A variable in bash is called a parameter.
There is three different type of parameter
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)
See also Bash - Parameter Expansion ${
if [ -z ${var+x} ]; then
echo "var is unset";
else
echo "var is set to '$var'";
fi
where
or -n: string is not null
if [ -n ${var+x} ]; then
echo "var is set to '$var'";
else
echo "var is unset";
fi