emergeme

emerge -v1 $(equery b  $(whereis $(objdump -p /home/petros/projects/CPlus/tracetool/build/gui/tracegui | grep NEEDED | cut -c 24-256 ) | cut -d':' -f1 ) -n | cut -d'(' -f1) -Op

emerge -a1Op $(echo -e $(equery b -e  /lib64/libcap.so.2) | nl -s= | cut -d'1' -f2)

echo  $(equery b -e  /lib64/libcap.so.2 ) | nl -s= | cut -d'1' -f2

echo $(equery b -e /lib64/libacl.so.1) | nl --body-numbering=n -s =
       sys-apps/acl-2.2.52-r1


#emerge all installed ebuilds with cuda USE flag. Also append = :)
emerge -1v $(fquery -C h cuda | sed 's/^/=/' | tail -n 9 | head -n 8)

 equery hasuse -pI perl
              View all Gentoo packages that have the "perl" USE 
flag, exluding installed packages.

       USE="perl"; for PKG in $(equery -q hasuse $USE); do echo $PKG: $(equery -q uses $PKG |grep $USE); done
              This Bash one-liner uses hasuse to find a list of 
packages that have a certain USE flag, and uses to check whether 
the flag is enabled or disabled. Modify USE="perl" to change the 
query.

This way I can emerge every package that has systemd USE flag
emerge -1 $(printf '%s\n' `equery h systemd` '\n' | cut -f1 -d']' |  awk '{print "=" $0}' | head -n -1)