Tclsh command line parameters

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By Ivan Pepelnjak

A Tcl script executed with the tclsh command can receive command line parameters: all words entered after the file name in the tclsh command line are passed to the Tcl script in the $argv list.

The Tcl list commands can be used to examine each individual parameter. When used in a scalar context, $argv returns the rest of the command line after the script name.

Scalar context example

The following script executes the show ip interface command and filters the outputs to include only the relevant lines (the lines with the address or protocol keywords):

ipconfig.tcl

puts [exec "show ip interface $argv ¦ include address¦protocol"] 

You can execute the script with the tclsh flash:ipconfig.tcl interface command. To simplify the user interface, configure an alias.

alias exec ipconfig flash:ipconfig.tcl

After the alias is configured, you can use ipconfig interface command to display interface’s IP address, line protocol status, NAT status and broadcast address:

router#ipconfig vlan1
Vlan1 is up, line protocol is up
  Internet address is 192.168.200.193/28
  Broadcast address is 255.255.255.255
  Helper address is not set
  Network address translation is enabled, interface in domain inside

List context example

The following script shuts down the specified interface:

shutdown.tcl

#
# ifname is set to first CLI parameter (interface name)
#
set ifname [lindex $argv 0]

if {[string equal $ifname ""]} { 
  puts "Usage: shutdown ifname"; return; 
}

if { [ catch { exec "show ip interface $ifname" } errmsg ] } {
  puts "Invalid interface $ifname, show ip interface failed"; return
}

ios_config "interface $ifname" "shutdown" 

The script performs the following steps:

  • Variable ifname is set to the value of the first command-line parameter. In most programming languages, this would be written as argv[0], Tcl extracts values from a list with the lindex command.
  • If the ifname is empty, the script aborts and prints the usage guidelines. In a more human-oriented programming language, the test would be written as if (ifname == “”). The version of Tcl running on Cisco IOS does not support string comparison operators; you have to use the string equal command to compare two strings.
  • The show ip interface ifname command is executed in a catch block. If the show command fails, the interface name is not correct and the script aborts.
  • IOS configuration commands interface ifname and shutdown are executed to disable the interface.

If you store this Tcl script into the router’s flash as shutdown.tcl and configure alias exec shutdown tclsh flash:shutdown.tcl, you can execute the command shutdown interface to shut down an interface without entering the router configuration mode.

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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