There are many situations where it is useful to ask the user for some information while a program is running. Tcl makes this a breeze. Check out these code snippets that demonstrate how to interact with the user and manipulate input.
# Present a message
puts "Enter a value: "
# Get input from standard input (keyboard) and store in $someVar
gets stdin someVar
# Output the results
puts "You entered: $someVar."
Note that gets also works on files not just standard input