Help, autocompletion, monitoring, shortcuts

The Cisco CLI has no mouse, no menu and no contextual help message. Everything happens on a command prompt and a handful of conventions, the same ones for decades. Knowing them turns a painful session into fast work.

This article follows a real session: first knowing where you are, then typing fast and right, making the console bearable, reading what the device holds, and finally changing then saving.

Getting your bearings: the CLI modes

It all starts with the prompt. It permanently shows two things: the device name, and the mode you are in. That mode determines which commands are accepted, and it is the number one cause of “invalid command” among beginners.

Prompt	Mode	What you do there	How to get in
`Switch>`	user EXEC	very limited read-only access	the default landing mode
`Switch#`	privileged EXEC	every `show`, reload, saving	`enable`
`Switch(config)#`	global configuration	settings for the whole device	`configure terminal`
`Switch(config-if)#`	interface configuration	settings for one specific port	`interface FastEthernet0/1`
`Switch(config-line)#`	line configuration	console, VTY, remote access	`line console 0`

You go down the hierarchy by naming what you want to configure, and back up with exit, one level at a time. CTRL + Z jumps straight back to privileged mode from any depth.

enable
Switch#configure terminal
Switch(config)#interface FastEthernet0/1
Switch(config-if)#exit
Switch(config)#

TipRead the prompt before hunting for the error

A rejected command is far more often a command typed in the wrong mode than a misspelled one. show running-config does not work in (config)#, ip address does not work in #. The useful reflex is reading the prompt before rereading the command.

Once the mode is identified, the command still has to be typed, and the device offers several aids for that.

Typing less and getting it wrong less

The question mark

? lists the options available at the point you have reached in the command.

show ?
  aaa                Show AAA values
  access-lists       List access lists
  arp                Arp table
  cdp                CDP information
  class-map          Show QoS Class Map
  clock              Display the system clock
  controllers        Interface controllers status
  crypto             Encryption module
  debugging          State of each debugging option
  dhcp               Dynamic Host Configuration Protocol status
  dot11              IEEE 802.11 show information
  file               Show filesystem information
  flash:             display information about flash: file system
  flow               Flow information
  frame-relay        Frame-Relay information
  history            Display the session command history
  hosts              IP domain-name, lookup style, nameservers, and host table
  interfaces         Interface status and configuration
  ip                 IP information
  ipv6               IPv6 information
  license            Show license information
  line               TTY line information
--More--

The --More-- marker means the list is taller than the window. Enter scrolls line by line, Space screen by screen, and Q aborts the display.

Tip? also works mid-word

Typed against the start of a word, sh?, it lists commands starting with those letters. Typed after a space, show ?, it lists the possible arguments. This is the device’s built-in documentation, available even with no network.

Autocompletion

The TAB key completes a command as soon as it is no longer ambiguous: sh becomes show, int becomes interface. As long as several commands share the same beginning, one more letter is needed to lift the ambiguity.

Abbreviations

You do not even need to complete: the CLI accepts any unambiguous abbreviation as is.

Abbreviation	Full command
`conf t`	`configure terminal`
`sh run`	`show running-config`
`sh ip int b`	`show ip interface brief`
`wr`	`write memory`
`int g0/0`	`interface GigabitEthernet0/0`
`no shut`	`no shutdown`

WarningAbbreviations help you, they do not document

They are perfect in an interactive session. In a script, a procedure or a capture meant for a colleague, write the full command: an abbreviation that turns ambiguous on another IOS version produces an error that is hard to understand for whoever reads it.

The error report

When a command is rejected, the device points at the exact spot of the problem with a caret.

sh ip routte
                 ^
% Invalid input detected at '^' marker.

The marker sits on the first character the parser could not interpret. Here everything before it is valid, the mistake really is on routte.

Keyboard shortcuts

Sequence	Description
`CTRL` + `A`	Start of line
`CTRL` + `E`	End of line
`CTRL` + `P` or `↑`	Previous command
`CTRL` + `N` or `↓`	Next command
`CTRL` + `F` or `→`	Cursor right
`CTRL` + `B` or `←`	Cursor left
`ESC` + `f`	Forward one word
`ESC` + `b`	Back one word
`CTRL` + `W`	Delete previous word
`CTRL` + `U`	Delete the whole line
`CTRL` + `R`	Redisplay the current line
`CTRL` + `Z`	Back to privileged mode
`CTRL` + `C`	Interrupt the command
`CTRL` + `SHIFT` + `6`	Forced break, for a stuck `ping` or `traceroute`
`TAB`	Complete the command

NoteThese are the Emacs shortcuts

CTRL+A start of line, CTRL+E end of line, CTRL+P previous, CTRL+N next: these are exactly the Emacs conventions, picked up by readline and therefore by bash. Reflexes built in a GNU/Linux terminal transfer straight onto a Cisco device.

Those aids are enough to type fast. The console still has to stop working against you.

Making the console usable

Three IOS default behaviours get in the way constantly. Each one is disabled with a single command.

DNS resolution of typos

In privileged mode, an unknown command is interpreted as a hostname to reach. The device then attempts a DNS lookup and locks the console for several tens of seconds.

showe
Translating "showe"...domain server (255.255.255.255)

no ip domain-lookup

Warning

This command disables all name resolution on the device, including the one you might need for a ping www.example.com. On a production device that genuinely uses DNS, prefer configuring a reachable server with ip name-server rather than cutting everything off.

Messages that cut into your typing

Every interface state change writes a message on the console, right in the middle of whatever you are typing.

