f $foo exists and is not null, return $foo. If it doesn't exist or is null, return bar. export foo=""
e... f $foo exists and is not null, return $foo. If it doesn't exist or is null, set $foo to bar and return b... If $foo exists and is not null, return bar. If it doesn't exist, or is null, return a null. export foo=... o exists and is not null, return its value. If it doesn't exist or is null, print the error message. If
