what does docker export dir mean - docker

Question

i am following this tutorial to run mssql in a docker.First the user pulls the image
docker pull microsoft/mssql-server-linux
second he does below
export DIR=/var/lib/mssql
sudo mkdir $DIR
finally he runs
docker run \
-d \
--name mssql \
-e 'ACCEPT_EULA=Y' \
-e 'SA_PASSWORD=' \
-p 1433:1433 \
-v $DIR:/var/opt/mssql \
microsoft/mssql-server-linux
Author explains second step as below
Create a directory on the host that will store data from the container and keep the value in an environment variable for convenience:
ask:
what does the author meant by that and what happens if we dont create the directory
I tried searching for different terms like below
docker container default path
docker file system
but not able to understand.Can some one shed some light on this

Answer 1

So here is thing. Consider below code
export DIR=/var/lib/mssql
sudo mkdir $DIR
I can rewrite it as
sudo mkdir /var/lib/mssql
But I will also have to change my RUN command to
docker run \
-d \
--name mssql \
-e 'ACCEPT_EULA=Y' \
-e 'SA_PASSWORD=' \
-p 1433:1433 \
-v /var/lib/mysql:/var/opt/mssql \
microsoft/mssql-server-linux
Now if you change the directory, then you you will have to update two places. Thats why DIR was used.
If you remove below from your docker run
-v /var/lib/mysql:/var/opt/mssql \
The data of your DB will be stored inside container at /var/opt/mssql and the data will only exist till the container is running. Next time you restart the container the DB will be blank.
That is why you map it to an outside directory on host. So when you restart the container or launch a new one then that directory content would be made available inside the container and the DB will have all the changes you made