Using if-else in Dockerfile

Docker is an essential tool for containerizing applications, making them portable and isolated. The Dockerfile is a key component in the Docker ecosystem, allowing developers to specify how their application should be containerized. Often, there is a need for conditional logic in Dockerfiles, much like you would find in programming scripts. However, Dockerfile syntax does not directly support if-else conditions.

In this article, we’ll explore how you can effectively use conditional logic in Dockerfiles, especially with external arguments.

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1. The Basics: ARG and ENV

Before diving into the conditional logic, let’s cover some basics.

• ARG: This instruction defines a variable that users can pass at build-time to the builder with the docker build command. The value of the ARG can be accessed during the Docker build, but not after the image has been built.
• ENV: This instruction sets a persistent environment variable for the image. It can be accessed during the build and when a container is run from the built image.

2. Using Shell Commands for Conditional Logic

The primary way to achieve conditional logic in a Dockerfile is by executing shell commands. The RUN instruction can be used with typical shell commands to achieve this.

Let’s see a practical example using an external argument.

Scenario:

We want to install either nginx or apache2 based on an external build argument.

Dockerfile:

# Use an Ubuntu base image
FROM ubuntu:20.04

# Avoid prompts with apt
ENV DEBIAN_FRONTEND=noninteractive

# Declare an argument for the web server
ARG WEBSERVER

# Use shell logic to determine which server to install
RUN if [ "$WEBSERVER" = "nginx" ]; then \
        apt-get update && apt-get install -y nginx; \
    elif [ "$WEBSERVER" = "apache" ]; then \
        apt-get update && apt-get install -y apache2; \
    else \
        echo "No valid webserver specified"; \
    fi

# Rest of the Dockerfile...

Building the Docker image:

To build the image and specify the web server:

docker build --build-arg WEBSERVER=nginx -t myimage:latest . 

This will build the Docker image with nginx installed. To install apache2, you’d change the WEBSERVER argument value accordingly.

3. Limitations and Considerations

• Readability: While using shell commands in the RUN instruction provides flexibility, it can decrease the readability of your Dockerfile if overused or if the logic gets too complex.
• Layers: Each RUN command creates a new layer in the Docker image. This can increase the size of the image. To mitigate this, try to combine commands where possible using &&.
• Default Values: If you’re expecting an ARG, it’s a good practice to provide a default value to avoid any unexpected behavior. For instance: ARG WEBSERVER=nginx.

Conclusion

Though Dockerfiles don’t support native if-else structures, we can cleverly use shell scripting within the RUN instruction to implement conditional logic. This approach, when used judiciously, can provide great flexibility in building Docker images tailored to different needs or environments. Always remember to keep the Dockerfile readable and to be mindful of the number of layers you’re adding.