AWS instance with only custom shell as ssh user

The shell provisioner can connect to instance over ssh using ssh-keypair. The default shell for ssh user is custom device cli. I can drop/escape into linux shell using “shell” command on the cli and issue any linux commands or prepend “shell” command to linux command. How to make the shell provisioner understand this.

Currently it executes linux commands and gets command not found. Same issue with shell/inline provisioner. It copies the file into /tmp directory but fails on chmod +x on the file.

I tried execution_command but couldn’t get syntax or override correct. Any example or pointer much appreciated.