The whalesay image could be improved. It would be nice if you didn’t have to think of something to say and you didn’t need to type as much to get whalesay to talk.
docker run docker/whalesay cowsay boo-boo
In this next section, you will improve the whalesay image by building a new version that “talks on its own” and requires fewer words to run.
In this step, you use a text editor to write a short Dockerfile. A Dockerfile is a recipe which describes the files, environment, and commands that make up an image. Your recipe is going to be very short.
You run these steps in a terminal window on Linux or macOS, or a command prompt on Windows. Remember that if you are using macOS or Windows, you are still creating an image which runs on Linux.
Make a new directory. If you’re on Windows, use md instead of mkdir.
$ mkdir mydockerbuild
This directory will contain all the things you need to build your image. Right now, it’s empty.
Change to your new directory. Whether you’re on Linux, macOS, or Windows, the cd command is the same.
$ cd mydockerbuild
Edit a new text file named Dockerfile in the current directory, using a text editor such as nano or vi on Linux or Mac, or notepad on Windows.
Linux or Mac:
$ nano Dockerfile
Windows:
C:\> notepad Dockerfile
Add a FROM statement by copying the following line into the file:
FROM docker/whalesay:latest
The FROM keyword tells Docker which image your image is based on. Whalesay is cute and has the cowsay program already, so we’ll start there.
Add a RUN statement which will install the fortunes program into the image.
RUN apt-get -y update && apt-get install -y fortunes
The whalesay image is based on Ubuntu, which uses apt-get to install packages. These two commands refresh the list of packages available to the image and install the fortunes program into it. The fortunes program prints out wise sayings for our whale to say.
Add a CMD statement, which tells the image the final command to run after its environment is set up. This command runs fortune -a and sends its output to the cowsay command.
CMD /usr/games/fortune -a | cowsay
Check your work. Your file should look just like this:
FROM docker/whalesay:latest RUN apt-get -y update && apt-get install -y fortunes CMD /usr/games/fortune -a | cowsay
Save the file and close the text editor. At this point, your software recipe is described in the Dockerfile file. You are ready to build a new image.
While you are in the mydockerbuild directory, build the image using the docker build command. The -t parameter gives your image a tag, so you can run it more easily later. Don’t forget the . command, which tells the docker build command to look in the current directory for a file called Dockerfile. This command works the same in Linux, macOS, or Windows.
$ docker build -t docker-whale . Sending build context to Docker daemon 2.048 kB ...snip... Removing intermediate container cb53c9d09f3b Successfully built c2c3152907b5
The command takes several seconds to run and reports its outcome. Keep reading to learn a little more about what the output means and what happens when you build an image.
The docker build -t docker-whale . command reads the Dockerfile in the current directory and processes its instructions one by one to build an image called docker-whale on your local machine. The command takes about a minute and its output looks really long and complex. In this section, you learn what each message means.
Docker checks to make sure it has everything it needs to build. This generates this message:
Sending build context to Docker daemon 2.048 kB
Docker checks to see whether it already has the whalesay image locally and pulls it from Docker hub if not. In this case, the image already exists locally because you pulled it in a previous task. This corresponds to FROM statement in the Dockerfile, and generates this message:
Step 1 : FROM docker/whalesay:latest ---> 6b362a9f73eb
At the end of each step, an ID is printed. This is the ID of the layer that was created by this step. Each line in a Dockerfile corresponds to a layer in the image. Your ID will be different.
Docker starts up a temporary container running the whalesay image (the Running in line below). In the temporary container, Docker runs the next command in the Dockerfile, which is the RUN command, which installs the fortune command. This generates a lot of lines of output, just like you would see if you were manually running the apt-get commands on an Ubuntu host.
