Teleporting a player from the overworld to the end

1.13+:

The introduction of the new execute in has made this task pretty easy:

/execute in the_end run tp <player> <location>

See: https://minecraft.gamepedia.com/Commands/execute#in

Before 1.13:

Not really. At least not directly. (something like /tp @p 0 50 0 dimension:end doesn't exist)

The only way would be to teleport them into an end-portal block (which you can create on any fixed position you'd like) and then to wherever you want (it takes some time to get to the end, so don't teleport them immediately).

This solution I came up with, is building upon a possibility hinted in the answer of Plagiatus above.

1) Build an End Portal and use it.

2) In the overworld, set up a command block with execute @p ~ ~ ~ setblock ~ ~ ~ end_portal.

Alternative: If you want the portal to be immediately removed, you need a more complex setup. You need the following commands to be executed in this order:

2a) execute @p ~ ~ ~ summon armor_stand ~ ~1 ~ {CustomName:"PortalPoint",NoGravity:1b,Marker:1b,Invisible:1,Invulnerable:1}

2b) execute @e[name=PortalPoint,type=armor_stand] ~ ~ ~ setblock ~ ~-1 ~ end_portal

2c) execute @e[name=PortalPoint,type=armor_stand] ~ ~ ~ setblock ~ ~-1 ~ air

2d) kill @e[name=PortalPoint,type=armor_stand]

The condition type=armor_stand makes sure that the command doesn't have any side effects, even if a player or another entity is coincidentally named PortalPoint. (Don't look at me like that. Odds are that this happens.)

3) In the End, place a command block right below the obsidian layer, with tp @a[r=4] x y z, where x, y and z are the coordinates you want the player to be teleported to. The radius is picked so that it doesn't reach outside the spawn area, but still covers the player, even if he manages to move a bit before the command gets executed. You'll still want to put this in a repeat command block that's constantly active, so players spawning into the End get teleported immediately.

4) Make sure both the End Portal in the overworld and the spawn area in the End are not accessible during normal gameplay, so players can neither use the Portal for a shortcut to the end nor reach the spawn area again after they've been teleported away. (Because they would be teleported away again, which would be odd and, depending on what you're doing, might mess up things.) Actually, I'd assume you can remove the original entrance Portal safely.

This, of course, assumes that the End Portals placed by the command have the same spawn area as the End Portal you originally use. I'd strongly assume this is the case, but I've never actually used multiple different End Portals, so I have no actual experience how they interact. I didn't test because I'm still mainly using 1.8 because of Forge, so I didn't get the 1.9+ versions of my resourcepack yet. Trying it out in 1.8 would probably be pointless - I could adapt the entity IDs and such, but there's been so much changed about the End since that it might easily break or fix the setup between the versions.

No, there is no way. Not that I know of in vanilla Minecraft anyway. If you are in one dimension, in this case the Overworld, you cannot teleport to another dimension. If you enter a /tp @p ~ ~ ~, it will teleport you somewhere in the Overworld, and there's no tag or extra data that will specify the End.

Now, you could just run a command that spawns a portal exactly at the player's location, and hook up a command that will run as soon as the player is in the End, teleporting them to the desired coordinates. It would take a lot of calculating, but it could technically be done.

This is trivial in Minecraft 1.13:

/execute in the_end run tp <player> <location>

You can spawn an end portal (like the one in the stronghold) one block above you:

 /setblock ~ ~1 ~ minecraft:end_portal

You can teleport to a player who's in another dimension by being in Gamemode 3 and using your Itemsbar to teleport.