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About Hariharan Narayanan

Hariharan Narayanan
Hari graduated from the School of Computer and Information Sciences in the University of Hyderabad. Over his career he has been involved in many complex projects in mobile applications, enterprise applications, distributed applications, micro-services, and other platforms and frameworks. He works as a consultant and is mainly involved with projects based on Java, C++ and Big Data technologies.

Docker Environment Variables Example

1. Introduction

In this post, we will discuss different ways to pass and set environment variables into a Docker container. Typically, when a new container is created Docker assigns it a few environment variables by default like so:

$ docker run alpine:latest env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=de2c993a62fb
HOME=/root

Let us now explore what options do we have to inject our custom environment variables into a Docker container.

2. Set environment variables within the container

Lets first look at the simplest way possible to set environment variables – open a terminal into the container and set environmnt an variable.
For this lets first create a very simple container based on alpine:linux and one that spawns an infinite-loop.

$ vi Dockerfile
FROM alpine:latest
MAINTAINER Hariharan Narayanan 
COPY ./infinite-loop.sh /
CMD ["sh", "/infinite-loop.sh"]
$
$ vi infinite_loop.sh
while (true); do echo hi; done
$
$ docker build --label=infinite_loop --tag=infinite_loop_1:latest .
Sending build context to Docker daemon 3.072 kB
Step 1 : FROM alpine:latest
 ---> baa5d63471ea
Step 2 : MAINTAINER Hariharan Narayanan 
 ---> Running in 52b7476a78de
 ---> 430c5ef6db0b
Removing intermediate container 52b7476a78de
Step 3 : COPY ./infinite-loop.sh /
 ---> 3ad24e361423
Removing intermediate container 25905f482a3d
Step 4 : CMD sh /infinite-loop.sh
 ---> Running in 1a7cf984561b
 ---> 2e386a31727b
Removing intermediate container 1a7cf984561b
Step 5 : LABEL "infinite_loop" ""
 ---> Running in 37ba69ab8515
 ---> 6f683b1c8437
Removing intermediate container 37ba69ab8515
Successfully built 6f683b1c8437
$
$ docker run --detach infinite_loop_1:latest 
59d4009b9cd1ec274d03799f9913622599fcbdf76eaef32c5f545088bb84c544

Now, open a shell into the container and set an environment variable called example_env_var and set to value xyz.

$ docker exec -it 59d4009b9cd1ec274d03799f9913622599fcbdf76eaef32c5f545088bb84c544 /bin/sh
/ # export example_env_var=xyz
/ # env
HOSTNAME=59d4009b9cd1
SHLVL=1
HOME=/root
example_env_var=xyz
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/

The command env verifies that the environment variable example_env_var is set to value xyz.

Now open another session into the same container and check the environment variables set again.

$ docker exec -it 59d4009b9cd1ec274d03799f9913622599fcbdf76eaef32c5f545088bb84c544 /bin/sh
/ # env
HOSTNAME=59d4009b9cd1
SHLVL=1
HOME=/root
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
PWD=/
/ # 

As can be seen, the new session does not have the environment variable example_env_var set. Lets explore if we can set environment variables in such a way that they are visible in all containers derived from this Docker image.

3. Set environment variables from the Docker CLI

The environment variable example_env_var=xyz was set above within a container session. This setting was not persistent across container sessions however. Docker engine CLI provides the --env option in docker run command to set environment variables. Lets try a few examples now.

3.1 Set a single environment variable

The environment variable example_env_var=xyz can be set in the docker run command using the --env option like so.

$ docker run --detach --env example_env_var=xyz infinite_loop_1:latest 
140311b1ab884142bbf055c16899b25b613d5810f99dd712710eb0da818b3c5b
$ docker exec -it 140311b1ab884142bbf055c16899b25b613d5810f99dd712710eb0da818b3c5b env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=140311b1ab88
example_env_var=xyz
HOME=/root

Now let’s check from another terminal if the environment variable is still set

$ docker exec -it 140311b1ab884142bbf055c16899b25b613d5810f99dd712710eb0da818b3c5b env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=140311b1ab88
example_env_var=xyz
HOME=/root

So this time the environment variable is set across sessions. Containers and images deriving from this image also will get these variables.

3.2 Set multiple environment variables

Multiple environment variables can be set by using multiple --env options.

