When we talk about packages, we often mean software titles that are available to the Gentoo users through the Portage tree. The Portage tree is a collection of ebuilds, files that contain all information Portage needs to maintain software (install, search, query, ...). These ebuilds reside in /usr/portage by default.

Whenever you ask Portage to perform some action regarding software titles, it will use the ebuilds on your system as a base. It is therefore important that you regularly update the ebuilds on your system so Portage knows about new software, security updates, etc.

The Portage tree is usually updated with rsync, a fast incremental file transfer utility. Updating is fairly simple as the emerge command provides a front-end for rsync:

# emerge --sync

If you are unable to rsync due to firewall restrictions you can still update your Portage tree by using our daily generated Portage tree snapshots. The emerge-webrsync tool automatically fetches and installs the latest snapshot on your system:

# emerge-webrsync

To search through the Portage tree after software titles, you can use emerge built-in search capabilities. By default, emerge --search returns the names of packages whose title matches (either fully or partially) the given search term.

For instance, to search for all packages who have "pdf" in their name:

$ emerge --search pdf

If you want to search through the descriptions as well you can use the --searchdesc (or -S) switch:

$ emerge --searchdesc pdf

When you take a look at the output, you'll notice that it gives you a lot of information. The fields are clearly labelled so we won't go further into their meanings:

*  net-print/cups-pdf
      Latest version available: 1.5.2
      Latest version installed: [ Not Installed ]
      Size of downloaded files: 15 kB
      Homepage:    http://cip.physik.uni-wuerzburg.de/~vrbehr/cups-pdf/
      Description: Provides a virtual printer for CUPS to produce PDF files.
      License:     GPL-2

Once you've found a software title to your liking, you can easily install it with emerge: just add the package name. For instance, to install gnumeric:

# emerge gnumeric

Since many applications depend on each other, any attempt to install a certain software package might result in the installation of several dependencies as well. Don't worry, Portage handles dependencies well. If you want to find out what Portage would install when you ask it to install a certain package, add the --pretend switch. For instance:

# emerge --pretend gnumeric

When you ask Portage to install a package, it will download the necessary source code from the internet (if necessary) and store it by default in /usr/portage/distfiles. After this it will unpack, compile and install the package. If you want Portage to only download the sources without installing them, add the --fetchonly option to the emerge command:

# emerge --fetchonly gnumeric

Many packages come with their own documentation. Sometimes, the doc USE flag determines whether the package documentation should be installed or not. You can check the existence of a doc USE flag with the emerge -vp <package name> command.

(alsa-lib is just an example, of course.)
# emerge -vp alsa-lib
[ebuild  N    ] media-libs/alsa-lib-1.0.14_rc1  -debug +doc 698 kB

The best way of enabling the doc USE flag is doing it on a per-package basis via /etc/portage/package.use, so that you get documentation only for packages that you are interested in. Enabling this flag globally is known to cause problems with circular dependencies. For more information, please read the USE Flags chapter.

Once the package installed, its documentation is generally found in a subdirectory named after the package under the /usr/share/doc directory. You can also list all installed files with the equery tool which is part of the app-portage/gentoolkit package.

# ls -l /usr/share/doc/alsa-lib-1.0.14_rc1
total 28
-rw-r--r--  1 root root  669 May 17 21:54 ChangeLog.gz
-rw-r--r--  1 root root 9373 May 17 21:54 COPYING.gz
drwxr-xr-x  2 root root 8560 May 17 21:54 html
-rw-r--r--  1 root root  196 May 17 21:54 TODO.gz

(Alternatively, use equery to locate interesting files:)
# equery files alsa-lib | less
media-libs/alsa-lib-1.0.14_rc1
* Contents of media-libs/alsa-lib-1.0.14_rc1:
/usr
/usr/bin
/usr/bin/alsalisp
(Output truncated)

When you want to remove a software package from your system, use emerge --unmerge. This will tell Portage to remove all files installed by that package from your system except the configuration files of that application if you have altered those after the installation. Leaving the configuration files allows you to continue working with the package if you ever decide to install it again.

However, a big warning applies: Portage will not check if the package you want to remove is required by another package. It will however warn you when you want to remove an important package that breaks your system if you unmerge it.

