DockerHow-ToBeginner · 3 min read

How to Use docker run -e to Set Environment Variables

Use docker run -e VARIABLE=value to set environment variables inside a Docker container at runtime. This lets you customize container behavior without changing the image. You can specify multiple -e options to set several variables.

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Syntax

The docker run -e option sets environment variables inside the container. You write it as -e VARIABLE=value. You can add many -e options to set multiple variables.

docker run: Command to start a container.
-e: Flag to set an environment variable.
VARIABLE=value: The name and value of the environment variable.
image: The Docker image to run.

bash

docker run -e VARIABLE=value image

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Example

This example runs an alpine container and sets the environment variable GREETING to Hello. The container prints the value of GREETING using echo.

bash

docker run --rm -e GREETING=Hello alpine sh -c 'echo $GREETING'

Output

Hello

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Common Pitfalls

Common mistakes include:

Forgetting to quote values with spaces or special characters.
Using = without a value, which sets an empty variable.
Trying to set variables without -e, which won't work.

Example of wrong and right usage:

bash

docker run --rm -e GREETING=Hello World alpine sh -c 'echo $GREETING'
# Wrong: 'World' is treated as a command, not part of the variable

docker run --rm -e "GREETING=Hello World" alpine sh -c 'echo "$GREETING"'
# Right: Quotes keep the value together

Output

Hello Hello World

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Quick Reference

Option	Description	Example
-e VARIABLE=value	Set environment variable inside container	-e GREETING=Hello
--env-file file	Set multiple variables from a file	--env-file env.list
--rm	Remove container after exit	--rm
image	Docker image to run	alpine

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Key Takeaways

Use -e VARIABLE=value to pass environment variables to Docker containers.

Quote values with spaces or special characters to avoid errors.

You can specify multiple -e options to set many variables.

Use --rm to clean up containers after they exit.

Environment variables customize container behavior without changing the image.