While RPM makes building packages as easy as possible, some of the default design decisions might not work well in a particular situation. Here are two situations where RPM's method of package building may cause problems:
You are unable to dedicate a system to RPM package building, or the software you're packaging would disrupt the build system's operation if it were installed.
You would like to package software, but you don't have root access to an appropriate build system.
Either of these situations can be resolved by directing RPM to build, install, and package the software in a different area on your build system. It requires a bit of additional effort to accomplish this, but taken a step at a time, it is not difficult. Basically, the process can be summed up by addressing the following steps:
Writing the package's spec file to support a build root.
Directing RPM to build software in a user-specified build area.
Specifying file attributes that RPM needs to set on installation.
The methods discussed here are not required in every situation. For example, a system administrator developing a package on a production system may only need to add support for a build root. On the other hand, a student wishing to build a package on a university system will need to get around the lack of root access by implementing every method described here.
Part of the process of packaging software with RPM is to actually build the software and install it on the build system. The installation of software can only be accomplished by someone with root access, so a non-privileged user will certainly need to handle RPM's installation phase differently. There are times, however, when even a person with root access will not want RPM to copy new files into the system's directories. As mentioned above, the reasons might be due to the fact that the software being packaged is already in use on the build system. Another reason might be as mundane as not having enough free space available to perform the install into the default directories.
Whatever the reason, RPM provides the ability to direct a given package to install into an alternate root. This alternate root is known as a build root. Several requirements must be met in order for a build root to be utilized:
A default build root must be defined in the package's spec file.
The installation method used by the software being packaged must be able to support installation in an alternate root.
The first part is easy. It entails adding the following line to the spec file:
Of course, you would replace <root> with the name of the directory in which you'd like the software to install.  If, for example, you specify a build root of /tmp/foo, and the software you're packaging installs a file bar in /usr/bin, you'll find bar residing in /tmp/foo/usr/bin after the build.
A note for you non-root package builders: make sure you can actually write to the build root you specify! Those of you with root access should also make sure you choose your build root carefully. For an assortment of reasons, it's not a good idea to declare a build root of "/"! We'll get into the reasons why shortly.
The final requirement for adding build root support is to make sure the software's installation method can support installing into an alternate root. The difficulty in meeting this requirement can range from dead simple to nearly impossible. There are probably as many different ways of approaching this as there are packages to build. But in general, some variant of the following approach is used:
The environment variable RPM_BUILD_ROOT is set by RPM and contains the value of the build root to be used when the software is built and installed.
The %install section of the spec file is modified to use RPM_BUILD_ROOT as part of the installation process.
If the software is installed using make, the makefile is modified to use RPM_BUILD_ROOT and to create any directories that may not exist at installation time.
Here's an example of how these components work together to utilize a build root. First, there's the definition of the build root in the spec file:
This line defines the build root as being /tmp/cdplayer. All the files installed by this software will be placed under the cdplayer directory. Next is the spec file's %install section:
%install make ROOT="$RPM_BUILD_ROOT" install
Since the software we're packaging uses make to perform the actual install, we simply define the environment variable ROOT to be the path defined by RPM_BUILD_ROOT. So far, so good. Things really start to get interesting in the software's Makefile, though:
install: cdp cdp.1.Z # chmod 755 cdp # cp cdp /usr/local/bin install -m 755 -o 0 -g 0 -d $(ROOT)/usr/local/bin/ install -m 755 -o 0 -g 0 cdp $(ROOT)/usr/local/bin/cdp # ln -s /usr/local/bin/cdp /usr/local/bin/cdplay ln -s ./cdp $(ROOT)/usr/local/bin/cdplay # cp cdp.1 /usr/local/man/man1 install -m 755 -o 0 -g 0 -d $(ROOT)/usr/local/man/man1/ install -m 755 -o 0 -g 0 cdp.1 $(ROOT)/usr/local/man/man1/cdp.1
In the example above, the commented lines were the original ones. The uncommented lines perform the same function, but also support installation in the root specified by the environment variable ROOT.
One point worth noting is that the Makefile now takes extra pains to make sure the proper directory structure exists before installing any files. This is often necessary, as build roots are deleted, in most cases, after the software has been packaged. This is why install is used with the -d option — to make sure the necessary directories have been created.
