Unexpected effect of the default build configuration in xcode

This was something that was tripping up my own Continuous Integration scripts as well and caused me to be very specific in xcodebuild commands when it came to determining which of my configurations to run. It wasn't until several months later I dug into the behavior to determine what was actually going on.

TL;DR version: The behavior you are observing is functioning correctly given the commands you are firing off to xcodebuild.

...now for the 'why'

As it turns out, the defaultConfigurationName value is a setting that was added to pre-Xcode 4 projects as a means to describe which of potentially many build configurations should be leveraged by command-line builds when a configuration hadn't been specified...exactly as its name suggests and entirely consistent with the expectation you've expressed in your question.

There is some nuance in that history -- specifically, why is Xcode 4 important? Xcode 4 was the first version of the developer toolchain that introduced the concept of "Schemes" as a manner of defining different ways in which an application would be built. Per Apple's definition (emphasis added):

An Xcode scheme defines a collection of targets to build, a configuration to use when building, and a collection of tests to execute.

Peering into the .xcscheme file associated with a project we see a bunch of configuration data consistent with the settings we see exposed in the Scheme editor of the modern Xcode interface. There is one section in the root of /Scheme for each of the six items that appear in the left side bar of the scheme editor accessible by selecting "Edit Scheme" from the scheme picker next to the Stop button:

• BuildAction maps to "Build"
• TestAction maps to "Test"
• LaunchAction maps to "Run"
• ProfileAction maps to "Profile"
• AnalyzeAction maps to "Analyze"
• ArchiveAction maps to "Archive"

Within all but the "Build" item in Xcode we find a "Build Configuration" drop down list. The value of this item is reflected in the .xcscheme file under the respective action's setting for buildConfiguration. Take note of the build configuration defined for what the "Run" action (via the Xcode Scheme Editor Interface) or the "LaunchAction" in the .xcscheme file. I'd hazard a guess that the CocoaPodsExample scheme has Debug set as the Build Configuration for the Run / LaunchAction.

In your original question, your xcodebuild commands indicate that you are using a Workspace to organize your project. Per documentation for xcodebuild:

To build an Xcode workspace, you must pass both the -workspace and -scheme options to define the build. The parameters of the scheme will control which targets are built and how they are built, although you may pass other options to xcodebuild to override some param- eters of the scheme.

...because you use a Workspace you are compelled to provide a -scheme parameter and therefore are leveraging the buildConfiguration value that is found under the "Run" item in the Xcode Scheme Editor, equivalently located in the "LaunchAction:buildConfiguration" setting in the Scheme's .xcscheme file. Whatever build configuration is defined in this scheme takes precedence over not setting -configuration in the xcodebuild command itself.

So why does defaultBuildConfiguration still exist?

There are still legacy projects and even some development teams that have not adopted workspaces in their Xcode projects. Were you to create a brand new test project, not incorporate it in a Workspace, and run xcodebuild build | grep "CONFIGURATION" from the folder that contains the .xcodeproj you'd see the defaultBuildConfiguration get picked up.

I created a Single View iOS Application named "Demo" and did nothing else to the blank project. The default project creates a scheme named "Demo" that has the Run Build Configuration set to Debug while the defaultConfigurationName defaults to Release (NOTE: There are user-specific paths in some of these commands -- If you are going to run them yourself, do be sure to update paths accordingly!):

$ xmllint --xpath //Scheme/LaunchAction/@buildConfiguration Demo.xcodeproj/xcuserdata/bmusial.xcuserdatad/xcschemes/Demo.xcscheme
        buildConfiguration="Debug"
$ grep defaultConfigurationName Demo.xcodeproj/project.pbxproj 
        defaultConfigurationName = Release;
        defaultConfigurationName = Release;
        defaultConfigurationName = Release;
$ xcodebuild build | grep "CONFIGURATION"
=== BUILD TARGET Demo OF PROJECT Demo WITH THE DEFAULT CONFIGURATION (Release) ===

The output of the BUILD TARGET has changed slightly: It now reads: DEFAULT CONFIGURATION (Release) instead of CONFIGURATION Release. The inclusion of DEFAULT and the parenthesis around the actual configuration are the only indications that xcodebuild has fallen back to the value from defaultConfigurationName. Incidentally, these same indicators are present in the output from your Workspace command equivalents.

As this is a Scheme-enabled project we can use -scheme to pull the scheme-configured build configuration, or we can explicitly override the configuration to use by setting -configuration:

$ xcodebuild -scheme Demo build | grep "CONFIGURATION"
=== BUILD TARGET Demo OF PROJECT Demo WITH CONFIGURATION Debug ===
=== BUILD TARGET DemoTests OF PROJECT Demo WITH CONFIGURATION Debug ===
$ xcodebuild -configuration Debug build | grep "CONFIGURATION"
=== BUILD TARGET Demo OF PROJECT Demo WITH CONFIGURATION Debug ===

The output of these commands goes back to the normal WITH CONFIGURATION style and the configuration not wrapped by parenthesis.

Hopefully this gives you a lens into what is going on with your own projects and you can adjust your command-line scripts accordingly. Once I sorted this out, I was able to be less specific in my CI scripts so that as the project evolved there was less off a need to update the build scripts in parallel with project configuration changes. Naturally, depending on the nature of your own project and the frequency with which build configuration may or may not change, you may want to defer to being very prescriptive in your xcodebuild commands...that is, compel a specific configuration via -configuration. If you run into any followup questions, I'm all ears!