It depends upon the command line interface. A Cisco router CLI is very discoverable. Sitting at the prompt, hit '?' and get a list of commands. Type the command, then '?', get a list of options. At each point in typing out the command, you can always get a list of what you can type next. And even better, you only have to type enough to distinguish from multiple commands/options. So 'show interface Ethernet/0' and 'sh in Eth/0' are the same command.

And on Unix, there's always the man command. Also, most commands will repond to "command -h" or "command --help", but yes, it's a far cry from the Cisco router command line.