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

I see several questions here.

1. “Can I write something that actually reflects this logic”

   Yes. There are a few ways you can do it. Here’s one:

   if [[ "$1" != "" ]]; then
       DIR="$1"
   else
       DIR=.
   fi
   

2. “What is the difference between this and DIR=${1-.}?”

   The syntax ${1-.} expands to . if $1 is unset, but expands like $1 if $1 is set—even if $1 is set to the empty string.

   The syntax ${1:-.} expands to . if $1 is unset or is set to the empty string. It expands like $1 only if $1 is set to something other than the empty string.

3. “Why can’t I do this? DIR="$1" || '.'”

   Because this is bash, not perl or ruby or some other language. (Pardon my snideness.)

   In bash, || separates entire commands (technically it separates pipelines). It doesn’t separate expressions.

   So DIR="$1" || '.' means “execute DIR="$1", and if that exits with a non-zero exit code, execute '.'”.