$ docker run --detach --env example_env_var=xyz --env example_env_var_2=abc --env example_env_var_3=1 --env example_env_var_4=true infinite_loop_1:latest 
$
$ docker exec -it be8120b6a7eeb90a0966f6d1b4bb7c07583ddd1dd2618033f9173da916db6e4f env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=be8120b6a7ee
example_env_var=xyz
example_env_var_2=abc
example_env_var_3=1
example_env_var_4=true
HOME=/root

This can become cumbersome when there are many environment variables to be set. This can be eased out by moving environment variables into a separate file, like this…

$ vi settings.env
example_env_var=xyz
example_env_var_2=abc
example_env_var_3=1
example_env_var_4=true
$
$ docker run --detach --env-file=settings.env infinite_loop_1:latest 
10ede841b2d3594a6aa5d12efe33144e8d6ee5b9ad10763dfaa97991c63a3162
$
$ docker exec -it 10ede841b2d3594a6aa5d12efe33144e8d6ee5b9ad10763dfaa97991c63a3162 env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=10ede841b2d3
example_env_var=xyz
example_env_var_2=abc
example_env_var_3=1
example_env_var_4=true
HOME=/root

We saw how to set environment variables from the docker run command by using the --env and --enf-file options. Let us try doing the same without using these options but using instead the options provided in the Dockerfile itself.

4. Set environment variables from Dockerfile

Docker engine provides the instruction ENV to set environment variables in the Dockerfile.
Lets redo the actions done above and introduce the same environment variables in the Dockerfile using the ENV ionstruction like so.

$ vi Dockerfile
FROM alpine:latest
MAINTAINER Hariharan Narayanan 

ENV example_env_var=xyz
ENV example_env_var_2=abc
ENV example_env_var_3=1
ENV example_env_var_4=true

COPY ./infinite-loop.sh /

CMD ["sh", "/infinite-loop.sh"]
$ 
$ docker build --label=infinite_loop --tag=infinite_loop_3:latest .
Sending build context to Docker daemon 4.096 kB
Step 1 : FROM alpine:latest
 ---> baa5d63471ea
Step 2 : MAINTAINER Hariharan Narayanan 
 ---> Using cache
 ---> 430c5ef6db0b
Step 3 : ENV example_env_var xyz
 ---> Running in de2ddabf92ed
 ---> 679eede67284
Removing intermediate container de2ddabf92ed
Step 4 : ENV example_env_var_2 abc
 ---> Running in f9bda788b180
 ---> 1e6a619df135
Removing intermediate container f9bda788b180
Step 5 : ENV example_env_var_3 1
 ---> Running in 6b90bf849305
 ---> 9ccfd0dff3dc
Removing intermediate container 6b90bf849305
Step 6 : ENV example_env_var_4 true
 ---> Running in b22534b4619b
 ---> 02ba8f83be20
Removing intermediate container b22534b4619b
Step 7 : COPY ./infinite-loop.sh /
 ---> 42d3f60549f0
Removing intermediate container 7cd3d47df6c4
Step 8 : CMD sh /infinite-loop.sh
 ---> Running in c2991b057887
 ---> cdd321499fd2
Removing intermediate container c2991b057887
Step 9 : LABEL "infinite_loop" ""
 ---> Running in d643b9354e8c
 ---> 87b2ae39b41e
Removing intermediate container d643b9354e8c
Successfully built 87b2ae39b41e
$
$ docker exec -it 09082da1a35470155d95b49125394a5505c6c1112d3a4b12f57dcbdbff93d057 env
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
HOSTNAME=09082da1a354
example_env_var=xyz
example_env_var_2=abc
example_env_var_3=1
example_env_var_4=true
HOME=/root

Multiple ENV values can also be combined into a single instruction like below. This saves multiple layers in the resulting Docker image.

$ vi Dockerfile
FROM alpine:latest
MAINTAINER Hariharan Narayanan 

ENV example_env_var=xyz \
    example_env_var_2=abc \
    example_env_var_3=1 \
    example_env_var_4=true

COPY ./infinite-loop.sh /

CMD ["sh", "/infinite-loop.sh"]

5. Summary

In this post, we saw different forms of injecting environment variables into Docker containers. Docker provides the --env option to inject environment variables through the docker run command. Multiple environment variables are injected through multiple --env options one for each variable. Environment variables can also be defined in a separate file and injected through the docker run --env-file call. In a Dockerfile, the ENV instruction is used to provide multiple environment variables. Each ENV call creates a new layer in the Docker image and so multiple environment variable definitions can be combined into one ENV call too.

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