How to use the /testfor command to detect block in an area

Question

How we use the /testfor command to detect if there's a block missing (or a new block) in an area with only one command block and comparator?

Answer 1

If you want to detect blocks, then /testforblocks is what you're looking for. /testfor is used to find entities, such as players, mobs or items. /testforblocks is used as follows:

testforblocks <x1> <y1> <z1> <x2> <y2> <z2> <x> <y> <z> [mode]

from Minecraft Wiki:

Arguments:

x1 y1 z1 and x2 y2 z2:

Specifies two opposing corners of the region to use as the pattern to test for (the "source region"). x1, z1, x2, and z2 must all be between -30,000,000 and 30,000,000 (inclusive, without the commas), and y1 and y2 must be between 0 and 255 (inclusive). May use tilde notation to specify coordinates relative to the command's position. The number of blocks in the source region must not exceed 524,288.

x y z Specifies the lower northwestern corner (the corner with the most-negative values) of the region to be checked (the "destination region"). x and z must both be between -30,000,000 and 30,000,000 (inclusive, without the commas), and y must be between 0 and 255 (inclusive). May use tilde notation to specify coordinates relative to the command's position. Source and destination regions may overlap.

mode (optional)

Specifies how to match blocks. Must be one of: all — every block in the source and destination regions must match exactly. masked — air blocks in the source region will match any block in the destination region. If not specified, defaults to all.

To use testforblocks, you will need an area on your map to compare against - this will need to be in a loaded chunk, so I would put it underground and out of sight somewhere.

Answer 2

Testfor is a command used to detect entities, which can be a player, a mob, item frame, item, etc. If using testfor you can hook another command block up to it, or use the execute command, which however, does not support data tags.

Answer 3

The command given in the accepted answer, testforblocks, was removed in version 1.13. The same functionality is now provided by the execute command.

Specifically, what would previously require

testforblocks <source_start_x> <source_start_y> <source_start_z> <source_end_x> <source_end_y> <source_end_z> <destination_x> <destination_y> <destination_z>

now instead requires

execute (if|unless) blocks <source_start_x> <source_start_y> <source_start_z> <source_end_x> <source_end_y> <source_end_z> <destination_x> <destination_y> <destination_z> (all|masked) run <command>

Note that the coordinates are used exactly the same as before:

• The six source coordinates select the start and end positions of a volume of blocks to be compared (the 'source volume').
  • Note that the coordinates are unordered corner coordinates, not a lower coordinate and a size.
• The three destination coordinates indicate the lowest northwest corner (i.e. lowest three coordinates) of the volume of blocks (the 'destination volume') to which the 'source volume' will be compared. The size of the 'destination volume' will be the same as the size of the 'source volume'.

The big advantages of the execute function compared to the old testforblocks function is that the execute function can be used to directly run a specific command as opposed to merely giving out a success/failure signal and that the blocks criterion can be combined with any of the other execute criteria, both of which can result in significantly more compact command block chains.

As before though, you will need to be mindful of where you keep your compared block areas. If the player is not supposed to see the area you are comparing against or there is a risk of the player interfering with the area (e.g. breaking blocks), then you will want to keep that area well hidden.