Minecraft – How to detect any block around an entity using the /execute command in Minecraft

It will succesfully run the command, but when you check the commandblock you aren't standing above a wool block, so it says '[21:28:27] Failed to execute 'detect' as Heroblind55246'. Try to do something like this:

/execute @a ~ ~ ~ detect ~ ~-4 ~ minecraft:wool 15 say hi

I hope it will work!

You have to use a command to apply a label to the player based on their data rather than using /testfor. For example, /scoreboard can assign a label:

/scoreboard players tag @a[tag=HasItem] remove HasItem
/scoreboard players tag @a[tag=!HasItem] add HasItem {SelectedItemSlot:0,Inventory:[{Slot:0b,tag:{display:{Name:"Fire Wand"}}}]}

And you'd then target players based on their label. The correct tag for fireball damage is ExplosionPower:

/execute @a[tag=HasItem] ~ ~ ~ /summon Fireball ~2 ~1 ~ {ExplosionPower:10,direction:[0.0,0.0,0.0]}

If using 1.8, you will have to use a score instead. Objective to create:

/scoreboard objectives add HasItem dummy

Commands to run for labeling:

/scoreboard players set @a HasItem 0
/scoreboard players set @a HasItem 1 {SelectedItem:{tag:{display:{Name:"Fire Wand"}}}}

And for targeting, you'd use the score parameter:

/execute @a[score_HasItem_min=1] ~ ~ ~ /summon Fireball ~2 ~1 ~ {ExplosionPower:10,direction:[0.0,0.0,0.0]}

You may be interested in the SelectedItem compound, which holds a copy of the player's currently-held item. This way you do not need to specifically test each individual hotbar slot:

/scoreboard players tag @a[tag=!HasItem] add HasItem {SelectedItem:{tag:{display:{Name:"Fire Wand"}}}}