1.6 “Seulement”. You should understand “avoir de quelque chose” somewhat as “enjoying something”. What part he can enjoy of the sky isn’t solely that which shines through the hole in the shutter.

As Stéphane suggested, it implies the “exclusivity” of what follows. I will elaborate with two distinct examples, and then discuss your quote.

First Example

Je n’ai que ça à faire

is the same as:

J’ai seulement ça à faire

which you can rephrase as:

Je n’ai rien d’autre à faire.

All of them being three ways of saying you have nothing else to do.

Second Example:

“Il ne fait que manger et dormir“

translates conveniently into:

“He does nothing but eating and sleeping

You can see a symmetrical structure that clarifies the translation between both languages. Nonetheless it rephrases awkwardly in french into the [ seulement / rien d’autre ] forms:

“Il ne fait rien d’autre que manger et dormir” where you still have to use the “que” after the “rien d’autre”), and “Seulement il mange et il dort” where you have to invert “eating and sleeping” into the active verbs on the sentence (and it looses the frenchness in my ears).

In Your Quote

“Qui n’a pas du ciel que ce qui brille par le trou du volet”

Is actually phrased in a complex style; I had to think it twice or thrice myself because there may appear to be more negatives than what there really are. Consequently, I have to slightly disagree with Mr. Stéphane, and I believe the context proves me right:

I would translate it in a rough non-poetic manner as “The only pice of sky he [experienced/watched/enjoyed/etc.] is none but the one that shines through the hole on his shutter”

The text speaks of “civilised” men as having a “cabined spirit [that] longs for liberty”, (that) “pines to stand in the great free sunlight”; but one might argue that (men) only “loves to fancy himself a wanderer” as no one is really willing to cut “the ties of civilised existence”, i.e. the “comforts […] of the life whose security is ample compensation for its monotony”. And yet we (men, humans) long for freedom, sunlight, liberty, and so on, as our ancestors did, the first humans, as we are seduced to throw the status quo, the ideology of our “peaceful years behind”, in favor of the wilderness.

Clearly, this archetype of a “civilised man” hasn’t enjoyed sunlight but for the scarce amount he gets thought the hole in his shutter. This implies that his windows and shutters are closed, leaving him in darkness and confinement, in an hyperbolic and yet totally allegorical manner. That’s why this man longs for “sunlight in the open”.

Lastly, and having nothing to do with your question, but with the text: does modern man “fancies himself as a wanderer” and “pines to stand in the great free sunlight”, because there is an anthropological connection with our ancestry, our animality; OR because we romanticise the concept, by contrast to our current lifestyle?

Quelle est la nature de que dans ‘qui n’a pas du ciel que ce qui brille par le trou du volet’

Que ne doit pas être considéré isolément. Il fait partie de la locution restrictive ne … que, adverbe de perspective.

Que signifie cette phrase ?

Qui à une vision complète du ciel et pas la vision limitée que l’on aurait à travers le trou d’un volet si l’on était dans l’obscurité d’une pièce aux volets clos.

Je ne comprends pas la métaphore dans le sujet. Comment peut quelqu’un posséder du ciel ?

Avoir ne signifie pas posséder ici mais plutôt avoir devant les yeux, percevoir.

Je présuppose que les volets sont fermés. Ainsi, peu d’éclairage perce les trous des volets.

Oui. On ne peut apercevoir qu’une partie limitée du ciel à travers un trou présent dans un volet.

Une expression proche est par le petit bout de la lorgnette, qui signifie que l’on a pas une vision d’ensemble de son environnement mais seulement une vision limitée (et grossie dans ce dernier cas).