Docker - Instruction Commands

Docker has a host of instruction commands. These are commands that are put in the Docker File. Let’s look at the ones which are available.

CMD Instruction

This command is used to execute a command at runtime when the container is executed.

Syntax

CMD command param1 

Options

• command − This is the command to run when the container is launched.
• param1 − This is the parameter entered to the command.

Return Value

The command will execute accordingly.

Example

In our example, we will enter a simple Hello World echo in our Docker File and create an image and launch a container from it.

Step 1 − Build the Docker File with the following commands −

FROM ubuntu 
MAINTAINER demousr@gmail.com 
CMD [“echo” , “hello world”] 

Here, the CMD is just used to print hello world.

Step 2 − Build the image using the Docker build command.

Step 3 − Run a container from the image.

ENTRYPOINT

This command can also be used to execute commands at runtime for the container. But we can be more flexible with the ENTRYPOINT command.

Syntax

ENTRYPOINT command param1 

Options

• command − This is the command to run when the container is launched.
• param1 − This is the parameter entered into the command.

Return Value

The command will execute accordingly.

Example

Let’s take a look at an example to understand more about ENTRYPOINT. In our example, we will enter a simple echo command in our Docker File and create an image and launch a container from it.

Step 1 − Build the Docker File with the following commands −

FROM ubuntu 
MAINTAINER demousr@gmail.com 
ENTRYPOINT [“echo”]

Step 2 − Build the image using the Docker build command.

Step 3 − Run a container from the image.

ENV

This command is used to set environment variables in the container.

Syntax

ENV key value 

Options

• Key − This is the key for the environment variable.
• value − This is the value for the environment variable.

Return Value

The command will execute accordingly.

Example

In our example, we will enter a simple echo command in our Docker File and create an image and launch a container from it.

Step 1 − Build the Docker File with the following commands −

FROM ubuntu 
MAINTAINER demousr@gmail.com 
ENV var1=Tutorial var2=point 

Step 2 − Build the image using the Docker build command.

Step 3 − Run a container from the image.

Step 4 − Finally, execute the env command to see the environment variables.

WORKDIR

This command is used to set the working directory of the container.

Syntax

WORKDIR dirname 

Options

• dirname − The new working directory. If the directory does not exist, it will be added.

Return Value

The command will execute accordingly.

Example

In our example, we will enter a simple echo command in our Docker File and create an image and launch a container from it.

Step 1 − Build the Docker File with the following commands −

FROM ubuntu 
MAINTAINER demousr@gmail.com 
WORKDIR /newtemp 
CMD pwd

Step 2 − Build the image using the Docker build command.

Step 3 − Run a container from the image.