Possible states an environment might transition between during startup. These states are not normally actionable by clients, but may be used to show a progress message to the user. An environment won't necessarily go through all of these states when starting. More states are likely to be added in the future.

STATE_UNSPECIFIED The environment's start state is unknown.
STARTING The environment is in the process of being started, but no additional details are available.
UNARCHIVING_DISK Startup is waiting for the user's disk to be unarchived. This can happen when the user returns to Cloud Shell after not having used it for a while, and suggests that startup will take longer than normal.
AWAITING_COMPUTE_RESOURCES Startup is waiting for compute resources to be assigned to the environment. This should normally happen very quickly, but an environment might stay in this state for an extended period of time if the system is experiencing heavy load.
FINISHED Startup has completed. If the start operation was successful, the user should be able to establish an SSH connection to their environment. Otherwise, the operation will contain details of the failure.