Note that if you specify more than one CMD instruction in your dockerfile, only the last one will be executed.įollowing are the different ways, through which you can execute a CMD command.ĬMD parameter1, parameter2 …. In case you specify a command while running a docker container, the default one will be ignored. This will be executed if you run a particular container without specifying some command. Using a CMD command, you will be able to set a default command. Examples include - RUN apt-get -y update (shell form) It can be written in both shell and exec forms.

It allows you to install packages and applications on top of an existing image layer and creates a new layer on top of it. The RUN command always gets executed in a new layer. Now that we have understood the two forms in-depth, let us continue with the three commands. The following dockerfile - ENV variable TutorialsPoint

It calls the command /bin/sh -c behind the scenes.Ĭonsider the commands below. When we try to execute an instruction using the shell form, normal shell processing takes place. When we try to run the image, the following output is generated. If we write a dockerfile with the following instructions, ENV variable TutorialsPoint When we execute an instruction in such a form, shell processing does not happen and the executable is called directly.Ĭonsider the commands below. This form is generally used for Entrypoint and CMD commands.

To understand all these three commands in depth, we first need to understand what are shell and exec forms. The final docker image can be considered as a layered structure where there is a core or a base image and on top of that, there are several layered intermediate images. Every such instruction creates a new intermediate image build and for each of them, caches are created. After that, we modify the base image either by including more images using FROM and AS commands or by modifying the images. The first instruction is usually pulling a base image such as an OS distribution like ubuntu, centos etc. When we try to build an image using dockerfile, the instructions are executed step by step. Understanding all the three commands conceptually will help to have a clearer understanding of the same. The commands RUN, CMD and Entrypoint usually cause a lot of confusion among docker developers.

Running docker image docker for mac