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I'm trying to find a more general method to create certain one-sided distributions using anydice.com, such that the values less than or equal to 0 show as 0, and the rest show normally, of a 2dy-8 dice roll.

As an example, for a 2d6-8, I will enter the following:

output (2d6-8>0)*dd1,d2,d3,d4,

while for 2d7-8, I will enter the following:

output (2d7-8>0)*dd1,d2,d3,d4,d5,d6.

These give me exactly what I want, but for higher values of y (2d12, for instance), this becomes more and more cumbersome, so I'm wondering if there is a more general (compact) way to write this out. Also, I do not want to have to make a custom function, but rather use only the single-line general "output xxx" language anydice employs.

Any help or suggestions is greatly appreciated.

You want to use the highest of NUMBER and NUMBER function in AnyDice instead, so that you don't need to manually specify the outcome based on your prior knowledge of how that roll should behave.

output [highest of 2d6-8 and 0]
output [highest of 2d7-8 and 0]
output [highest of 2d12-8 and 0]

Each of these will treat all negative results of the 2dX-Y roll as being equal to 0 instead.

In general, if AnyDice didn't happen to already have a suitable function built in (as noted in Xirema's answer), the answer would be to make a custom function. Like this, for example:

function: ROLL:n but not less than zero 
 if ROLL < 0 
 result: 0
 else 
 result: ROLL
 


output [2d12 - 8 but not less than zero]

The reason why you need a (custom or built-in) function is that if you try to use a die roll in an if statement outside of a function, AnyDice will refuse to do run the code and print an error message instead:

calculation error

Boolean values can only be numbers, but you provided "d?".

Depending on what you want, you might need to create a function.

However, if you pass that same die roll to a function that expects a number (i.e. has :n after the parameter name), then AnyDice will automatically call the function for every possible value of the roll and collect the results back into a custom die. Inside the function, the parameter (e.g. ROLL above) will be a fixed number, and thus expressions involving it (like ROLL < 0) will have a definite fixed value that can be safely used in an if statement.

(If you want to look at the individual dice in a rolled pool of two or more, you can also make the function expect a sequence by replacing the :n with :s. In this particular case it wouldn't make any difference, however, since applying math operations like - 8 to a dice pool in AnyDice will automatically add up all the dice together before doing the math. So there's no way to get the individual d12s back out of 2d12 - 8.)