`CMD` and `ENTRYPOINT`

Objectives
Defining a default command
Adding CMD to our Dockerfile
Build and test our image
Overriding CMD
Using ENTRYPOINT
Adding ENTRYPOINT to our Dockerfile
Implications of JSON vs string syntax
Build and test our image
Using CMD and ENTRYPOINT together
CMD and ENTRYPOINT together
Build and test our image
Overriding the image default parameters
Overriding ENTRYPOINT

Objectives

In this lesson, we will learn about two important Dockerfile commands:

CMD and ENTRYPOINT.

These commands allow us to set the default command to run in a container.

Defining a default command

When people run our container, we want to greet them with a nice hello message, and using a custom font.

For that, we will execute:

figlet -f script hello

-f script tells figlet to use a fancy font.
hello is the message that we want it to display.

Adding `CMD` to our Dockerfile

Our new Dockerfile will look like this:

FROM ubuntu
RUN apt-get update
RUN ["apt-get", "install", "figlet"]
CMD figlet -f script hello

CMD defines a default command to run when none is given.
It can appear at any point in the file.
Each CMD will replace and override the previous one.
As a result, while you can have multiple CMD lines, it is useless.

Build and test our image

Let's build it:

$ docker build -t figlet .
...
Successfully built 042dff3b4a8d
Successfully tagged figlet:latest

And run it:

$ docker run figlet
 _          _   _       
| |        | | | |      
| |     _  | | | |  __  
|/ \   |/  |/  |/  /  \_
|   |_/|__/|__/|__/\__/

Overriding `CMD`

If we want to get a shell into our container (instead of running figlet), we just have to specify a different program to run:

$ docker run -it figlet bash
root@7ac86a641116:/#

We specified bash.
It replaced the value of CMD.

Using `ENTRYPOINT`

We want to be able to specify a different message on the command line, while retaining figlet and some default parameters.

In other words, we would like to be able to do this:

$ docker run figlet salut
           _            
          | |           
 ,   __,  | |       _|_
/ \_/  |  |/  |   |  |  
 \/ \_/|_/|__/ \_/|_/|_/

We will use the ENTRYPOINT verb in Dockerfile.

Adding `ENTRYPOINT` to our Dockerfile

Our new Dockerfile will look like this:

FROM ubuntu
RUN apt-get update
RUN ["apt-get", "install", "figlet"]
ENTRYPOINT ["figlet", "-f", "script"]

ENTRYPOINT defines a base command (and its parameters) for the container.
The command line arguments are appended to those parameters.
Like CMD, ENTRYPOINT can appear anywhere, and replaces the previous value.

Why did we use JSON syntax for our ENTRYPOINT?

Implications of JSON vs string syntax

When CMD or ENTRYPOINT use string syntax, they get wrapped in sh -c.
To avoid this wrapping, we can use JSON syntax.

What if we used ENTRYPOINT with string syntax?

$ docker run figlet salut

This would run the following command in the figlet image:

sh -c "figlet -f script" salut

Build and test our image

Let's build it:

$ docker build -t figlet .
...
Successfully built 36f588918d73
Successfully tagged figlet:latest

And run it:

$ docker run figlet salut
           _            
          | |           
 ,   __,  | |       _|_
/ \_/  |  |/  |   |  |  
 \/ \_/|_/|__/ \_/|_/|_/

Using `CMD` and `ENTRYPOINT` together

What if we want to define a default message for our container?

Then we will use ENTRYPOINT and CMD together.

ENTRYPOINT will define the base command for our container.
CMD will define the default parameter(s) for this command.
They both have to use JSON syntax.