Set your CLI’s binary as the image’s entrypoint. This is most beneficial when you’re creating utility containers to encapsulate CLI programs. Separating the entrypoint from its arguments helps you hide complexity in your containers. ENTRYPOINT exec binary -param -another-param Benefits of Docker’s Entrypoint Approach You can launch your process with exec to avoid this. Docker will signal the shell to stop, instead of the process within. Arguments given to docker run will be ignored your container will always use the entrypoint as-is.īecause your binary runs within a shell, Docker lifecycle commands like docker stop may work erratically or not at all. You can’t use CMD so users won’t be able to issue overrides. This gives your entrypoint access to environment variables defined by the shell. Using shell mode causes your binary to be executed as a subprocess of /bin/sh -c. # exec modeĮNTRYPOINT ĮNTRYPOINT binary -param -another-param In shell mode, the command is specified as one string. Exec mode is characterised by the use of an array construct to specify parameters. Entrypoint Modes: Shell or Execĭocker actually supports two different forms of ENTRYPOINT: exec mode and shell mode. Any arguments provided after the image name will be interpreted as the CMD string for the container. docker run has transparent support for command overrides. If you want to provide default arguments, but expect the user to override them, include CMD too.Īs an image user, you can normally stick to overriding CMD. If you’re an image author, you should use ENTRYPOINT when defining what your container will run. If a container’s misbehaving, overriding its entrypoint can grant you shell access you couldn’t otherwise obtain. Setting a custom entrypoint can be useful though, particularly when debugging. It can go against the image author’s intentions. Overriding entrypoints should be a rare occurrence. In our example, a shell session will be started, instead of the date command.
The entrypoint defined in the container’s image will be ignored in favour of the command you’ve specified. Pass the -entrypoint flag to docker run: docker run -entrypoint /bin/sh my-image You can force Docker to start an image using a custom entrypoint. It will be overridden when the container is run with different arguments. Use CMD to define default arguments for that executable. You should use ENTRYPOINT to define your container’s primary executable. The CMD is always appended to the ENTRYPOINT, so the final command becomes date +%B. This works because the image’s entrypoint remains intact. The default CMD will be overridden with +%B, causing the container to display the name of the current month. docker run lets you specify a different command for an individual container instance: docker run my-image +%B Monday).ĬMD is designed to be overridden. The +%A argument to date displays the current day of the week (e.g. This example results in the container running date +%A. It provides default arguments for the command defined by ENTRYPOINT. The CMD instruction is something of a misnomer. You can add execute permissions using chmod +x my-script.sh. If you’re using a custom script, make sure it’s got the executable bit set.
Your container won’t start if you specify an invalid entrypoint. As date is not a long-living foreground process, the container will exit immediately afterwards.Įntrypoints must be executable binaries or scripts. ENTRYPOINT Ī container created with this Dockerfile will run the date command. It will become the foreground process, instead of the default shell session. Setting the ENTRYPOINT directive in a Dockerfile instructs Docker to run a specific command when the container starts. You want headless services to start their workload straightaway. For many containers, it’s more desirable to have a different process launch by default. This means you’ll end up in a shell session when you start the container. The image’s entrypoint defines the process which will be run when the container starts.ĭocker defaults the entrypoint to /bin/sh -c. We’ll look at ENTRYPOINT first as it’s processed before CMD when starting a new container. Effective use of these directives makes your container easier to use by shortening the length of commands you supply. The CMD and ENTRYPOINT can be overridden individually within each image. Both have a role in determining the command which will run when the container starts. The CMD and ENTRYPOINT instructions are two commonly confused Dockerfile directives.