Question: The spell Summon Greater Demon lets the caster summon a Demon and give it orders:

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> You utter foul words, summoning one demon from the chaos of the Abyss.[...] When you summon it and on each of your turns thereafter, you can issue a verbal command to it (requiring no action on your part), telling it what it must do on its next turn. [...]
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> At the end of each of the demon’s turns, it makes a Charisma saving throw. The demon has disadvantage on this saving throw if you say its true name. On a failed save, the demon continues to obey you. On a successful save, your control of the demon ends for the rest of the duration, and the demon spends its turns pursuing and attacking the nearest non-demons to the best of its ability.
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The Demon obeys the caster's orders, at least initially. As my first order, can I command the Demon to tell me its true name - thus inflicting disadvantage on the Demon's subsequent attempts to break free of my control?

Answer: 
It would appear so
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It's hard to know if it's intended or not, but it is a consistent way to interpret the rules, depending on how you interpret p.54 of the Monster Manual, which tells us that:

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> A demon can be forced to disclose its name if charmed
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There are two ways of reading this into your question:

1. This specific rule overrides the general rules of *Summon Greater Demon*, and tells us that **the only way** to get a demon to disclose its true name is by charming it. The demon under your control via the casting of the spell is not technically charmed, so you'd need to charm it in order to learn its name
2. This rule describes one possible way of learning a Demon's name, but there are others. This makes sense because the paragraph in question also lists 'ancient scrolls' as a possible way of learning a Demon's name. It's consistent that threats, bargains or **other means of control** could also be used to illicit a demon's name from the demon itself.

I'm sympathetic to the second ruling; I don't see anything to suggest that the *charmed* condition is the only way to get a demon to tell you its name. Since the spell places no limit on what the controlled demon can be commanded to do (ie. commands are limited by what the demon can do), it follows that you can command the demon to tell you its name.

This seems counterintuitive and might be unintended - it would be unusual for the spell to specify that 'the demon has disadvantage on this saving throw if you say its true name' when the spell gives the means of learning its true name, albeit at the cost of an action at the start of the hour-long casting time.

I would~~n't~~ allow it at my table, but from what I can see it's totally aligned with the RAW

*Edit*: I've been reading some [other opinions](https://www.reddit.com/r/dndnext/comments/8g5qqc/am_i_missing_something_exploiting_summon_greater/) on this, and I'm not 100% sure that it isn't intended.

It's also worth noting the absence of any rules which dissuade the caster from harming the demon, or causing it to cause harm to itself. Many creature-control spells allow the saving throw to be repeated or cause the spell to end if the caster or their allies harm the charmed creature, and explicitly forbid commands that will directly imperil the creature. This spell has no such caveats: You could literally summon a demon and command it to stab itself in the face with zero penalty. Considering that this is the extent of your control over the creature, it's consistent to infer that you could ask the demon to tell you its name.
Game mechanics wise, forcing a caster to use TWO 4th level spell slots (SGD and Charm Monster) and TWO Turns to try to get the demon to follow orders longer (and thus not kill your team) when it has one turn in between these actions to save and break free is overly punitive. Most fights are over in a few rounds. Even if the caster gets its name, it will still break free often times before the end of the battle (see an advantage, straight roll, disadvantage table, plug in the cha modifier and magic resistance trait). It will never last a full hour since it can make a save multiple times a minute depending on how many are in initiative. Plus, some of them have a 30% chance to summon another demon as an action. Think in terms of action economy. Getting a demon out, chance to save, charm (which can fail) is not fun. The player is losing out on doing everything else only to get something they will statistically lose control over shortly.

The widespread number of punitive interpretations of this spell make it not worth bothering with. Run it as written and don't bring in rules from other places. The spell is the specific rule, and it doesn't say anywhere that it should take an action to give you the name:

Player characters can say things in battle for free and take actions too. Why is the demon treated differently? It has its own initiative and should be given the same combat criteria as players who can yell "help" or "over here" or "the Wall is an illusion" while also casting a spell or swinging a maul two times and moving 30 feet. Familiars get this treatment.

Commanding it to give you its name first should be protocol or the spell is junk and nothing more than a stink bomb, in which case you're safer casting sickening radiance or any other persistent area of effect that do more damage and that you maintain control over.

DMs making it an action or have an extra save or requiring charm or having the cleric in your group throw a fit and call you evil are all ways to make people pass this spell up or leave the table. Putting all these unwritten rules into the spell ruins it.

It is there so you can access a sack of hit points (less reliably than say Summon Shadowspawn, a lower level spell), acquire access to spells at lower save DCs (watch out for that DC 12!) you may not otherwise have or increase your spells known economy at a risk, and do a little damage if you get the barlgura out, whose attacks aren't even magical, which is a big deal by the time you get this spell.

Tldr: it can say it's name if commanded on the first turn, and it doesn't take an action because players can say things too without taking an action. The spell does not state it makes a save at that time either. It only makes a save at the end of the demon's turn. More punitive interpretations make the spell horrible. It is a cool spell. Let it play. It's not going to wreck the game.