I'm getting different results while using -build all and -build .* to build my headless cdt projects How does eclipse see these two options? how do they differ?
The most significant difference is that all builds the workspace rather than iterating over projects. It builds the workspace with "Build All Configurations" preference set to true. Therefore non CDT projects will be built too with all.
Additionally, the order of the build may be different, all should be respecting the workspace build order, while specifying projects with regular expressions will only respect dependency order.