compose/docs/reference/exec.md

exec

Usage: exec [options] SERVICE COMMAND [ARGS...]

Options:
-d                Detached mode: Run command in the background.
--privileged      Give extended privileges to the process.
--user USER       Run the command as this user.
-T                Disable pseudo-tty allocation. By default `docker-compose exec`
                  allocates a TTY.
--index=index     index of the container if there are multiple
                  instances of a service [default: 1]

This is equivalent of docker exec. With this subcommand you can run arbitrary commands in your services. Commands are by default allocating a TTY, so you can do e.g. docker-compose exec web sh to get an interactive prompt.