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 HOME=/ PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin HOSTNAME=9c6f6cd077b2 FOO=bar
Multiple variables can be added with multiple -e switches.
# docker run -e FOO=bar -e BAR=foo busybox env HOME=/ PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin HOSTNAME=6cf2d6e8acb3 FOO=bar BAR=foo
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 #!/bin/sh if [ -z "$FOO" ]; then echo "FOO is empty" else echo "FOO is $FOO" fi
I can now build that Dockerfile.
# ls Dockerfile run.sh # docker build -t testenv . Uploading context 3.584 kB Uploading context Step 0 : FROM busybox ---> 4c0f792ebd1e Step 1 : ADD run.sh run.sh ---> 6154a355bdd6 Step 2 : RUN chmod +x run.sh ---> Running in 20784036cde1 ---> 784a1682769b Step 3 : CMD ./run.sh ---> Running in d86187fc3a6f ---> 1c952ed5cc6d Successfully built 1c952ed5cc6d Removing intermediate container c598898e1706 Removing intermediate container 20784036cde1 Removing intermediate container d86187fc3a6f
Now that the image has been built, I can run a container based off it, and the run.sh script should execute.
# docker run testenv FOO is empty
And now adding the FOO environment variable to the docker run command:
# docker run -e FOO=bar testenv FOO is bar # docker run -e FOO=$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.