The -e flag

Learn to pass environment variables with -e.

The -e flag

Environment variables are the way to run the same image with different settings in different environments (development, test, production). -e NAME=value gives the container a variable; several -e flags can be written:

docker run --rm -e AD=Ayşe -e DIL=tr alpine sh -c 'echo "$AD $DIL"'
# Ayşe tr

If no value is written (-e TOKEN), the variable is taken from the host variable of the same name.

Seeing the variables

All environment variables inside a container are seen with env. docker inspect shows them too; that is why passwords given with -e can be read by anyone with access to the container:

$ docker exec db env | grep POSTGRES
POSTGRES_PASSWORD=gizli
POSTGRES_DB=app
$ docker inspect -f '{{.Config.Env}}' db

In production, Docker secrets or Kubernetes Secrets are preferred for secret values.

Exercise A configured database

Start a container named db from postgres:16 in the background with three environment variables: POSTGRES_USER=app, POSTGRES_PASSWORD=gizli, POSTGRES_DB=magaza_db.

Test yourself

In docker run -e TOKEN img, where does TOKEN's value come from?
- From the TOKEN environment variable on the host
- It is always empty
- From the image
- It errors
Show answer

From the TOKEN environment variable on the host — This is one way to keep a password out of the command history.

Give the container an environment variable.

docker run --rm ____ DIL=tr alpine env

Show answer

docker run --rm -e DIL=tr alpine env

Match.

-e NAME=value

Assign a variable
-e AD

Take from the host
env

List the variables
docker inspect

Shows the settings

Next lesson: The ENV instruction

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