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. Here / is the mount point of Macintosh HD volume. -target accepts path like &quot;/Volumes/Macintosh HD&quot;, or /dev/disk0 also.