GitHub Codespaces devcontainer.json postCreateCommand execution consistency

The postCreateCommand in devcontainer.json executes only during the creation or full rebuild of a container. It does not trigger automatically when a codespace is resumed from a stopped state, even if the devcontainer.json file was modified while the codespace was inactive.

Execution Behavior by State Transition

To maintain environment consistency, GitHub Codespaces distinguishes between creating a filesystem and starting a process. The postCreateCommand is tied to the filesystem creation lifecycle.

• Full Rebuild: This action destroys the existing container and creates a new one from the image and devcontainer.json. The postCreateCommand will run.
• Resuming a Stopped Codespace: This action restarts the existing container instance. The postCreateCommand will not run.
• Configuration Updates: Modifying devcontainer.json does not trigger an automatic rebuild upon start. The changes remain dormant until a manual "Rebuild Container" is initiated.

Comparison of Lifecycle Commands

Trigger Event	postCreateCommand	postStartCommand
Initial Build	Executes	Executes
Full Rebuild	Executes	Executes
Resume from Stop	Skipped	Executes
Container Restart	Skipped	Executes

Verification and Implementation

If you are unsure if your command is executing, you can verify the behavior by adding a timestamped log to your configuration:

"postCreateCommand": "echo \"Created at $(date)\" > /tmp/creation_log.txt"

After stopping and resuming your codespace, check /tmp/creation_log.txt. If the timestamp does not update upon resume, the command is behaving as expected. To apply changes made to devcontainer.json, use the Command Palette (Ctrl+Shift+P) and select Codespaces: Rebuild Container.

Diagnostic Note: Please specify if you are using a custom Dockerfile or a pre-built image, as image layer caching can sometimes mask the perceived duration of the creation phase, though it does not skip the postCreateCommand execution itself.