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This was discussed and agreed in #246. It will probably break a few outstanding PRs as well as a few external links but it's for the better good long term.
275 lines
5.3 KiB
Markdown
275 lines
5.3 KiB
Markdown
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class: title
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# `CMD` and `ENTRYPOINT`
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---
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## Objectives
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In this lesson, we will learn about two important
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Dockerfile commands:
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`CMD` and `ENTRYPOINT`.
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These commands allow us to set the default command
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to run in a container.
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---
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## Defining a default command
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When people run our container, we want to greet them with a nice hello message, and using a custom font.
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For that, we will execute:
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```bash
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figlet -f script hello
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```
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* `-f script` tells figlet to use a fancy font.
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* `hello` is the message that we want it to display.
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---
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## Adding `CMD` to our Dockerfile
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Our new Dockerfile will look like this:
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```dockerfile
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FROM ubuntu
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RUN apt-get update
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RUN ["apt-get", "install", "figlet"]
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CMD figlet -f script hello
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```
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* `CMD` defines a default command to run when none is given.
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* It can appear at any point in the file.
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* Each `CMD` will replace and override the previous one.
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* As a result, while you can have multiple `CMD` lines, it is useless.
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---
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## Build and test our image
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Let's build it:
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```bash
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$ docker build -t figlet .
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...
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Successfully built 042dff3b4a8d
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Successfully tagged figlet:latest
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```
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And run it:
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```bash
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$ docker run figlet
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_ _ _
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|/ \ |/ |/ |/ / \_
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```
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---
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## Overriding `CMD`
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If we want to get a shell into our container (instead of running
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`figlet`), we just have to specify a different program to run:
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```bash
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$ docker run -it figlet bash
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root@7ac86a641116:/#
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```
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* We specified `bash`.
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* It replaced the value of `CMD`.
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---
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## Using `ENTRYPOINT`
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We want to be able to specify a different message on the command line,
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while retaining `figlet` and some default parameters.
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In other words, we would like to be able to do this:
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```bash
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$ docker run figlet salut
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_
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/ \_/ | |/ | | |
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\/ \_/|_/|__/ \_/|_/|_/
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```
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We will use the `ENTRYPOINT` verb in Dockerfile.
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---
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## Adding `ENTRYPOINT` to our Dockerfile
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Our new Dockerfile will look like this:
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```dockerfile
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FROM ubuntu
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RUN apt-get update
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RUN ["apt-get", "install", "figlet"]
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ENTRYPOINT ["figlet", "-f", "script"]
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```
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* `ENTRYPOINT` defines a base command (and its parameters) for the container.
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* The command line arguments are appended to those parameters.
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* Like `CMD`, `ENTRYPOINT` can appear anywhere, and replaces the previous value.
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Why did we use JSON syntax for our `ENTRYPOINT`?
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---
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## Implications of JSON vs string syntax
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* When CMD or ENTRYPOINT use string syntax, they get wrapped in `sh -c`.
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* To avoid this wrapping, we can use JSON syntax.
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What if we used `ENTRYPOINT` with string syntax?
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```bash
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$ docker run figlet salut
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```
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This would run the following command in the `figlet` image:
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```bash
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sh -c "figlet -f script" salut
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```
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---
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## Build and test our image
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Let's build it:
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```bash
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$ docker build -t figlet .
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...
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Successfully built 36f588918d73
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Successfully tagged figlet:latest
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```
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And run it:
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```bash
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$ docker run figlet salut
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_
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/ \_/ | |/ | | |
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\/ \_/|_/|__/ \_/|_/|_/
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```
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---
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## Using `CMD` and `ENTRYPOINT` together
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What if we want to define a default message for our container?
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Then we will use `ENTRYPOINT` and `CMD` together.
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* `ENTRYPOINT` will define the base command for our container.
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* `CMD` will define the default parameter(s) for this command.
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* They *both* have to use JSON syntax.
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---
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## `CMD` and `ENTRYPOINT` together
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Our new Dockerfile will look like this:
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```dockerfile
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FROM ubuntu
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RUN apt-get update
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RUN ["apt-get", "install", "figlet"]
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ENTRYPOINT ["figlet", "-f", "script"]
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CMD ["hello world"]
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```
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* `ENTRYPOINT` defines a base command (and its parameters) for the container.
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* If we don't specify extra command-line arguments when starting the container,
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the value of `CMD` is appended.
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* Otherwise, our extra command-line arguments are used instead of `CMD`.
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---
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## Build and test our image
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Let's build it:
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```bash
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$ docker build -t figlet .
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...
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Successfully built 6e0b6a048a07
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Successfully tagged figlet:latest
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```
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Run it without parameters:
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```bash
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$ docker run figlet
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_ _ _ _
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|/ \ |/ |/ |/ / \_ | | |_/ \_/ | |/ / |
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| |_/|__/|__/|__/\__/ \/ \/ \__/ |_/|__/\_/|_/
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```
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---
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## Overriding the image default parameters
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Now let's pass extra arguments to the image.
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```bash
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$ docker run figlet hola mundo
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_ _
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|/ \ / \_|/ / | / |/ |/ | | | / |/ | / | / \_
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| |_/\__/ |__/\_/|_/ | | |_/ \_/|_/ | |_/\_/|_/\__/
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```
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We overrode `CMD` but still used `ENTRYPOINT`.
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---
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## Overriding `ENTRYPOINT`
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What if we want to run a shell in our container?
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We cannot just do `docker run figlet bash` because
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that would just tell figlet to display the word "bash."
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We use the `--entrypoint` parameter:
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```bash
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$ docker run -it --entrypoint bash figlet
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root@6027e44e2955:/#
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```
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