%LINK-1-CHANGED: Interface FastEthernet0/1, changed state to up

Two possible answers, and the second one is almost always the right one.

line console 0
logging synchronous

Messages keep showing up, but the device redisplays your current line right below. You keep the information without losing your typing. This is the setting to apply by default on any console.

no logging console

Messages stop showing up entirely. Comfortable during a long configuration, but you also lose the useful alerts, such as an interface refusing to come up. Keep it for cases where the noise is truly unmanageable, and turn it back on afterwards.

Pagination

The --More-- prompt forces a keypress on every screen, which prevents copying an entire configuration in one block.

terminal length 0

The value 0 removes pagination for the current session only. It returns to normal on reconnect, which makes it a risk-free setting.

The console is now cooperative. Time to read the device.

Reading the device configuration

The general configuration

show running-config
Building configuration...

Current configuration : 1080 bytes
!
version 15.0
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname Switch
!
!
!
!
!
!
spanning-tree mode pvst
spanning-tree extend system-id
!
interface FastEthernet0/1
!
interface FastEthernet0/2
--More--

This command shows the configuration active in memory: hostname, services, interfaces, spanning-tree, passwords. It is the starting point of any diagnosis.

The VLAN configuration

show vlan

VLAN Name                             Status    Ports
---- -------------------------------- --------- -------------------------------
1    default                          active    Fa0/1, Fa0/2, Fa0/3, Fa0/4
                                                Fa0/5, Fa0/6, Fa0/7, Fa0/8
                                                Fa0/9, Fa0/10, Fa0/11, Fa0/12
                                                Fa0/13, Fa0/14, Fa0/15, Fa0/16
                                                Fa0/17, Fa0/18, Fa0/19, Fa0/20
                                                Fa0/21, Fa0/22, Fa0/23, Fa0/24
                                                Gig0/1, Gig0/2
1002 fddi-default                     active
1003 token-ring-default               active
1004 fddinet-default                  active
1005 trnet-default                    active

VLAN Type  SAID       MTU   Parent RingNo BridgeNo Stp  BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1    enet  100001     1500  -      -      -        -    -        0      0
1002 fddi  101002     1500  -      -      -        -    -        0      0
1003 tr    101003     1500  -      -      -        -    -        0      0
1004 fdnet 101004     1500  -      -      -        ieee -        0      0
1005 trnet 101005     1500  -      -      -        ibm  -        0      0
--More--

Every interface from Fa0/1 to Fa0/24 and Gig0/1 to Gig0/2 belongs to VLAN 1, the default one.

Note

VLANs 1002, 1003, 1004 and 1005 are reserved by the system for Token Ring and FDDI. Those technologies being obsolete, these VLANs are never used and cannot be deleted.

Filtering the output

A configuration several hundred lines long is unreadable as is. The | character filters the output of any show command.

sh run | include interface
interface GigabitEthernet0/0/0
interface GigabitEthernet0/0/1
interface GigabitEthernet0/0/2
interface Vlan1

Four filters cover nearly every need:

Filter	Effect	Example
`include`	keeps only the lines containing the pattern	`sh run \| include hostname`
`exclude`	drops the lines containing the pattern	`sh run \| exclude !`
`begin`	starts the output at the first matching line	`sh run \| begin interface`
`section`	shows the whole configuration block	`sh run \| section interface Fa0/1`

Tipsection is the most useful of the four

include returns only the matching line, which rarely helps for an interface whose configuration spans five lines. section returns the whole block, indentation included, and gives the full context straight away.

With the reading done, on to changes.

Changing, undoing and saving

Renaming the device

hostname MY_SWITCH

The change is immediate and visible in the prompt, which avoids configuring the wrong device in a room holding ten of them.

Protecting privileged mode

enable secret MY_PASSWORD

Importantsecret, not password

enable password stores the password in clear text in the configuration, readable by anyone running a show running-config. enable secret stores a hash of it. Both can coexist, and in that case secret wins. There is no reason to use enable password today.

Undoing with no

The no prefix removes, disables or clears any command entered before. It is the universal undo mechanism of IOS.

no ip address 192.168.1.1 255.255.255.0

Removes the IP address assigned to the interface.

no router ospf 1

Disables OSPF and deletes its entire configuration.

Warning

no on a whole block, such as no router ospf 1, wipes every line it contained at once. There is no undo for the undo: the only way back is the saved configuration, which is what the next section is about.

Running a command from another mode with do

show commands belong to privileged mode and are rejected in configuration mode. The do prefix runs them without leaving the current mode.

show run
                 ^
% Invalid input detected at '^' marker.

Switch(config)#do show run
Building configuration...

This avoids the exit then configure terminal round trip, and therefore avoids losing the interface context you were working in.

Saving, the step everyone forgets

A Cisco device keeps two distinct configurations, and that is the most expensive source of mistakes in the CLI.

Configuration	Where it lives	What happens to it on reboot
`running-config`	in RAM	lost
`startup-config`	in non-volatile memory	reloaded as is

Every command in this article changes the running-config, and takes effect immediately. None of them survives a power cut until it has been copied over.

copy running-config startup-config

write memory

Both commands do the same thing, the second being the historical shorthand, hence the wr in the abbreviations table. To check what will actually be reloaded:

show startup-config

TipNot saving can be a strategy

On a risky remote change, such as re-addressing or an access control list, not saving is a safety net: if you lose your hand on the device, a reboot restores the last working configuration. Only save once connectivity is confirmed.

These reflexes, modes, built-in help, filters and saving, come back as is in the other articles of the series, which apply them to VLAN, routing and remote access configuration.