Dockerfiles : building Docker images automatically III - RUN

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Continued from Dockerfile - Build Docker images automatically II - revisiting FROM, MAINTAINER, build context, and caching

In this chapter, we're going to learn more on how to automate this process via instructions in Dockerfiles, especially, the RUN instruction.

Dockerfie - RUN

This section is from http://docs.docker.com/reference/builder/.

RUN has 2 forms:

RUN <command> (the command is run in a shell - /bin/sh -c - shell form)
RUN ["executable", "param1", "param2"] (exec form)

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

Layering RUN instructions and generating commits conforms to the core concepts of Docker where commits are cheap and containers can be created from any point in an image's history, much like source control.

The exec form makes it possible to avoid shell string munging, and to RUN commands using a base image that does not contain /bin/sh.

Note: To use a different shell, other than '/bin/sh', use the exec form passing in the desired shell. For example, RUN ["/bin/bash", "-c", "echo hello"].
Note: The exec form is parsed as a JSON array, which means that you must use double-quotes (") around words not single-quotes (').
Note: Unlike the shell form, the exec form does not invoke a command shell. This means that normal shell processing does not happen. For example, RUN [ "echo", "$HOME" ] will not do variable substitution on $HOME. If you want shell processing then either use the shell form or execute a shell directly, for example: RUN [ "sh", "-c", "echo", "$HOME" ].

Dockerfile 'RUN' sample

Here is our Dockerfile we're going to playing with in this chapter. We'll run instructions from this file step by step by uncommenting and commenting each line.

FROM debian:latest
MAINTAINER devops@bogotobogo.com

# 1 - RUN
RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -yq apt-utils
RUN DEBIAN_FRONTEND=noninteractive apt-get install -yq htop
RUN apt-get clean

We have three instructions for RUN, and each of these instruction will create a new container, and at the completion of each instruction, it will become an image.

The following enviroment setting is to block any terminal output caused by some errors:

DEBIAN_FRONTEND=noninteractive

The 2nd instruction, htop is to monitor processes in linux system. Then, we removes all packages from the package cache using apt-get clean.

Let's run docker build with v2 instead of v1:

$ docker image build -t bogodevops/demo:v2 .
Sending build context to Docker daemon 33.56 MB
Sending build context to Docker daemon 
Step 0 : FROM debian:latest
 ---> f6fab3b798be
Step 1 : MAINTAINER k@bogotobogo.com
 ---> Using cache
 ---> 511bcbdd59ba
Step 2 : RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -yq apt-utils
 ---> Running in 10ffa5b21a27
...
Setting up apt-utils (0.9.7.9+deb7u6) ...
 ---> e6e2c03b8efc
Removing intermediate container 10ffa5b21a27
Step 3 : RUN DEBIAN_FRONTEND=noninteractive apt-get install -yq htop
 ---> Running in 2fe900ff207c
...
Setting up htop (1.0.1-1) ...
 ---> fac6e3168cfe
Removing intermediate container 2fe900ff207c
Step 4 : RUN apt-get clean
 ---> Running in 990373d72cc9
 ---> 327d400a953c
Removing intermediate container 990373d72cc9
Successfully built 327d400a953c

Listing images:

$ docker images -a
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
bogodevops/demo     v2                  327d400a953c        7 minutes ago       96.16 MB
<none>              <none>              fac6e3168cfe        7 minutes ago       96.16 MB
<none>              <none>              e6e2c03b8efc        7 minutes ago       95.12 MB
bogodevops/demo     v1                  511bcbdd59ba        2 hours ago         85.1 MB
debian              latest              f6fab3b798be        2 weeks ago         85.1 MB
<none>              <none>              f10807909bc5        2 weeks ago         85.1 MB
<none>              <none>              511136ea3c5a        17 months ago       0 B

As we discussed in the previous chapter, if we run this again, it will be completed much faster thanks to caching:

$ docker image build -t bogodevops/demo:v2 .
Sending build context to Docker daemon 33.56 MB
Sending build context to Docker daemon 
Step 0 : FROM debian:latest
 ---> f6fab3b798be
Step 1 : MAINTAINER k@bogotobogo.com
 ---> Using cache
 ---> 511bcbdd59ba
Step 2 : RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -yq apt-utils
 ---> Using cache
 ---> e6e2c03b8efc
Step 3 : RUN DEBIAN_FRONTEND=noninteractive apt-get install -yq htop
 ---> Using cache
 ---> fac6e3168cfe
Step 4 : RUN apt-get clean
 ---> Using cache
 ---> 327d400a953c
Successfully built 327d400a953c

docker run - launching container

$ docker container run -it --rm bogodevops/demo:v2 /bin/bash
root@cf6430ffba1b:/# exit 
exit

If we drop the :v2 tag in the command:

$ docker container run -it --rm bogodevops/demo /bin/bash
Unable to find image 'bogodevops/demo' locally
Pulling repository bogodevops/demo
2014/11/24 18:55:36 Error: image bogodevops/demo not found

So, to make it work, we need to build default as latest:

$ docker image build -t bogodevops/demo .

Now, if look at the images:

$ docker images -a
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
bogodevops/demo     v2                  327d400a953c        32 minutes ago      96.16 MB
bogodevops/demo     latest              327d400a953c        32 minutes ago      96.16 MB

we have a new bogodevops/demo image tagged as 'latest'. So, from now on, we can execute docker run without the 'tag' since it'll look for 'latest' tag by default:

$ docker container run -it --rm bogodevops/demo /bin/bash
root@88d48b65ebd7:/#

Now, we're in our Docker container for Debian, and htop has been installed.

root@88d48b65ebd7:/# htop

root@88d48b65ebd7:/# exit
exit

We should not see any container:

$ docker container ps -a
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

No containers are hanging around!

docker run - CMD

Continued in Dockerfile - Build Docker images automatically IV - CMD.