Step 2 : RUN apt-get -y update && apt-get install -y fortunes ---> Running in 05d4eda04526 Ign http://archive.ubuntu.com trusty InRelease Get:1 http://archive.ubuntu.com trusty-updates InRelease [65.9 kB] Get:2 http://archive.ubuntu.com trusty-security InRelease [65.9 kB] Hit http://archive.ubuntu.com trusty Release.gpg Hit http://archive.ubuntu.com trusty Release Get:3 http://archive.ubuntu.com trusty-updates/main Sources [480 kB] Get:4 http://archive.ubuntu.com trusty-updates/restricted Sources [5921 B] Get:5 http://archive.ubuntu.com trusty-updates/universe Sources [214 kB] Get:6 http://archive.ubuntu.com trusty-updates/main amd64 Packages [1160 kB] Get:7 http://archive.ubuntu.com trusty-updates/restricted amd64 Packages [20.4 kB] Get:8 http://archive.ubuntu.com trusty-updates/universe amd64 Packages [505 kB] Get:9 http://archive.ubuntu.com trusty-security/main Sources [157 kB] Get:10 http://archive.ubuntu.com trusty-security/restricted Sources [4621 B] Get:11 http://archive.ubuntu.com trusty-security/universe Sources [54.5 kB] Get:12 http://archive.ubuntu.com trusty-security/main amd64 Packages [700 kB] Get:13 http://archive.ubuntu.com trusty-security/restricted amd64 Packages [17.0 kB] Get:14 http://archive.ubuntu.com trusty-security/universe amd64 Packages [191 kB] Hit http://archive.ubuntu.com trusty/main Sources Hit http://archive.ubuntu.com trusty/restricted Sources Hit http://archive.ubuntu.com trusty/universe Sources Hit http://archive.ubuntu.com trusty/main amd64 Packages Hit http://archive.ubuntu.com trusty/restricted amd64 Packages Hit http://archive.ubuntu.com trusty/universe amd64 Packages Fetched 3640 kB in 11s (329 kB/s) Reading package lists... Reading package lists... Building dependency tree... Reading state information... The following extra packages will be installed: fortune-mod fortunes-min librecode0 Suggested packages: x11-utils bsdmainutils The following NEW packages will be installed: fortune-mod fortunes fortunes-min librecode0 0 upgraded, 4 newly installed, 0 to remove and 92 not upgraded. Need to get 1961 kB of archives. After this operation, 4817 kB of additional disk space will be used. Get:1 http://archive.ubuntu.com/ubuntu/ trusty/main librecode0 amd64 3.6-21 [771 kB] Get:2 http://archive.ubuntu.com/ubuntu/ trusty/universe fortune-mod amd64 1:1.99.1-7 [39.5 kB] Get:3 http://archive.ubuntu.com/ubuntu/ trusty/universe fortunes-min all 1:1.99.1-7 [61.8 kB] Get:4 http://archive.ubuntu.com/ubuntu/ trusty/universe fortunes all 1:1.99.1-7 [1089 kB] debconf: unable to initialize frontend: Dialog debconf: (TERM is not set, so the dialog frontend is not usable.) debconf: falling back to frontend: Readline debconf: unable to initialize frontend: Readline debconf: (This frontend requires a controlling tty.) debconf: falling back to frontend: Teletype dpkg-preconfigure: unable to re-open stdin: Fetched 1961 kB in 19s (101 kB/s) Selecting previously unselected package librecode0:amd64. (Reading database ... 13116 files and directories currently installed.) Preparing to unpack .../librecode0_3.6-21_amd64.deb ... Unpacking librecode0:amd64 (3.6-21) ... Selecting previously unselected package fortune-mod. Preparing to unpack .../fortune-mod_1%3a1.99.1-7_amd64.deb ... Unpacking fortune-mod (1:1.99.1-7) ... Selecting previously unselected package fortunes-min. Preparing to unpack .../fortunes-min_1%3a1.99.1-7_all.deb ... Unpacking fortunes-min (1:1.99.1-7) ... Selecting previously unselected package fortunes. Preparing to unpack .../fortunes_1%3a1.99.1-7_all.deb ... Unpacking fortunes (1:1.99.1-7) ... Setting up librecode0:amd64 (3.6-21) ... Setting up fortune-mod (1:1.99.1-7) ... Setting up fortunes-min (1:1.99.1-7) ... Setting up fortunes (1:1.99.1-7) ... Processing triggers for libc-bin (2.19-0ubuntu6.6) ... ---> dfaf993d4a2e Removing intermediate container 05d4eda04526
When the RUN command finishes, a new layer is created and the intermediate container is removed.
A new intermediate container is created, and Docker adds a layer for the CMD line in the Dockerfile, and removes the intermediate container.
Step 3 : CMD /usr/games/fortune -a | cowsay ---> Running in a8e6faa88df3 ---> 7d9495d03763 Removing intermediate container a8e6faa88df3 Successfully built 7d9495d03763
You have now built an image called docker-whale.
Now you can verify that the new image is on your computer and you can run it.
In a terminal window or command prompt, type docker images, which lists the images you have locally.
$ docker images REPOSITORY TAG IMAGE ID CREATED SIZE docker-whale latest c2c3152907b5 4 minutes ago 275.1 MB docker/whalesay latest fb434121fc77 4 hours ago 247 MB hello-world latest 91c95931e552 5 weeks ago 910 B
Run your new image by typing docker run docker-whale.
$ docker run docker-whale
______________________________________
< You will be successful in your work. >
--------------------------------------
\
\
\
## .
## ## ## ==
## ## ## ## ===
/""""""""""""""""___/ ===
~~~ {~~ ~~~~ ~~~ ~~~~ ~~ ~ / ===- ~~~
\______ o __/
\ \ __/
\____\______/
The whale is now a lot smarter. It comes up with its own thoughts and it takes less typing to run. You may also notice that Docker didn’t have to download anything, because you built the image locally.
Just for fun, run it again.
_______________________________________
/ If everything seems to be going well, \
| you have obviously overlooked |
\ something. /
---------------------------------------
\
\
\
## .
## ## ## ==
## ## ## ## ===
/""""""""""""""""___/ ===
~~~ {~~ ~~~~ ~~~ ~~~~ ~~ ~ / ===- ~~~
\______ o __/
\ \ __/
\____\______/
Your whale now has a mind of its own, but if you don’t like what it says, just run it again!
Now that you have learned to build an image and run it, you can learn to share the image by creating a Docker Hub account.
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Licensed under the Apache License, Version 2.0.
Docker and the Docker logo are trademarks or registered trademarks of Docker, Inc. in the United States and/or other countries.
Docker, Inc. and other parties may also have trademark rights in other terms used herein.
https://docs.docker.com/v1.13/engine/getstarted/step_four/