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Docker ENTRYPOINT command
After docker image is created you will have to run image. You should define what should happen when you run your image. For example when you run your image it should start application servers, it should start Database services. Sometimes you may need to set environment specific settings before starting any services... so on. To execute your desired tasks when docker image is run you have to set your executable instructions from either CMD command or from ENTRYPOINT command. In our previous chapter Docker CMD Command we studied about CMD command. In this chapter we learn ENTRYPOINT command.
To run you image you should execute below docker command.
docker run <imagename> |
Now when you image is run if you want to start your spring boot application you may have to create your dockerfile as below:
FROM ubuntu:latest RUN apt-get update && \ apt-get install -y curl \ wget \ openjdk-8-jdk ENV JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java ARG JAR_FILE=target/*.jar COPY ${JAR_FILE} app.jar ENTRYPOINT ["java","-jar","/app.jar"] |
When docker run command is executed it runs the command java –jar /app.jar command on Ubuntu terminal.
Below is another simple example, we just echo a message to console when docker image is run.
FROM ubuntu:latest ENTRYPOINT ["echo", "Hello World"] |
Instructions set from ENTRYPOINT command cannot be overridden by providing instructions to docker run command. If any parameters are provided in docker run command, all those are appended to ENTRYPOINT command parameters.
Docker ENTRYPOINT command syntax
The ENTRYPOINT command has three types of syntax, as shown below:
Shell Syntax
ENTRYPOINT <command> |
Example:
ENTRYPOINT echo "Hello World" |
JSON Array Syntax
ENTRYPOINT ["executablecommand", "argument1", " argument2", ...] |
executablecommand : This is the executable command should be supported form the base image.
argument1, argument2…: Arguments for the executablecommand
Example:
ENTRYPOINT ["echo", "Hello World"] |
JSON Array Syntax with default parameters from CMD instruction
The third type of syntax is similar to the previous type. However, in this type we set the default parameters from CMD instruction. Please read Docker CMD command chapter from here.
CMD ["argument1", " argument2", ...] ENTRYPOINT ["executablecommand"] |
Example:
FROM ubuntu:latest CMD "Hello World" ENTRYPOINT ["echo"] |
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