# emerge --unmerge gnumeric

When you remove a package from your system, the dependencies of that package that were installed automatically when you installed the software are left. To have Portage locate all dependencies that can now be removed, use emerge's --depclean functionality. We will talk about this later on.

To keep your system in perfect shape (and not to mention install the latest security updates) you need to update your system regularly. Since Portage only checks the ebuilds in your Portage tree you first have to update your Portage tree. When your Portage tree is updated, you can update your system with emerge --update world. In the next example, we'll also use the --ask switch which will tell Portage to display the list of packages it wants to upgrade and ask you if you want to continue:

# emerge --update --ask world

Portage will then search for newer version of the applications you have installed. However, it will only verify the versions for the applications you have explicitly installed (the applications listed in /var/lib/portage/world) - it does not thoroughly check their dependencies. If you want to update every single package on your system, add the --deep argument:

# emerge --update --deep world

Since security updates also happen in packages you have not explicitly installed on your system (but that are pulled in as dependencies of other programs), it is recommended to run this command once in a while.

If you have altered any of your USE flags lately you might want to add --newuse as well. Portage will then verify if the change requires the installation of new packages or recompilation of existing ones:

# emerge --update --deep --newuse world

Some packages in the Portage tree don't have any real content but are used to install a collection of packages. For instance, the kde package will install a complete KDE environment on your system by pulling in various KDE-related packages as dependencies.

If you ever want to remove such a package from your system, running emerge --unmerge on the package won't have much effect as the dependencies remain on the system.

Portage has the functionality to remove orphaned dependencies as well, but since the availability of software is dynamically dependent you first need to update your entire system fully, including the new changes you applied when changing USE flags. After this you can run emerge --depclean to remove the orphaned dependencies. When this is done, you need to rebuild the applications that were dynamically linked to the now-removed software titles but don't require them anymore.

All this is handled with the following three commands:

# emerge --update --deep --newuse world
# emerge --depclean
# revdep-rebuild

revdep-rebuild is provided by the gentoolkit package; don't forget to emerge it first:

# emerge gentoolkit

As we stated before, Portage is extremely powerful and supports many features that other software management tools lack. To understand this, we explain a few aspects of Portage without going into too much detail.

With Portage different versions of a single package can coexist on a system. While other distributions tend to name their package to those versions (like freetype and freetype2) Portage uses a technology called SLOTs. An ebuild declares a certain SLOT for its version. Ebuilds with different SLOTs can coexist on the same system. For instance, the freetype package has ebuilds with SLOT="1" and SLOT="2".

There are also packages that provide the same functionality but are implemented differently. For instance, metalogd, sysklogd and syslog-ng are all system loggers. Applications that rely on the availability of "a system logger" cannot depend on, for instance, metalogd, as the other system loggers are as good a choice as any. Portage allows for virtuals: each system logger provides virtual/syslog so that applications can depend on virtual/syslog.

Software in the Portage tree can reside in different branches. By default your system only accepts packages that Gentoo deems stable. Most new software titles, when committed, are added to the testing branch, meaning more testing needs to be done before it is marked as stable. Although you will see the ebuilds for those software in the Portage tree, Portage will not update them before they are placed in the stable branch.

Some software is only available for a few architectures. Or the software doesn't work on the other architectures, or it needs more testing, or the developer that committed the software to the Portage tree is unable to verify if the package works on different architectures.

Each Gentoo installation adheres to a certain profile which contains, amongst other information, the list of packages that are required for a system to function normally.

[blocks B     ] mail-mta/ssmtp (is blocking mail-mta/postfix-2.2.2-r1)

!!! Error: the mail-mta/postfix package conflicts with another package.
!!!        both can't be installed on the same system together.
!!!        Please use 'emerge --pretend' to determine blockers. 

Ebuilds contain specific fields that inform Portage about its dependencies. There are two possible dependencies: build dependencies, declared in DEPEND and run-time dependencies, declared in RDEPEND. When one of these dependencies explicitly marks a package or virtual as being not compatible, it triggers a blockage.

To fix a blockage, you can choose to not install the package or unmerge the conflicting package first. In the given example, you can opt not to install postfix or to remove ssmtp first.

