linux - When to wrap quotes around a shell variable?

Could someone tell me whether or not I should wrap quotes around variables in a shell script?

For example, is the following correct:

xdg-open $URL 
[ $? -eq 2 ]

or

xdg-open "$URL"
[ "$?" -eq "2" ]

And if so, why?

Answer

General rule: quote it if it can either be empty or contain spaces (or any whitespace really) or special characters (wildcards). Not quoting strings with spaces often leads to the shell breaking apart a single argument into many.

$? doesn't need quotes since it's a numeric value. Whether $URL needs it depends on what you allow in there and whether you still want an argument if it's empty.

I tend to always quote strings just out of habit since it's safer that way.