Minecraft – How to use the /testfor command

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In Minecraft and in the answers I see stuff like this:

/testfor @a[x=X,y=Y,z=-Z,r=R]

Then the people say to hook it up to a /setblock command or a /fill command or etc.

What do people mean when they say "hook up to"? How do you hook up a /testfor command block to another /setblock command? I understand the whole /testfor thing except the part on how to hook it up to another command block.

Best Answer

To use a /testfor command block, simply hook it up to your circuit with a redstone comparator.

Arrangement of Command Block and Comparator

When the testfor condition is met, the comparator will output a signal (out of the end facing away from the block). Simply place another command block or a redstone circuit there and it will do what you want when you want.

Related Solutions

Minecraft – use the /testfor command In Minecraft

As per the Minecraft Wiki Commands page, the syntax of the /testfor command is either of:

/testfor <playername>
/testfor <selector>

So, e.g., a command block that fires the command /testfor notch, with a redstone comparator attached to it, would output a signal from the comparator, assuming that Notch is playing on that server.

Using the Command Block selectors syntax lets you do things like /testfor @a[r=3], which will cause the comparator to emit a signal strength equal to the number of players within three meters. Or /testfor @a[m=2], which would cause the comparator to emit a signal strength equal to the number of Creative Mode players currently on the server.

Minecraft – How to Use /testfor Command to Check Item in Player’s Inventory

It's very difficult to see whether someone has an object anywhere in their inventory. That would take 36 command blocks, one for each slot! You can, however, take the object, then see if taking the object worked.

The Commands

Set up a fast clock (but not as fast as a fill clock)
One command block does /clear @p iron_ingot 0 1 (clears 1 iron ingot from the nearest player. You can choose which player, but it must be exactly one. Also, you must use the same selector throughout all these commands.)
Run a comparator from that command block to another command block running this command: /scoreboard players add @p ironIngots 1 (Increases the player's ironIngots score by 1. Make sure you create ironIngots first!)
Create another command block that does whatever you want. In the case of giving them a diamond, you would do this: /give @p[score_ironIngots_min=10] diamond. Note the target selector; it gives the player the diamond only if their ironIngots score is 10 or greater.
The last thing you want to do is take the ironIngots score away so they don't keep getting items. Just run /scoreboard players remove @p[score_ironIngots_min=10] ironIngots 10. This removes 10 iron ingots from the player if they have an ironIngots score of 10 or greater. Any player who matches this will have received a diamond in the previous step.

How to Set Them Up

The commands listed above must run in order. It is actually very simple to put them in order. Just open the debug menu and look at the stylish new crosshair. Align the command blocks along either the green or red line so that one of those lines points down the first to the last command block.

Notes

You don't have to press a button to use this, you just have to be the nearest player. You can change the command to the nearest player within 10 blocks, nearest player on a team, etc.
The commands will take a short amount of time to finish, but for 10 iron ingots, it will only take half a second. It's one tick (1/20 of a second) per item taken away.
It will take away iron ingots even if you don't have ten. However, you can submit one, then later submit five, then submit four more later, and it will still give you the diamond. You can also submit 14 at a time and the extra 4 will be saved for later.