Bash: How to set a variable from argument, and with a default value

Question

It is pretty clear that with shell scripting this sort of thing can be accomplished in a huge number of ways (more than most programming languages) because of all the different

Answer 1

How about this:

DIR=.
if [ $# -gt 0 ]; then
  DIR=$1
fi

$# is the number of arguments given to the script, and -gt means "greater than", so you basically set DIR to the default value, and if the user has specified an argument, then you set DIR to that instead.