Gentoo portage. How to find out why some particular package is installed

You can use app-portage/genlop for this.

genlop -l --date some_date

will list all packages merged on or after that date. (You can also specify an end date.)

To get a list of packages suitable for emerge --unmerge, try something like:

genlop -ln --date 2011/10/02 | perl -ne '/>>> (.*)/ and print " =$1";'

Do double-check that list before actually unmerging, removing system packages by accident is not fun.

Another way of getting a list of things merged after a given date is looking at the BUILD_TIME saved in the portage database.

#!/bin/bash
stime=$(date -d "$1" +%s)

for dir in /var/db/pkg/*/* ; do
    if [ -f $dir/BUILD_TIME ] ; then
        btime=$(<$dir/BUILD_TIME)
        if [ $btime -ge $stime ] ; then
            package=$(basename $dir)
            category=$(basename $(dirname $dir))
            echo $category/$package
        fi
    fi
done

Call this with a date (i.e. ./script "2001/09/30 21:32") and you'll get a list of packages merged since that date.

Portage doesn't store whether a merge was a new install or an update in its database. You could reconstruct that information from the emerge.log file assuming you have all your system's history there.

A simpler way of handling all this would be to use package sets. Create a set each time you try out a new recipe, and use that to do your cleanups. (Depclean is still necessary.)

# echo dev-perl/IO-AIO > /etc/portage/sets/my_set
# emerge -a @my_set

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild  N     ] dev-perl/IO-AIO-2.33 

Would you like to merge these packages? [Yes/No] y
>>> Recording @my_set in "world" favorites file...

...
>>> Installing (1 of 1) dev-perl/IO-AIO-2.33
>>> Auto-cleaning packages...

>>> No outdated packages were found on your system.

 * GNU info directory index is up-to-date.
# emerge -a --unmerge @my_set
 * This action can remove important packages! In order to be safer, use
 * `emerge -pv --depclean <atom>` to check for reverse dependencies before
 * removing packages.

>>> These are the packages that would be unmerged:

 dev-perl/IO-AIO
    selected: 2.33 
   protected: none 
     omitted: none 

All selected packages: dev-perl/IO-AIO-2.33

>>> 'Selected' packages are slated for removal.
>>> 'Protected' and 'omitted' packages will not be removed.

Would you like to unmerge these packages? [Yes/No] 

While I would recommend lkraav's solution, here is another way:

$ ls /usr/portage/app-portage/eix/
ChangeLog  eix-0.25.5.ebuild  eix-0.29.6.ebuild  eix-0.30.1.ebuild
Manifest   eix-0.29.3.ebuild  eix-0.30.0.ebuild  metadata.xml

this will of course only give you list of all ebuilds available for eix in the portage tree (without all the nice info which eix does provide). If you are using layman more ebuilds will be available in overlays (basically in other folders).