How can I make a dispenser that has unlimited arrows?

The replaceitem command is what you're looking for. It can add items directly to chests/dispensers/etc.

replaceitem block [x] [y] [z] [slot] [item] [amount] [data] [dataTag]

x,y,z should be the position of the block. Tilde notation can be used to specify a block relative to the command block

Slot refers to the slot being filled. It needs to be in the form slot.container.x

Item refers to the item, so it should be minecraft:arrow

Amount refers to the number

Data and dataTag refer to extra info about the item. Arrows don't have these values- they would be necessary if, for example, you wanted to pelt people with spruce saplings.

So the command to reload the dispenser above a command block with two arrows would be:

replaceitem block ~ ~1 ~ slot.container.0 minecraft:arrow 2

Make sure this command is on the same circuit and fires when the dispenser does.

You asked to have this for your base so I assume your playing survival. In survival there is no way to do this without hacks or other third party materials, but there is a way to get pretty close to infinite arrows in a dispenser. What you would want to do is have a dispenser attached to a bunch of hoppers filled with arrows like this: .The arrows in the hoppers will automatically refill the dispenser when arrows are shot. You can have the dispensers attached in any way you want it doesn't need to be sidewards.

You could like make the dispenser as usual but have a backup chest of arrows with redstone and stuff so it could refill when needed. Hope this helps :-)

Well you could use /clone xyz(of block) xyz(same) xyz (away) and you could put an arrow in one of the dispensers and build a Redstone clock

I came across this post while trying to use the replaceitem command to put an 8 min water breathing potion in a container. I don't know if the answer above is for a different version of Minecraft or what, but the syntax of the command in the answer above does not work for the X-Box one version of Minecraft. Here's the syntax that will work for X-Box one:

replaceitem block 3889 101 1064 slot.container 4 potion 1 20

Explanation:

replaceitem: the name of the command

block: part of the replaceitem command syntax

3889: the x coordinate of the container to put the item in

101: the y coordinate of the container to put the item in

1064: the z coordinate of the container to put the item in

slot.container: stating that you are wanting to put an item in a slot within the container

4: the slot number of the container you want to put the item in

potion: the type of item you are wanting to put in the slot (types are listed in the namespaceid column on the page here)

1: the number of items to put in the slot

20: the data value of the item you want to put in the slot

to use this yourself you would have to come up with the x y z coordinates of the container in your game, the slot number within the container you want to put the item in as well as the data value of the specific item you want to place.

Hope this helps.