How to Have Minecraft Command Blocks output comparator signal in 1.13 [closed]

In 1.9 and after, you can click on the command block and click Unconditional to change it to a conditional command block. The way those work is if the previous command block returns something that is not null, then the conditional command block will then run its command.

So say you have an execute command that runs an if command, and another command that is set to Conditional. If the if command returns true, then the rest of the execute command will run and the conditional command block will run, but if the if command returns false, then the execute command will quit, and the game will move on to the next unconditional command block, which means that the conditional command block will not run.

Hope this helps!