por Building an image in Docker involves using a script that specifies the instructions related to these tasks. This script has two different types of instructions that contribute to defining the work performed in the container. These instructions are called ENTRYPOINT and CMD.
Although both options are characterized by specify the programs that will run once the container starts, ENTRYPOINT and CMD instructions have a series of differentiating elements that distinguish them from each other. This implies that Each option has its own capabilities and is suitable for performing its tasks in different use cases, environments and applications.
On the one hand, Docker CMD takes care of the process of defining the default executable of the Docker image, which can be run as a base container without having to add command line arguments. On the other hand, the option of ENTRYPOINT in the Dockerfiles it is used for the purpose of set the executables that will run whenever the container is started. ENTRYPOINT cannot be ignored or overridden, even when the container It is executed with the command line arguments indicated for that task.
Differences between ENTRYPOINT and CMD
Among the differences between the ENTRYPOINT and CMD docker options, it is found that the ENTRYPOINT statements are added in command line parameter formatby treating them as if they were arguments to these commands, while the CMD options daemon of docker It does not take them into account in cases where there are predetermined parameters related to command docker run.
This means that CMD fulfills the function of defining pre-established commands and parameters for a given platform container, so it is the most useful instruction if the user requires a default command that contains the overridden property In a very simple way. It should be noted that when a Dockerfile has more than one CMD docker statement, you can only apply the script of the last of these.
On the other hand, the docker option ENTRYPOINT is ideal for defining a container that includes a specific executable. It should be noted that it is not possible to override an ENTRYPOINT when starting containers, unless the user adds the call –entrypoint flag.
Although the docker options ENTRYPOINT and CMD docker are used to configure the execution of a container in Docker and both instructions need to specify which commands and parameters will be used to carry out their tasks, ENTRYPOINT statements cannot be overridden by adding a parameter to the command line of the command docker runso if the user uses this option, it means that the container they are working with has a design and configuration specifically developed to fulfill such a function.
Combination of ENTRYPOINY and CMD
Despite the many differences between the ENTRYPOINT docker and CMD docker options, these They are not mutually exclusive, so you can have both within a Dockerfile. In addition, the user has the possibility of combining the ENTRYPOINT and CMD instructions in cases where a container with an executable (defined with the ENTRYPOINT statement) and a default parameter (specified by the CMD docker option) that can be easily modified. An example of this may be setting specific environment variables when the system contains an application.
When the user performs the combination of the ENTRYPOINT docker and CMD instructions, It is important that you keep these options in the format execto guarantee its proper functioning within the system.
What is the next step?
In this article you have been able to learn what the ENTRYPOINT docker and CMD docker instructions are, as well as their most important characteristics and differences within the Docker container platform. Now, the time has come to take the next step and, to do so, we advise you continue with your training process in relation to this and other tools with the support of our DevOps & Cloud Computing Full Stack Bootcamp. Thanks to this bootcamp, in just 6 months you will acquire the ability to master everything essential to become a professional in the IT sector and, thus, stand out from your competitors. Sign up now and keep learning!