liuxg@liuxg:~/snappy/desktop/helloworld-classic$ inst
[sudo] password for liuxg: 
error: cannot perform the following tasks:
- Mount snap "hello" (unset) (snap "hello" requires classic or confinement override)
liuxg@liuxg:~/snappy/desktop/helloworld-classic$ sudo snap install *.snap --classic
error: cannot find signatures with metadata for snap "hello_1.0_amd64.snap"
liuxg@liuxg:~/snappy/desktop/helloworld-classic$ ls
bin  hello_1.0_amd64.snap  parts  prime  setup  snapcraft.yaml  stage
liuxg@liuxg:~/snappy/desktop/helloworld-classic$ sudo snap install *.snap --classic
error: cannot find signatures with metadata for snap "hello_1.0_amd64.snap"
liuxg@liuxg:~/snappy/desktop/helloworld-classic$ sudo snap install *.snap --classic --dangerous
hello 1.0 installed
liuxg@liuxg:~/snappy/desktop/helloworld-classic$