What is the Difference Between Docker ENV and RUN Export?

Understanding the Difference Between Docker ENV and RUN Export

We need to know the difference between Docker ENV and RUN export. This is very important for managing environment variables in our Docker containers. Docker ENV helps us set environment variables that stay in the container. On the other hand, RUN export sets temporary environment variables while we build the container. This difference is very important for us as developers. It helps us make our Dockerfiles better and clearer.

In this article, we will look at the differences between Docker ENV and RUN export. We will see how to use each one and when to use them. Here are the topics we will cover:

  • What is the Difference Between Docker ENV and RUN Export
  • Understanding Docker ENV Command and Its Usage
  • Exploring the RUN Export Command in Docker
  • Key Differences Between Docker ENV and RUN Export
  • When to Use Docker ENV Over RUN Export
  • Practical Examples of Docker ENV and RUN Export
  • Frequently Asked Questions

At the end of this article, we will understand how to use these commands. This will help us manage environment variables well in our Docker containers.

Understanding Docker ENV Command and Its Usage

The ENV command in Docker helps us set environment variables inside a Docker container. We can access these variables by applications running in the container. This allows us to change settings and customize how our application works.

Syntax

ENV <key>=<value> ...

Usage

  • Defining Environment Variables: We can set variables that affect how applications behave inside the container.
  • Persistent Across Layers: The environment variables we set with ENV are available for all instructions after it in the Dockerfile and while the container runs.

Example

Here is an example of using the ENV command in a Dockerfile:

FROM ubuntu:latest

# Set environment variables
ENV APP_HOME /usr/src/app
ENV NODE_ENV production

# Create application directory
RUN mkdir -p $APP_HOME

WORKDIR $APP_HOME

# Copy application files
COPY . .

# Install dependencies
RUN npm install

# Command to run the application
CMD ["npm", "start"]

In this example: - We set APP_HOME to /usr/src/app. We use this later to create the application directory. - We set NODE_ENV to production. This affects how the Node.js application runs.

Benefits

  • Flexibility: We can easily change settings without changing the application code.
  • Security: We can keep sensitive data like API keys and secrets safe with environment variables.
  • Portability: Our applications can run in different environments like development, staging, and production with little changes.

Using the ENV command well can help us improve the settings and security of Docker containers. For more information on Docker, we can look at articles like What Are the Benefits of Using Docker in Development.

Exploring the RUN Export Command in Docker

The RUN command in Docker helps us to run commands when we build an image in a Dockerfile. We can use this command to install packages, change files, or prepare the environment for our application.

The RUN export command is not a separate command in Docker. It helps us to set environment variables for a short time during the build. When we use RUN to run commands, any environment variables we set with export will only work for that specific layer of the image we are building.

Example of Using RUN with Export

FROM ubuntu:20.04

# Update the package repository and install curl
RUN apt-get update && apt-get install -y curl

# Set a temporary environment variable
RUN export MY_VAR="Hello World" && echo $MY_VAR

# Verify that MY_VAR is not available in the next RUN command
RUN echo $MY_VAR # This will not output "Hello World"

In this example: - The first RUN command installs curl. - The second RUN command sets the environment variable MY_VAR and shows it. - The third RUN command cannot access MY_VAR. This shows that the variable does not last across layers.

If we want to set an environment variable that lasts in all layers of our Docker image, we should use the ENV command instead.

Differences Between RUN and Export

  • Scope: RUN runs a command. export sets environment variables but only for that command.
  • Persistence: Environment variables from export do not last in the next layers unless we use ENV.

Using RUN with export is good for temporary settings or configurations when we build the image. But if we need variables in the final image, we should use the ENV command.

For more information on Docker commands, check out What is the Dockerfile and How Do You Create One.

Key Differences Between Docker ENV and RUN Export

Docker ENV and RUN Export are two commands we use in Dockerfiles. They have different purposes when we build Docker images. Knowing these differences is important for building and managing Docker images well.

  1. Purpose:
    • ENV: This command sets environment variables that stay in the container when it runs. We can use these variables in the application running inside the container.
    • RUN: This command runs a command while we build the image. The result of this command goes into the image layer, but it does not set environment variables that last for the container runtime.
  2. Syntax:

    • ENV:

      ENV <key> <value>

    • RUN:

      RUN <command>
  3. Scope:
    • ENV: The variables we set with ENV are available in all the later layers of the image. Any process running in the container can access these variables.
    • RUN: The command runs and we can see the output. But any environment variables set in the command do not stay in the image.
  4. Example:

    • Using ENV to set a variable:

      FROM ubuntu:latest
ENV APP_HOME /usr/src/app
WORKDIR $APP_HOME

    • Using RUN to install a package:

      RUN apt-get update && apt-get install -y curl
  5. Use Cases:
    • ENV: It is good for setting configuration details or paths that our application needs when it runs.
    • RUN: We use this for installing packages, running scripts, or doing build actions that change the image.
  6. Layering:
    • ENV makes a new layer with the environment variable set. This can add to the image size.
    • RUN also makes a new layer. But it can make the image bigger if we do not manage it well, like using && to connect commands.
  7. Persistence:
    • ENV values stay when we run the container from the image.
    • RUN commands do not keep runtime variables unless we export them clearly.

