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Question mark

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

The question mark is a Cisco IOS CLI help trigger that displays valid commands, keywords, or completions for the current input context.

The question mark (?) activates context-sensitive help in the Cisco IOS CLI. It shows commands for the current mode, keywords for a command, or completions for partially typed input. section 5.2.4

R1> ?                                                         #1
Exec commands:                                                #2
  <1-99>           Session number to resume                   #2
  access-enable    Create a temporary Access-List entry       #2
  access-profile   Apply user-profile to interface            #2
  clear            Reset functions                            #2
  connect          Open a terminal connection                 #2
. . .
#1 Uses context-sensitive help to view the available commands
#2 A list of available commands is displayed.

Displays the commands available in the current EXEC or configuration mode without pressing Enter. section 5.2.4

Finding a show command

At R1>, show is incomplete by itself. Type show ? to view its keywords, then show clock ? to discover detail or execute show clock with Enter.

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Explains three help uses and distinguishes question-mark listings from Tab completion and Enter execution.
Examples
Demonstrates ?, show ?, show clock ?, e?, en?, and the resulting Tab and Enter behavior.
What it assumes
Assumes basic Cisco IOS CLI interaction while teaching command modes, keywords, completions, and carriage-return notation.
What you can do after
Operate Cisco IOS CLI help to discover valid input without memorizing every available command.
How it connects
Connects Cisco IOS CLI help with EXEC mode, configuration mode, show commands, keywords, output modifiers, and command completion.
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.