Environment variables with Docker

I've been working with Docker for a little while, and was wondering how environment variables work, so I took a few minutes to look into it.

Docker supports setting environment variables with the -e switch.

Below is the simplest example. As can be seen, the FOO variable is indeed set to bar within the container.

# docker run -e FOO=bar busybox env

Multiple variables can be added with multiple -e switches.

# docker run -e FOO=bar -e BAR=foo busybox env

Using environment variables in a script in a docker image

Below is a simple docker file that adds a script called run.sh to the image.

# file Dockerfile
FROM busybox
ADD run.sh run.sh
RUN chmod +x run.sh
CMD ./run.sh

This is what is in the run.sh script. If FOO is not set, the container should report "FOO is empty" and otherwise it should print what the variable has been set to.

# file run.sh

if [ -z "$FOO" ]; then
    echo "FOO is empty"
    echo "FOO is $FOO"

I can now build that Dockerfile.

# ls
Dockerfile  run.sh
# docker build -t testenv .
Uploading context 3.584 kB
Uploading context 
Step 0 : FROM busybox
 ---> 4c0f792ebd1e
Step 1 : ADD run.sh run.sh
 ---> 6154a355bdd6
Step 2 : RUN chmod +x run.sh
 ---> Running in 20784036cde1
 ---> 784a1682769b
Step 3 : CMD ./run.sh
 ---> Running in d86187fc3a6f
 ---> 1c952ed5cc6d
Successfully built 1c952ed5cc6d
Removing intermediate container c598898e1706
Removing intermediate container 20784036cde1
Removing intermediate container d86187fc3a6f

Now that the image has been built, I can run a container based off it, and the run.sh script should execute.

# docker run testenv
FOO is empty

And now adding the FOO environment variable to the docker run command:

# docker run -e FOO=bar testenv
FOO is bar
# docker run -e FOO=$RANDOM testenv
FOO is 10567
# docker run -e FOO=$RANDOM testenv
FOO is 8898

Awesome. Now with this ability, we could, for example, create one Wordpress container and have it configure wp-config.php with the correct MySQL connection variables, and have that container connect out to the MySQL server with the right user and password, rather than creating an image for each Wordpress site.

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