You get a codechef command right out of the box.:

$ codechef
Usage: codechef [OPTIONS] COMMAND [ARGS]...

  Codechef CLI

  --version                       Show the version and exit.
  -ll, --log-level [ERROR|WARNING|INFO|DEBUG]
                                  Log Level
  --help                          Show this message and exit.

  contest      Information about contests.
  login        Start the login process, receive tokens and store them
  submissions  Submit your solution to a problem and view it's status.

Use codechef login to login and authenticate the app. After that you can start using the different commands.