How to get help in router command prompt

Cisco IOS Software uses several command-line input help facilities, among these context-sensitive help is the most powerful feature of cisco ios.

Context-Sensitive Help

One of the more powerful features of the IOS is context-sensitive help. Context sensitive help is supported at all modes within the IOS, including User EXEC, Privilege EXEC, and Configuration modes. You can use this feature in a variety of ways. If you are not sure what command you need to execute, at the prompt, type either help or ?. The Cisco device then displays a list of commands that can be executed at the level in which you are currently located, along with a brief description of each command. 
Here is an example from a router’s CLI at User EXEC mode:

Router>?

Exec commands:

  <1-99>      Session number to resume

  connect     Open a terminal connection

  disconnect  Disconnect an existing network connection

  enable      Turn on privileged commands

  exit        Exit from the EXEC

  ipv6        ipv6

  logout      Exit from the EXEC

  ping        Send echo messages

  resume      Resume an active network connection

  show        Show running system information

  ssh         Open a secure shell client connection

  telnet      Open a telnet connection

  terminal    Set terminal line parameters

  traceroute  Trace route to destination

Router>

If you see -- More -- at the bottom of the screen, this indicates that more help information is available than can fit on the current screen. On a Cisco device,
if you press the SPACEBAR, the IOS pages down to the next screen of help information; 
if you press the ENTER key, help scrolls down one line at a time Any other keystroke breaks out of the help text.

For more detailed help, you can follow a command or parameter with a space and a ?. This causes the CLI to list the available options or parameters that are included for the command. For instance, you could type copy followed by a space and a ? to see all of the parameters available for the copy command:

Router#copy ?

  running-config  Copy from current system configuration

  startup-config  Copy from startup configuration

  tftp:           Copy from tftp: file system

Router#copy

In this example, you can see at least the first parameter necessary after the copy command. Please note that additional parameters may appear after the first one, depending on the next parameter that you enter.

f you’re not sure how to spell a command, you can enter the first few characters and immediately follow these characters with a ?. Typing e?, for instance, lists all the commands that begin with e at the current mode:

Router# e?

enable erase exit

Router# e

This example shows that three commands begin with the letter e in Privilege EXEC mode.

Console Error Messages

error messages: Identifies problems with any Cisco IOS commands that are incorrectly entered so that you can alter or correct them.

Error:-% Invalid input detected at '^' marker.

Errors certainly creep up when you enter commands. Whenever you mistype a command, the IOS tells you that it has encountered a problem with the previously executed command. For instance, this message indicates a CLI input error:

Router#copy running-config stertup-config

                             ^

% Invalid input detected at '^' marker.

Router#

As you can see in this example that we have typed stert on the place of startup.

You should examine the line between the command that you typed in and the error message. Somewhere in this line, you'll see a ^ character. This is used by the IOS to indicate that an error exists in the command line at that spot.

Error:-% Incomplete command.

This error indicates that you have not entered all the necessary parameters for the command. The syntax of the command is correct, but more parameters are necessary.

Router#copy running-config % Incomplete command.

Router#copy running-config ? startup-config Copy to startup configuration tftp: Copy to current system configuration

Router#copy running-config startup-config Destination filename [startup-config]? Building configuration... [OK]

Router#

In this case, you can use the context-sensitive help feature to help you figure out what parameter or parameters you forgot.

Error:-% Ambiguous command: "show i"

You will see this error message if you do not type in enough characters to make a command or parameter unique.

Router#show i

% Ambiguous command: "show i"

Router#show i?

interfaces  ip  ipv6

Router#show i

In this example, apparently, more than one parameter for the show command begins with the letter i. As shown above you can use context-sensitive help to figure out what parameter to use.

% Unknown command or computer name, or unable to find computer address

If you enter a command that the IOS does not understand, you'll see this error message. If you see this, use the context-sensitive help to figure out the correct command to enter.