Can I select the architecture of a program from the command line? Many of the unix executable binary programs are delivered with more than one architecture:

mac:~ mike$ file `which bc`
/usr/bin/bc: Mach-O universal binary with 2 architectures
/usr/bin/bc (for architecture x86_64):  Mach-O 64-bit executable x86_64
/usr/bin/bc (for architecture i386):    Mach-O executable i386


Is there a way I can select or force a specific architecture to run (mainly for testing to explore the effects that some larger programs or longer running processes have when executing the same workload over different codebases / memory capabilities?)
You can use the arch program to select which architecture of a universal binary to run.

If you wanted to use the 32-bit version of the executable:

arch -arch i386 /usr/bin/bc


And if you wanted the 64-bit version of the executable:

arch -arch x86_64 /usr/bin/bc


Note this doesn't work with applications (folders that end in .app and have manifests and what not so OS X recognizes them as applications), just universal binaries. If you had a app with multiple architectures you'd want to use Launch Services to launch it with a specific architecture.