Sometimes you’ll want to emerge a set of packages but skip one of them. For example, if you…
emerge -p world
you could get something like this…
Calculating world dependencies …done!
[ebuild U ] sys-devel/gnuconfig-20030708
[ebuild U ] sys-devel/libtool-1.4.3-r1
[ebuild U ] sys-libs/db-1.85-r1
…..
[ebuild U ] dev-perl/Digest-MD5-2.26 [2.24]
[ebuild U ] dev-perl/Digest-SHA1-2.04 [2.02]
Now suppose that you want to emerge all of these packages but, say, Digest-MD5.
To do that, simply inject the desired package, so emerge thinks that it is already installed.
emerge -i dev-perl/Digest-MD5
According to the man pages…
Injecting a package inserts a 'stub' for that package so that Portage thinks that it is installed. It is handy if you need, say, a binary version of XFree86 for esoteric hardware, or you just like to roll your own packages. You must specify a category and particular version of a package for injecting.