Docker: CMD: Provide default command

CMD is used to provide default command to a running container.

CMD command can be written in below three forms.

a.   CMD ["executable","param1","param2"] (exec form, this is the preferred form)

b.   CMD ["param1","param2"] (as default parameters to ENTRYPOINT)

c.    CMD command param1 param2 (shell form)

Let’s try with an example.

Step 1: Create a directory ‘sleeper’.

Step 2: Navigate to the directory ‘sleeper’. Create Dockerfile with below content.

Dockerfile

FROM ubuntu

CMD sleep 5

Step 3: Go to the directory where Dockerfile is located, execute below command to generate the image.

docker build -t ubuntu_sleeper .

$docker build -t ubuntu_sleeper .
Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM ubuntu
 ---> 94e814e2efa8
Step 2/2 : CMD sleep 5
 ---> Running in 7f5bd2287317
Removing intermediate container 7f5bd2287317
 ---> 2678ecea665f
Successfully built 2678ecea665f
Successfully tagged ubuntu_sleeper:latest

Step 4: Run the container ubuntu_sleeper by executing below command.

docker run ubuntu_sleeper

You can observe that the container will sleep for 5 seconds and exit.

How to override the default command?

You can pass the command to run while running the container itself.

Syntax

docker run {image} {commandToExecute}

$docker run ubuntu_sleeper echo "I am overriding sleep command."
I am overriding sleep command.

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