In Minecraft 1.9 command blocks get an overhaul.
There will be three modes of command blocks: Impulse :
Regular behavior, command gets activated via redstone Repeat : The command in this command block gets activated every tick (superfast clock) Chain : This is the one you're looking for, see below
Command blocks also have a direction now, they can face north, south, east, west, up or down, this is important for the chain command blocks.
When any command block gets activated, it will activate the chain command block that it is pointing into.
This means you can build chains out of command blocks, whose commands get activated in the order they are pointing into each other.
gaming.stackexchange.com/users/30132/Lars
This is not a good idea.
If you have commands that must be executed in a specific order, separate them with repeaters or comparators so that they execute in a specific order.
If you trigger a lot of command blocks with interfering actions at the same time, they will execute in a specific order, but you can't know what this is in advance, it may change without notice (if the server is restarted for instance), and will almost certainly cause you a lot of headaches.
gaming.stackexchange.com/users/3610/fredley
It starts with the lowest z coordinate, then runs them in order of increasing x coordinate, then starts again with the next z coordinate.
I do not know how the y coordinate is involved.
gaming.stackexchange.com/users/70248/Bloons113744
You really don't want to be using redstone dust in your command block contraptions, or in this case, even repeaters.
I discuss this further in this answer .
There are much better solutions these days, and even better solutions to come in the near future.
So for 1.8, what you want to be using is a 20Hz fill clock .
There are 2 great features about fill clocks, plus some other advantages.
The first feature is that every command runs every game tick, i.e. 20 times a second.
The second feature is that execution is well defined, at least when everything is in the same chunk.
If your command blocks are in a single line along the X or Z axis, the command blocks will be executed from low coordinate to high coordinate.
One of the other advantages is that fill clocks generally produce less lag than redstone.
But fill clocks are old hat now.
The new hotness is command block chaining, which is coming in 1.9, and is available now in snapshots.
These are even better than fill clocks because while both run at 20Hz, the new command blocks produce even less lag and command execution is even easier to determine.
If you're working with command blocks, you should be using 1.9, even though, at the time of writing, it hasn't been fully released.
gaming.stackexchange.com/users/1351/MBraedley
In fact, it is actually the lowest z, then executes that row of x, then goes +z by 1, then executes that row of x
and so on...
for y-execution, it goes bottom-up, so it starts at the lowest point, then goes to the lowest z point, the executes that row of x.
It's actually not that hard, and really compact instead of a line of repeaters running from each command block.
gaming.stackexchange.com/users/80534/SkyBlue_
If the command blocks trigger each other by placing and removing redstone blocks, wires, circuit-breaking opaque blocks and/or inputless torches, then they're predictable (at least as long as the chunks never unload -- see https://bugs.mojang.com/browse/MC-711 ).
They'll be triggered from bottom to top, then from west to east, then north to south (ascending order of Y, then X, then Z), and a command block will run each game tick if its signal has switched from off to on at least once since it last ran.
(The same rule applies to note blocks, which are the only type of output block that can run at more than 5 Hz.)
This is what makes 20Hz /setblock
clocks possible.
It's only when you use repeaters, comparators, switching torches, or moving parts that the unpredictability arises.
gaming.stackexchange.com/users/93873/Pr0methean
TL;DR :
The answer varies from where you activate the command blocks.
I did an experiment with these where all command blocks are programmed with /say
Whatever at
and then their coordinates.
Next I activated them from all sides.
The lever at 165 -245:
The lever at 169 -240:
The lever at 173 -245:
The lever at 169 -250:
Then, I made sure the test was consistent.
As you can see, the first time it activated in this order: 166 -245 168 -245 167 -245 169
-245 169 -244 169
-246 170 -245 169 -243 169 -242 169 -241 169 -247 169
-248 169 -249 172 -245 171 -245
The second time it activated in this order: 166 -245 168 -245 167 -245 169
-245 169
-244 169 -246 170 -245 169 -243 169
-242 169
-241 169 -247 169
-248 169 -249 172 -245 171 -245
Those two are exactly the same.
In conclusion, the order depends on where the signal was activated.
gaming.stackexchange.com/users/118326/Rainbow Dash
If I make some  Command Blocks  with commands (let's say  say whatever ), connect them with redstone dust ( you can place redstone dust on Command Blocks by holding   Shift ), then produce a redstone signal to trigger them at the same time, is there a specific order in which they are executed? Does it depend on coordinates, directions, or what?
Order of command block execution if triggered at the same time?
gaming.stackexchange.com
<minecraft-java-edition><minecraft-commands>
gaming.stackexchange.com/questions/109232
gaming.stackexchange.com/users/38938/Alvin Wong
You can build chains out of command blocks and you can get them to execute in a specific order. The order depends on where the signal was activated. Command blocks do get triggered in a specific order. That order can be tracked via different methods, such as knowing the numeric value of coordinates.
The community details the chain of triggering, regarding command blocks execution order.