Minecraft – Testing For A Mob With Less than “x” Hearts

This command should work. If the time is always the same in the output and doesn't change, the command might be succeeding but not outputting to the console. Try using a comparator directly next to the command block to check whether it is finding the Minecart Command Block. If the comparator is lit, the command is succeeding. If the comparator is not lit, the command is either incorrect or the block could not find the entity you're looking for. If it continues outputting

[00:44:19] Unknown command. Try /help for a list of commands

then try opening the command block, pressing CTRL + A to highlight all, then pressing CTRL + C to copy. Next, break the block and place a new one, then open this new block and do CTRL + V to paste. Sometimes when I have this problem it helps to "reset" the block in this way.

Assuming you have the "sentry gun" shooting arrows, you would use the command:

/summon Arrow x y z {damage:1,Motion:[x.0,y.0,z.0]}

You would have to plug into the first set of coordinates where you want the arrows to be shooting from, and the direction the arrows are shooting in the second. TIP: The higher the number you set for your direction (second set of coordinates, the faster the arrow will travel! You can set anything for the damage (except dont get too crazy :P)

As for your "Tool Box" Idea, you can do the test for command on a redstone ticker:

testfor @e[x,y,z,r,type=Item] {Item:{id:minecraft:iron_block}}

(Unfortunately, you cant test for a custom named block/entity - at least to my knowledge) Run a comparator out of this command block, to another command block (does not NEED to be a command block). This will start a fast ticking redstone clock on another command block. This command block will have the command:

testfor @e[x,y+1,z,r,type=Item] {Item:{id:minecraft: ... }}

Make sure that the coordinates you plug into the last testfor command have y+1 the y coordinate of the testfor command before that. This means it will test for the item on top of your "tool box". Run a comparator out of that command block to start the /summon Arrow command, or in other words, start up your sentry! (The arrows are seen only as little black dots in this photo, but thats because I have the speed ramped up :)

Basically, its a system of Testing positive for an iron block at x y z - starting ticker to test for ... entity at x y+1 z - testing positive for ... entity at x y+1 z - starting ticker to shoot "sentry gun"

Hope this helps