Something about Docker

Almost every 6 months, I would look up the differences between Docker ENTERPOINT and CMD. Now it comes to my blog

0 Setting Env variable in Linux

Starting with a sublt differences between two ways of setting env var

  # Only exist in this shell
  var = 1
  # Exist in other processes
  export var = 1

Another differences is that env or printenv can show var set by export, but NOT directly = assignment

1 Shell form vs Executable form

Shell form directly feed in commands

ENV name Darwin
CMD echo “Hello World”
ENTRYPOINT /bin/echo "Welcome, $name"

Executable mode directly runs the executable binaries, without going through shell validation and processing.

CMD ["yum", "-y", "install", "httpd"]
ENV name Darwin
# Welcome, $name
ENTRYPOINT ["/bin/echo", "Welcome, $name"]
# Welcome, Darwin
ENTRYPOINT ["/bin/bash", “-c” "echo Welcome, $name"]

2 ENTRYPOINT vs CMD

|-|ENTRYPOINT|CMD| |–|–|–| |order|First|Second| |Default|can NOT be overwrite in docker run CLI|CAN be overwrite in docker run CLI| |Ignore|will NOT be ignored| will be ignored with run parameters|

CMD ["Darwin"]
ENTRYPOINT ["echo", "Hello,"]
#docker run container
#output: Hello, Darwin
#
#docker run container Kevin
#output: Hello, Kevin # CMD ignored

3 ARG vs ENV

This picture explains all.

• ARG only exists during the build time
• ENV exists in both build and run time,so you can see it in both image(with docker inspect) and container.

A side note, the .env file is used for docker-composer.yaml to get env var values.

Both ARG and ENV can be reset by CLI