Installing .pkg with terminal ?

/usr/sbin/installer

The installer command is used to install Mac OS X installer packages to a specified domain or volume. The installer command installs a single package per invocation, which is specified with the -package parameter ( -pkg is accepted as a synonym). It may be either a single package or a metapackage. In the case of the metapackage, the packages which are part of the default install will be installed unless disqualified by a package's check tool(s).

See man installer for the full functionality. Often

sudo installer -pkg /path/to/package.pkg -target /

is all that's needed. The target is a "device" (see the man page for details or run installer -dominfo). Here / is the main drive, it also accepts devices like "/Volumes/Macintosh HD", or /dev/disk0.

Just in case it's needed; if you want to installer a .pkg without root access:

installer -pkg myapp.pkg -target CurrentUserHomeDirectory

will install the package in ~/Applications.