Bash - Parameter

About

A variable in bash is called a parameter.

There is three different type of parameter

variables: referenced by a name (Ex: ${myVariable})
positional parameters (ie command/script arguments): referenced by a number (Ex: ${1})
auto-set parameters: a special symbol (Special Character) that has different special meanings and uses

A parameter can be then referenced by:

A parameter is set if it has been assigned a value. (The null string is a valid value)

if [ -z ${var+x} ]; then
  echo "var is unset"; 
else 
  echo "var is set to '$var'"; 
fi

where

${var+x} is a parameter expansion which evaluates to nothing if var is unset, and substitutes the string x otherwise.
Bash - If then else

or -n: string is not null

if [ -n ${var+x} ]; then
  echo "var is set to '$var'"; 
else 
  echo "var is unset"; 
fi