 18 (Authentication Failed)