PS а зачем перед world еще @ ?
Не принципиально, сейчас по крайней мере, но технически правильно
@ говорит о том, что следующее за ним слово так называемый «set» — набор предопределённых пакетов.
man emerge
...
set A set is a convenient shorthand for a large group of packages. Three sets are currently always available: selected, system
and world. selected contains the user-selected "world" packages that are listed in /var/lib/portage/world, and nested sets
that may be listed in /var/lib/portage/world_sets. system refers to a set of packages deemed necessary for your system to
run properly. world encompasses both the selected and system sets. [See FILES below for more information.] Other sets can
exist depending on the current configuration. The default set configuration is located in the /usr/share/portage/con‐
fig/sets directory. User sets may be created by placing files in the /etc/portage/sets/ directory (see portage(5)). Note
that a set is generally used in conjunction with --update. When used as arguments to emerge sets have to be prefixed with @
to be recognized. Use the --list-sets action to display a list of available package sets.
...