Use shell variables for variable parts of instructions

[paradigm]

Currently instructions on the website heavily uses the ~(...~) syntax to hint
end-users that they may want to consider changing something. While this is
good, a preferable alternative might be to have the user set a shell variable
to hold this value. This would allow much more of the code to be
copy/paste-able. The ~(...~) syntax can be retained to remind the end-user
to change something in case the user leaves and returns without re-setting the
variable.