Detailed explanation of the specific use of the ENV instruction in Dockerfile

1. The ENV instruction in the Dockerfile is used to define the environment variables of the image. Here is an example:

RUN set -ex && apt-get update && apt-get install -y iputils-ping
ENV PATH /usr/local/bin:$PATH
ENV LANG C.UTF-8
ENV TERM xterm
ENV PYTHON_VERSION 3.5.3
ENV name1=ping name2=on_ip
CMD $name1 $name2

Note: When defining environment variables, you can reference already defined environment variables.

In the ENV directive, you can directly reference the following environment variables:

• HOME, user home directory
• HOSTNAME, the default container hostname
• PATH,
• TERM, default xterm

2. Due to the hierarchical file system of the image, the environment variables defined by ENV can only be applied in subsequent layers. The example is as follows:

ENV abc=hello
ENV abc=bye def=$abc
ENV ghi=$abc

illustrate:

In the above definition, def = hello, ghi = bye

3. After starting the container, you can view the environment variables in the container instance through the env command

env

Reference Links:

https://docs.docker.com/engine/reference/builder/

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