To list all the available commands in any mode, use ?

sonic# ?
  configure  Enter configuration mode
  exit       Exit from the CLI
  image      Image related operations
  no         No commands under exec mode
  show       Show running system information
  system     System command
sonic#

sonic(config)# ?
  aaa             Configures AAA
  end             Exit to EXEC mode
  exit            Exit from current mode
  feature         Configure feature commands
  hostname        Enter name of the host
  interface       Select an interface
  ip              Global IPv4 configuration commands
  ipv6            Global IPv6 configuration commands
  lldp            Configure lldp
  logging         Configures logging
  mac             Global MAC configuration commands
  mclag           Global mclag configuration commands
  mirror-session  Mirror session configuration
  no              Negate a command or set its defaults
  ntp             Configures NTP
  qos             Global QoS configuration commands
  radius-server   Configures RADIUS client
  sflow           Configure sFlow
  snmp-server     Configures SNMP server
  tacacs-server   Configures TACACS servers
  wred            Configures a weighted random early detection (WRED) profile
sonic(config)#