You may also see blocking packages with specific atoms, such as <media-video/mplayer-bin-1.0_rc1-r2. In this case, updating to a more recent version of the blocking package would remove the block.

It is also possible that two packages that are yet to be installed are blocking each other. In this rare case, you should find out why you need to install both. In most cases you can do with one of the packages alone. If not, please file a bug on Gentoo's bugtracking system.

!!! all ebuilds that could satisfy "bootsplash" have been masked. 

!!! possible candidates are:

- gnome-base/gnome-2.8.0_pre1 (masked by: ~x86 keyword)
- lm-sensors/lm-sensors-2.8.7 (masked by: -sparc keyword)
- sys-libs/glibc-2.3.4.20040808 (masked by: -* keyword)
- dev-util/cvsd-1.0.2 (masked by: missing keyword)
- games-fps/unreal-tournament-451 (masked by: package.mask)
- sys-libs/glibc-2.3.2-r11 (masked by: profile)

When you want to install a package that isn't available for your system, you will receive this masking error. You should try installing a different application that is available for your system or wait until the package is put available. There is always a reason why a package is masked:

• ~arch keyword means that the application is not tested sufficiently to be put in the stable branch. Wait a few days or weeks and try again.
• -arch keyword or -* keyword means that the application does not work on your architecture. If you believe the package does work file a bug at our bugzilla website.
• missing keyword means that the application has not been tested on your architecture yet. Ask the architecture porting team to test the package or test it for them and report your findings on our bugzilla website.
• package.mask means that the package has been found corrupt, unstable or worse and has been deliberately marked as do-not-use.
• profile means that the package has been found not suitable for your profile. The application might break your system if you installed it or is just not compatible with the profile you use.

emerge: there are no ebuilds to satisfy ">=sys-devel/gcc-3.4.2-r4".

!!! Problem with ebuild sys-devel/gcc-3.4.2-r2
!!! Possibly a DEPEND/*DEPEND problem. 

The application you are trying to install depends on another package that is not available for your system. Please check bugzilla if the issue is known and if not, please report it. Unless you are mixing branches this should not occur and is therefore a bug.

!!! The short ebuild name "aterm" is ambiguous.  Please specify
!!! one of the following fully-qualified ebuild names instead:

    dev-libs/aterm
    x11-terms/aterm

The application you want to install has a name that corresponds with more than one package. You need to supply the category name as well. Portage will inform you of possible matches to choose from.

!!! Error: circular dependencies: 

ebuild / net-print/cups-1.1.15-r2 depends on ebuild / app-text/ghostscript-7.05.3-r1
ebuild / app-text/ghostscript-7.05.3-r1 depends on ebuild / net-print/cups-1.1.15-r2 

Two (or more) packages you want to install depend on each other and can therefore not be installed. This is most likely a bug in the Portage tree. Please resync after a while and try again. You can also check bugzilla if the issue is known and if not, report it.

!!! Fetch failed for sys-libs/ncurses-5.4-r5, continuing...
(...)
!!! Some fetch errors were encountered.  Please see above for details.

Portage was unable to download the sources for the given application and will try to continue installing the other applications (if applicable). This failure can be due to a mirror that has not synchronised correctly or because the ebuild points to an incorrect location. The server where the sources reside can also be down for some reason.

Retry after one hour to see if the issue still persists.

!!! Trying to unmerge package(s) in system profile. 'sys-apps/portage'
!!! This could be damaging to your system.

You have asked to remove a package that is part of your system's core packages. It is listed in your profile as required and should therefore not be removed from the system.

Sometimes, when you attempt to emerge a package, it will fail with the message:

>>> checking ebuild checksums
!!! Digest verification failed:

This is a sign that something is wrong with the Portage tree -- often, it is because a developer may have made a mistake when committing a package to the tree.

When the digest verification fails, do not try to re-digest the package yourself. Running ebuild foo manifest will not fix the problem; it will almost certainly make it worse!

Instead, wait an hour or two for the tree to settle down. It's likely that the error was noticed right away, but it can take a little time for the fix to trickle down the Portage tree. While you're waiting, check Bugzilla and see if anyone has reported the problem yet. If not, go ahead and file a bug for the broken package.

Once you see that the bug has been fixed, you may want to re-sync to pick up the fixed digest.