# Variables are undefined by default and do not need explicit declaration
 
# Check to see whether it is defined
if {![info exists var]} {puts "var is undefind at first check"}
 
# Give it a value
set var "Screwy Squirrel"
 
# Check to see whether it is defined
if {![info exists var]} {puts "var is undefind at second check"}
 
# Remove its value
unset var
 
# Check to see whether it is defined
if {![info exists var]} {puts "var is undefind at third check"}
 
# Give it a value again
set var 12345
 
# Check to see whether it is defined
if {![info exists var]} {puts "var is undefind at fourth check"}
 
puts "Done"