How to execute at a player holding a specific item in Minecraft? [duplicate]

I know I'm a little late to the party, but I would use snowballs as a gun, retexture them to look like a weapon, then instantly summon a snowball at that snowball's location- the snowball will give the fireball it's exact momentum before being deleted. First off, put into the chat this command:

/scoreboard objectives add holding.gun dummy

Then, attach these commands to a repeating/chain command block string (in order)

/scoreboard players set @a holding.gun 2 {SelectedItem:{id:snowball,tag:{put any other NBT tags in here}}}

/scoreboard player remove @a[score_holding.gun_min=1] holding.gun 1

/execute @a[score_holding.gun_min=1] ~ ~ ~ execute @e[type=Snowball,tag=!snowball,r=2] ~ ~ ~ summon Fireball ~ ~ ~ {ExplosionPower:0,direction:[0.0,0.0,0.0]}

/scoreboard players tag @e[type=Snowball] add snowball

Actually, to testfor yourself holding a certain item, all you need is one command block with the following command (for example, we can test for ourself holding an arrow)

/testfor @e[type=Player] {SelectedItem:{id:"minecraft:arrow"}}

The execute command sadly does not support data tags, but can be matched up with a test for command block on a repeater clock with a secondary command of /summon, or so put as

/testfor @a {SelectedItem:{tag:{display:{Name:"Firestarter"}}}}

, then do

/summon Fireball ~ ~ ~ {ExplosionPower:0,direction:[0.0,0.0,0.0]}

You can also add particles to make it more detailed, or setblock to fire like this:

/setblock ~1 ~ ~ fire 

with the /testfor command, and do this:

/execute @e[type=Fireball] ~ ~ ~ /particle <Particle Name> ~ ~ ~ 0.25 0.25 0.25 <Speed> <Count> 

,although 0.25 can be replaced with how many blocks away from the fireball the particles can reach. You can also do:

/execute @e[type=Fireball] ~ ~ ~ /setblock ~ ~-1 ~ fire

to set the block below the fire charge on fire.

This is what you need to do...

/testfor @e {SelectedItem:{tag:{Display{Name: "Firestarter"}}}}
/execute @e {SelectedItem:{tag:{Display{Name: "Firestarter"}}}} ~ ~ ~
/summon Fireball ~ ~ ~ {ExplosionPower0,direction:[0.0,0.0,0.0]}

That should work :)