Docker

Docker- Difference between ARG and ENV

Posted Nidhi Gupta

In this blog, we will explore what is the Difference between ARG and ENV and the use of both the instructions.

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Agenda:

  1. How to pass CMD line arguments to Dockerfile ?
  2. How to pass CMD line arguments to docker-compose.yml file ?

What is ARG ?

  • ARG instruction defines a variable that can be used to build a Docker image.
  • ARG values are not available after the image is built.
  • In the running container you can’t access the ARG variables.
  • Once ARG variable is defined in the Dockerfile, you can always override the values by passing command line argument as shown below
  • If you need to pass two variables using CMD line then you need to use --build-arg two times
docker build -t test --build-arg VARIABLE_1=7 --build-arg VAR_2=8 .
  • We can have multiple ARG instruction in the Dockerfile. 
  • ARG is the only instruction that can precede the FROM instruction in the Dockerfile.

Example of using ARG argument in the Dockerfile

FROM alpine:3.7
ARG VARIABLE_1=5
RUN echo $VARIABLE_1

Run the Dockerfile

docker build .

Output: We will see the value which is set in the Dockerfile

Sending build context to Docker daemon 2.048 kB
Step 1/3 : FROM alpine:3.7
 ---> 6d1ef012b567
Step 2/3 : ARG VARIABLE_1=5
 ---> Using cache
 ---> 02b0388547b1
Step 3/3 : RUN echo "print variable value:" $VARIABLE_1
 ---> Running in 77671e6ff726
print variable value: 5
---> 59e00ed8f143
Removing intermediate container 77671e6ff726
Successfully built 59e00ed8f143

Run the Dockerfile by passing arguments

docker build --build-arg VARIABLE_1=7 .

Output: We can see the value is override with the command line argument

Sending build context to Docker daemon 2.048 kB
Step 1/3 : FROM alpine:3.7
 ---> 6d1ef012b567
Step 2/3 : ARG VARIABLE_1=5
 ---> Using cache
 ---> 02b0388547b1
Step 3/3 : RUN echo "print variable value:" $VARIABLE_1
 ---> Running in ecc5f5891b37
print variable value: 7
---> 1a44ebca2814
Removing intermediate container ecc5f5891b37
Successfully built 1a44ebca2814

What is ENV?

  • ENV is to provide default values for your future environment variables inside the container
  • We can’t change the ENV variable using command line argument directly
  • If we need to change the ENV variable using CMD line then we have to use ARG and place ARG variable in ENV variable
  • In the below Dockerfile, we have created two variables , one as ARG and one as ENV
  • Using ARG Value in ENV Variable
FROM alpine:3.7
ARG VARIABLE_1=5
ENV VARIABLE_2=$VARIABLE_1
RUN echo "print variable value:" $VARIABLE_1
RUN echo " print ENV variable : " $VARIABLE_2

Build the docker image directly

  • You will both variable having same values as defined for ARG variable
docker build .
Sending build context to Docker daemon 2.048 kB
Step 1/5 : FROM alpine:3.7
---> 6d1ef012b567
Step 2/5 : ARG VARIABLE_1=5
---> Using cache
---> 02b0388547b1
Step 3/5 : ENV VARIABLE_2 $VARIABLE_1
Step 4/5 : RUN echo "print variable value:" $VARIABLE_1
---> Running in 9d650a1bc85d
print variable value: 5
Step 5/5 : RUN echo " print ENV variable : " $VARIABLE_2
print ENV variable : 5
Successfully built 2a3bf7db012e

Build the docker image using CMD line argument

  • --build-arg VARIABLE_1=7 
  • Output will show 7 for both the variables
docker build -t test --build-arg VARIABLE_1=7 .
Sending build context to Docker daemon 2.048 kB
Step 1/5 : FROM alpine:3.7
---> 6d1ef012b567
Step 2/5 : ARG VARIABLE_1=5
---> Using cache
---> 02b0388547b1
Step 3/5 : ENV VARIABLE_2 $VARIABLE_1
---> Using cache
---> 663cb87b7a1a
Step 4/5 : RUN echo "print variable value:" $VARIABLE_1
---> Running in fcb9e45e94ce


print variable value: 7
---> 6c74a45a9687
Removing intermediate container fcb9e45e94ce
Step 5/5 : RUN echo " print ENV variable : " $VARIABLE_2
---> Running in c90ef900ada2


print ENV variable : 7
---> 1b35e794624d
Removing intermediate container c90ef900ada2
Successfully built 1b35e794624d

Build image by using ENV variable as CMD line argument

  • Build will fail and you will see the error “[Warning] One or more build-args [VARIABLE_2] were not consumed
docker build -t test --build-arg VARIABLE_2=7 .
Sending build context to Docker daemon 2.048 kB
Step 1/5 : FROM alpine:3.7
---> 6d1ef012b567
Step 2/5 : ARG VARIABLE_1=5
---> Using cache
---> 02b0388547b1
Step 3/5 : ENV VARIABLE_2 $VARIABLE_1
---> Using cache
---> 243b8cb9e272
Step 4/5 : RUN echo "print variable value:" $VARIABLE_1
---> Using cache
---> e797761367a0
Step 5/5 : RUN echo " print ENV variable : " $VARIABLE_2
---> Using cache
---> 2a3bf7db012e
[Warning] One or more build-args [VARIABLE_2] were not consumed

Run the container using this image

  • We are using -it for the interactive terminal session
  • We will try to echo the variables which we were using in our Dockerfile
  • As mentioned already, ARG argument is not available inside the Docker containers and ENV argument is accessible inside the container
$ docker run -it test
/ # echo $VARIABLE_1
/ # echo $VARIABLE_2
5

How to pass parameters to docker-compose file

  • In docker-compose file you use the mariadb and don’t want to hardcode the version directly in the file
  • Below is the sample code how you use the variable MARIADB_VERSION
version: '2'
services:
  mariadb:
    image: 'bitnami/mariadb:${MARIADB_VERSION}'

How to pass the value of the variable in this docker-compose file

  • While running the command manually you can pass the parameter as shown below
MARIADB_VERSION=1.2 docker-compose up -d
  • In automation, you can declare your variable in your property file and use the variable name as shown below:
MARIADB_VERSION=$(MARIADB_VERSION) docker-compose up -d

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