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I have a query regarding the execution of commands in a container from the Dockerfile. 

#Dockerfile Content: FROM ubuntu:14.04 MAINTAINER RAGHU RUN echo "hello World"

Build procedure

 docker build -t helloworld . Sending build context to Docker daemon 20.04 MB Step 1 : FROM ubuntu:14.04 ---> b1719e1db756 Step 2 : MAINTAINER RAGHU ---> Using cache ---> 1704b62d66e2 Step 3 : RUN echo "hello World" ---> Running in 2b513872628e hello World ---> ff559047fd19 Removing intermediate container 2b513872628e Successfully built ff559047fd19

Query 1: Why does the below execution has not resulted in any result ? I am expecting to print "hello World". What wrong has happened ?

 root@labadmin-VirtualBox:/home/labadmin# docker run ff559047fd19 root@labadmin-VirtualBox:/home/labadmin# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES a8eede1874a2 ff559047fd19 "/bin/bash" 27 seconds ago Exited (0) 26 seconds ago tiny_williams root@labadmin-VirtualBox:/home/labadmin# docker logs a8eede1874a2 #Above command has not resulted in any logs for this container.

Query 2 :I am able to run the execution steps in the following way. Why it has executed the command in the below container and why not when I run the container a8eede1874a2 ?

 root@labadmin-VirtualBox:/home/labadmin# docker run -it ff559047fd19 root@486595ac9110:/# echo "hello world" hello world root@486595ac9110:/# exit exit root@labadmin-VirtualBox:/home/labadmin/RAGHU/welcome-page# docker ps -a CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 486595ac9110 ff559047fd19 "/bin/bash" 22 seconds ago Exited (0) 7 seconds ago goofy_noyce a8eede1874a2 ff559047fd19 "/bin/bash" About a minute ago Exited (0) About a minute ago tiny_williams root@labadmin-VirtualBox:/home/labadmin# docker logs 486595ac9110 root@486595ac9110:/# echo "hello world" hello world root@486595ac9110:/# exit exit root@labadmin-VirtualBox:/home/labadmin/RAGHU/welcome-page#
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Docker images are built layer by layer using (you guessed it!) docker containers. To get Hello World to print when you're trying to run a container you'll need to specify an ENTRYPOINT or a CMD within your Dockerfile.

Taken from Dockerfile reference:

Note: don’t confuse RUN with CMD. RUN actually runs a command and commits the result; CMD does not execute anything at build time, but specifies the intended command for the image.

Moving on swiftly to your queries:

Query 1: Why does the below execution has not resulted in any result ? I am expecting to print "hello World". What wrong has happened ?

In your Dockerfile, when you say RUN echo "hello world" what docker actually does is it creates an intermediate container, executes the command and saves the state of the container as a layer on which it'll run the next command. In your example you can see that "hello world" was actually printed in Step 3.

 Step 3 : RUN echo "hello World" ---> Running in 2b513872628e hello World

And for your second query:

Query 2: I am able to run the execution steps in the following way. Why it has executed the command in the below container and why not when I run the container a8eede1874a2 ?

So when you've created container with the hash of a8eede1874a2 what you're actually doing is well... nothing. You haven't specified CMD or ENTRYPOINT or a command through the command line. Your container started and stopped since there was nothing to execute.

Now in your second example you're executing an interactive shell in which you're running the echo "hello world".

Example Dockerfile:

FROM ubuntu:14.04 MAINTAINER RAGHU CMD "echo hello World"

Useful references:

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Question 1:

According to Dockerfile reference: The RUN instruction will execute any commands in a new layer on top of the current image and commit the results. The resulting comitted image will be used for the next step in the Dockerfile.

So when you execute RUN echo "hello World", echo "hello World" will print out a "hello World". And the execute result, which is executed successfully with return code 0, has been added as a new lay to the top of previous layer. In your case, the previous layer is the execute result of MAINTAINER RAGHU.

When you create container with your image ff559047fd19, RUN echo "hello World" will NOT be executed again, because this command will only be executed during image building process. Which is the step 3:

 Step 3 : RUN echo "hello World" ---> Running in 2b513872628e hello World

Question 2:

You were create a new container with your image ff559047fd19. Because of ff559047fd19 is based on ubuntu, so you are be able to attach to your new container's bash.

 root@486595ac9110:/# echo "hello world" hello world

Here, you are simply execute an echo command in your new container's bash.

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