I'd like to have side-by-side installations of XCode, and use the DEVELOPER_DIR environment variable to select between them.
It's fine to have two or more versions of Xcode installed. The usual way to select between them is to use the xcode-select command, but the man page for that command does seem to say that you can accomplish the same thing for just the current session by setting the DEVELOPER_DIR environment variable yourself. Here's what the documentation for the --switch option says:
Sets the active developer directory to the given path, for example
/Applications/Xcode-DP.app. This command must be run with superuser
permissions (see sudo(8)), and will affect all users on the system. To
set the path with-out superuser permissions or only for the current
shell session, use the DEVELOPER_DIR environment variable instead
(see ENVIRONMENT).
However, you should also heed the advice given in the ENVIRONMENT section:
Note that for historical reason, the developer directory is considered
to be the Developer content directory inside the Xcode application
(for example /Applications/Xcode.app/Contents/Developer). You can set
the environment variable to either the actual Developer contents
directory, or the Xcode application directory -- the code-select
provided shims will automatically convert the environment variable
into the full Developer content path.
So the <whatever> in your question should specify the full path to the Developer directory, not just to the Xcode application.
In other words, while xcrun clearly does depend on the value of DEVELOPER_DIR, do the tools themselves depend on it too? Is there a meaningful difference between:
• DEVELOPER_DIR=<whatever> command CC=$(xcrun -f --sdk macosx clang)
Using xcrun to find the tools so that you can invoke them yourself and worrying about when to set and unset DEVELOPER_DIR seems overly complicated. Per the xcode-select documentation, all the "shims" respect the DEVELOPER_DIR setting. "Shims" here refers to the Xcode commands in /usr/bin. As long as that environment variable is set correctly, you can just invoke '/usr/bin/clang' instead of whatever xcrun finds, and /usr/bin/clang will call through to the clang version in DEVELOPER_DIR. Presumably, you're asking this question because you're writing a build script of some kind, so there should be no problem setting DEVELOPER_DIR early in that script and then just using the /usr/bin commands.
• command CC=$(DEVELOPER_DIR=<whatever> xcrun -f --sdk macosx clang)
Programs inherit the environment from their parent processes. Here you're finding the right copy of clang to use, but you'll invoke it in an environment where DEVELOPER_DIR isn't set. Whether this matters depends on whether the tool you're using invokes any other tools. I honestly don't know whether any of the tools do invoke other tools, but it seems logical that a tool like xcodebuild would invoke many of those tools and therefore depend on DEVELOPER_DIR being set correctly.
In short, there's no downside to setting DEVELOPER_DIR as needed in your build script, and trying to specify it only in your call to xcrun seems less reliable in general than the alternative approach.