Knowing these key differences between Docker ENV and RUN Export helps us optimize our Dockerfiles for better performance and easier maintenance. For more tips on Docker best practices, we can read What are the Benefits of Using Docker in Development.

When to Use Docker ENV Over RUN Export

Choosing between Docker ENV and RUN export depend on what we need and how long we want the environment variables to last.

Docker ENV: - Scope: When we use ENV in a Dockerfile, the environment variables stay available for all commands that come after it. They also stay in the running container. - Use Case: - We use it when we want to set default values. These values can be changed at runtime with the -e flag or --env-file. - It is good for configuration that our app needs while it runs.

Example:

FROM alpine
ENV APP_ENV=production
CMD ["echo", "$APP_ENV"]

RUN export: - Scope: The variables we export in a RUN command only work while that command runs. They do not stay in the final image. - Use Case: - We use it for temporary variables. These are for a specific command or during building. They do not need to last when we run the container.

Example:

FROM alpine
RUN export TEMP_VAR=temporary && echo "$TEMP_VAR"
CMD ["echo", "$TEMP_VAR"]  # This will not print TEMP_VAR

In short, we use ENV for settings that need to stay throughout the container’s life. We use RUN export for temporary settings just for the build. Knowing this helps us make Docker images that work well and are easy to manage. For more about Docker environment variables, check this Docker tutorial.

Practical Examples of Docker ENV and RUN Export

Example of Docker ENV

We use the ENV instruction in a Dockerfile to set environment variables. These variables are available to the running container. Here is a simple example:

# Dockerfile
FROM ubuntu:latest

# Set environment variables
ENV APP_HOME /usr/src/app
ENV NODE_ENV production

# Create app directory
RUN mkdir -p $APP_HOME

# Set the working directory
WORKDIR $APP_HOME

# Copy files
COPY . .

# Install dependencies
RUN npm install

# Start the application
CMD ["npm", "start"]

In this example, we set APP_HOME and NODE_ENV as environment variables. We can access them inside the container.

Example of RUN Export

We use the RUN instruction to execute commands in the Dockerfile. It can set environment variables but only for the build phase. For example:

# Dockerfile
FROM ubuntu:latest

# Install dependencies and set environment variable
RUN apt-get update && apt-get install -y curl \
    && export MY_VAR="Hello World" \
    && echo $MY_VAR

# Check the variable
RUN echo $MY_VAR # This will not output "Hello World"

In this case, MY_VAR is only available when we run the RUN command. It will not stay in the final image.

Multi-stage Example

We can use both ENV and RUN together in a multi-stage build:

# First stage
FROM node:14 AS builder

WORKDIR /app
COPY package.json ./
RUN npm install

# Second stage
FROM node:14

# Set environment variable
ENV NODE_ENV production

WORKDIR /app
COPY --from=builder /app/node_modules ./node_modules
COPY . .

CMD ["node", "server.js"]

In this example, we set NODE_ENV for the final image. The dependencies are installed in a separate build stage.

These examples show how we can use ENV for permanent environment variables. We use RUN for temporary variables during the image build. For more information on Docker commands, we can check What is Docker and Why Should You Use It.

Frequently Asked Questions

1. What is the purpose of the Docker ENV command?

The Docker ENV command sets environment variables in a Docker image. We can use these variables in applications that run inside the container. This helps us change settings without changing the code. With Docker ENV, our applications can have the right settings and credentials. This makes it easier to manage different environments like development, testing, and production.

2. How does the RUN export command differ from Docker ENV?

The RUN export command is used in a Dockerfile. It sets environment variables only for the build process. Unlike Docker ENV, the variables set with RUN export do not stay in the final image. They are only available during the build stage. So, we need to pick the right command based on if we want the variable to stay for the container’s runtime or just for the build.

3. Can I access environment variables set with Docker ENV in my application?

Yes, we can access environment variables set with Docker ENV in our application running in the container. This is very important for apps that need settings like database connection strings or API keys. Using Docker ENV helps us keep private information out of our code while giving our application the right context to work well.

4. When should I use RUN export instead of Docker ENV?

Use RUN export when we need to set variables just for a short time during the Docker image build. This is especially for installation commands or scripts. If we want the environment variables to be available when the container runs, we should use Docker ENV. Knowing how long we need the variables will help us choose the right command.

5. Are there any limitations to using Docker ENV for sensitive information?

Docker ENV makes it easy to set environment variables. But we should know that anyone with access to the Docker image or container can see these variables. For sensitive information like API keys or passwords, it is better to use Docker Secrets or Docker Compose with securely stored environment variables. This way, we can improve security while still using Docker for application settings.

For more information about Docker, check out What is Docker and Why Should You Use It? and What are the Benefits of Using Docker in Development?.