Insert responses to command prompts in Tclsh

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By Ivan Pepelnjak

Some IOS CLI commands (for example, the clear counter command) require user confirmations. The typeahead text-string Tcl command can be used to insert the simulated user response before an IOS CLI command is executed.

The typeahead command might not work correctly in earlier IOS releases. All the tests in this article were performed with the IOS release 12.4(15)T5.

Simulated typeahead is accepted only by the IOS commands executed through the exec Tcl command; if you execute an IOS CLI command directly from the tclsh script, Tcl cannot control its input/output and thus cannot insert the user response.

The typeahead text is consumed as needed. You can execute more than one IOS CLI command with multiple exec Tcl commands without replenishing the typeahead buffer; Cisco IOS obviously opens a single VTY session for the duration of the tclsh script and executes all IOS CLI commands requested with the exec command in that session.

For example, the following code sequence will clear the interface counters even though the clear counter command is the second command in the sequence.

Sample typeahead script

typeahead "y"
exec {show ip route}
exec {clear counters fa 0/0} 

The typeahead text shall include all required user input, including the newline character. To include the newline character in the typeahead text, use the “\n” sequence. For example, the reload command asks two questions when the running configuration has been changed:

Test#reload
System configuration has been modified. Save? [yes/no]: y
Building configuration...
[OK]
Proceed with reload? [confirm]y

To reload a router from the Tcl script you can use the following code which answers “yes” to both questions (reloading a router without saving the configuration is better done through the EEM action reload command).

Reloading the router

typeahead "y\ny"
exec {reload} 

If the typeahead command is not followed by an exec command, the typeahead text stays in the buffer and appears as user-typed input on the command line after the tclsh script completes. For example, the following script clears interface counters and executes the show ip route command after the tclsh script has completed.

typeahead "y"
exec {clear counters fa 0/0}
typeahead "show ip route\n"
This document is part of the Tclsh on Cisco IOS tutorial.If you need professional help in planning, developing or deploying Tclsh scripts in your network, contact the author or NIL's Professional Services team
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