CODEWITHSUNDEEP
docker
KikiYu
KikiYu

Reputation: 3267

What is the default user of instructions in Dockerfile?

I have done an experiment to check this out. The following is my Dockerfile:

FROM ubuntu:bionic                                

RUN echo $UID > a.out && echo "hello" > a.out     
RUN useradd -m user      
RUN echo $UID > b.out                             

USER user

The result of a.out and b.out is shown below.

user@39370ee77a16:/$ cat a.out
hello
user@39370ee77a16:/$ cat b.out

My question is:

  1. Is root the default user?
  2. Why $UID in Dockerfile shows nothing?

Thanks!

Upvotes: 4

Views: 3451

Answers (2)

Roger Gustavsson
Roger Gustavsson

Reputation: 1709

What has not been fully addressed in other answers (so far) is your second question:

Why $UID in Dockerfile shows nothing?

$UID is a bash internal variable. When the image is created from the dockerfile, bash is not used, so $UID is not defined, hence using it returns an empty string.

To make this answer complete, even if it has already been answered:

Is root the default user?

That depends on how the image in the FROM statement was setup.

Upvotes: 3

Adiii
Adiii

Reputation: 59946

It depend upon the base image, but in you case the default user is root. you can verify from the base image or inside Dockerfile.

FROM ubuntu:bionic 
RUN whoami

In your case, you see hello because you override the content, try with

RUN echo $UID >> a.out && echo "hello" >> a.out

You can change user using

USER my_user

Upvotes: 5

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