Start a process when the container starts
When you are working in a development container, you may want to execute a command or start something each time the container starts. The easiest way to do this is using the
postStartCommand property in
devcontainer.json. For example, if you wanted to run
yarn install every time you connected to the container to keep dependencies up to date, you could add the following:
"postStartCommand": "yarn install"
Video: Run npm install when a container is created
In other cases, you may want to start up a process and leave it running. This can be accomplished by using
nohup and putting the process into the background using
&. For example:
"postStartCommand": "nohup bash -c 'your-command-here &'"
Video: Run 'npm start' whenever the container is started
Those familiar with Linux may expect to be able to use the
systemctl command to start and stop background services managed by something called
systemd has overhead and is generally not used in containers as a result.
In many cases, there is a command you can run instead (for example,
sshd). And on Debian/Ubuntu, there are often scripts under
/etc/init.d that you can run directly.
"postStartCommand": "/etc/init.d/ssh start"
These systems also include a
service command that will use
/etc/init.d scripts based on what is installed.
"postStartCommand": "service ssh start"
Video: Start SSH service in a container
Adding startup commands to the Docker image instead
When referencing a Dockerfile in
devcontainer.json, the default entrypoint and command is overridden. First, disable this behavior using the
overrideCommand property defaults to
true because many images will immediately exit if a command is not specified. Instead, we will need to handle this in our Dockerfile.
Next, consider this Dockerfile:
FROM mcr.microsoft.com/devcontainers/base:1-ubuntu COPY docker-entrypoint.sh / RUN chmod +x /docker-entrypoint.sh ENTRYPOINT [ "/docker-entrypoint.sh" ] CMD [ "sleep", "infinity" ]
CMD here makes sure the container stays running by default. Keeping your startup steps in the
ENTRYPOINT allows you to safely override the command when using
docker run with your image or using Docker Compose. This resolves to the following:
/docker-entrypoint.sh sleep infinity
Next, create a
#!/usr/bin/env bash echo "Hello from our entrypoint!" exec "$@"
Anything you execute in this file will then fire each time the container starts. However, it's important to include the last
exec "$@" line since this is what will cause the command
sleep infinity in our example to fire.