Update: 2020-09-24

Three updates since I wrote the post:

gcloud alpha cloud-shell get-mount-command ${DIR}

It’s possible to use sshfs to mount the Cloud Shell home directory locally:

DIR=/path/to/dir
gcloud alpha cloud-shell get-mount-command ${DIR}

Which generates something of the form:

sshfs [[USERNAME]]@[[HOST]]: ${DIR} \
-p [[PORT]] \
-oIdentityFile=~/.ssh/google_compute_engine \
-oStrictHostKeyChecking=no

You may then code --new-window ${DIR}

curl command may lack .sshHost

curl’ing the cloudshell.googleapis.com endpoint will result in a null value for .sshHost if the Cloud Shell VM must be recreated. The gcloud command avoids this by using an operation to poll the endpoint until the VM exists. The hacky alternative is to run gcloud alpha cloud-shell ssh to force the VM to be created before running the curl command.