How to display compiler output or custom build steps output when building with xcode?

You'll find (and you can watch during the build) the complete build output in the Log Navigator. That's the right most icon of the small icons just below the Run and Build buttons.

Pre-action output at least appears in system.log and is visible in Console.app.

EDIT: as pointed out in the comment below this answer only works for Build phase scripts, not pre action and post action scripts.

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In Xcode 8 you need need to select your latest build in "Navigator -> Report Navigator". In the main area you will be able to see the complete build log including your output.

Here is a simple "Hello world" echo