Ken Muse

The New DevContainer initializeCommand

Over the last couple of weeks, there’s been an interesting change to the Dev Container lifecyle script spec. According to the original specification, initializeCommand was defined as:

A command string or list of command arguments to run on the host machine before the container is created.

This makes it seem that it run one time, when the container is created. This command is unique in that it runs on the host, not in the container. Last year, there were several issues raised around the behavior of the initializeCommand. The problem with this definition is that it’s not entirely accurate. The initializeCommand isn’t actually limited to just when the container is created. The community observed that it is executed more frequently that documented, including each time the container is restarted. This behavior exists in both VS Code and GitHub Codespaces. As a result, some developers have come to depend on the behavior of a script that runs on the host before starting the container.

That’s one challenge with development platforms. Once a behavior is established, it’s hard to change it without breaking something. As a result, the maintainers of the spec have decided to update the definition to reflect its actual behavior. The new definition is:

A command string or list of command arguments to run on the host machine during initialization, including during container creation and on subsequent starts. The command may run more than once during a given session.

This change makes it official that the initializeCommand is not limited to just container creation

This is a good change, as it reflects the actual behavior of the initializeCommand. Of course, all things in software development have tradeoffs. There is community feedback such as this requesting a lifecycle script that is guaranteed to run only when the container is being built. This might eventually lead to a new lifecycle script that is more explicit about only running before the container is initially built. Until then, if you need to ensure the script is only run once, you’ll need to add that logic to the script itself. A common trick is to use a file to track if the script has been run before. For initializeCommand, this could be a file that exists in the .devcontainer folder, the home directory for the user, or even in a dedicated volume. Because it runs on the host environment, it will need a marker that is accessible on the host.

A similar trick is already used with other lifecycle scripts by some of the features. To minimize the need to re-run some commands, marker files are created within the container and accessible to the command. If the marker file exists, the command can skip the work. This allows the scripts to be idempotent. Running the same command multiple times will result in the same state for the system. Knowing that the behavior of this command has changed is a great reminder of why its important for scripts to be idempotent whenever possible.