Let's see how it works:
# rpm -ba cdplayer-1.0.spec * Package: cdplayer Executing: %prep + cd /usr/src/redhat/BUILD … + exit 0 Executing: %build + cd /usr/src/redhat/BUILD + cd cdplayer-1.0 … + exit 0 + umask 022 Executing: %install + cd /usr/src/redhat/BUILD + cd cdplayer-1.0 + make ROOT=/tmp/cdplayer install install -m 755 -o 0 -g 0 -d /tmp/cdplayer/usr/local/bin/ install -m 755 -o 0 -g 0 cdp /tmp/cdplayer/usr/local/bin/cdp ln -s ./cdp /tmp/cdplayer/usr/local/bin/cdplay install -m 755 -o 0 -g 0 -d /tmp/cdplayer/usr/local/man/man1/ install -m 755 -o 0 -g 0 cdp.1 /tmp/cdplayer/usr/local/man/man1/cdp.1 + exit 0 Executing: special doc + cd /usr/src/redhat/BUILD + cd cdplayer-1.0 + DOCDIR=/tmp/cdplayer//usr/doc/cdplayer-1.0-1 + rm -rf /tmp/cdplayer//usr/doc/cdplayer-1.0-1 + mkdir -p /tmp/cdplayer//usr/doc/cdplayer-1.0-1 + cp -ar README /tmp/cdplayer//usr/doc/cdplayer-1.0-1 + exit 0 Binary Packaging: cdplayer-1.0-1 Finding dependencies... Requires (2): libc.so.5 libncurses.so.2.0 usr/doc/cdplayer-1.0-1 usr/doc/cdplayer-1.0-1/README usr/local/bin/cdp usr/local/bin/cdplay usr/local/man/man1/cdp.1 93 blocks Generating signature: 0 Wrote: /usr/src/redhat/RPMS/i386/cdplayer-1.0-1.i386.rpm + umask 022 + echo Executing: %clean Executing: %clean + cd /usr/src/redhat/BUILD + cd cdplayer-1.0 + exit 0 Source Packaging: cdplayer-1.0-1 cdplayer-1.0.spec cdplayer-1.0.tgz 82 blocks Generating signature: 0 Wrote: /usr/src/redhat/SRPMS/cdplayer-1.0-1.src.rpm #
Looking over the output from the %install section, we first see that the RPM_BUILD_ROOT environment variable in the make install command, has been replaced with the path specified earlier in the spec file on the BuildRoot: line. The ROOT environment variable used in the makefile now has the appropriate value, as can be seen in the various install commands that follow.
Note, also, that we use install's -d option to ensure that every directory in the path exists before we actually install the software. Unfortunately, we can't do this and install the file in one command.
Looking at the section labeled Executing: special doc, we find that RPM is doing something similar for us. It starts by making sure there is no pre-existing documentation directory. Next, RPM creates the documentation directory and copies files into it.
The remainder of this example is identical to that of a package being built without a build root being specified. However, although the output is identical, there is one crucial difference. When the binary package is created, instead of simply using each line in the %files list verbatim, RPM prepends the build root path first. If this wasn't done, RPM would attempt to find the files, relative to the system's root directory, and would, of course, fail. Because of the automatic prepending of the build root, it's important to not include the build root path in any %files list entry. Otherwise, the files would not be found by RPM, and the build would fail.
Although RPM has to go through a bit of extra effort to locate the files to be packaged, the resulting binary package is indistinguishable from the same package created without using a build root.
Once the necessary modifications have been made to support a build root, it's necessary for the package builder to keep some issues in mind. The first is that the build root specified in the spec file can be overridden. RPM will set the build root (and therefore, the value of $RPM_BUILD_ROOT) to one of the following values:
The value of buildroot in the spec file.
The value of buildroot in an rpmrc file.
The value following the --buildroot option on the command line.
Because of this, it's important that the spec file and the makefile be written in such a way that no assumptions about the build root are made. The main issue is that the build root must not be hard-coded anywhere. Always use the RPM_BUILD_ROOT environment variable!
Another issue to keep in mind is cleaning up after the build. Once software builds and is packaged successfully, it's probably no longer necessary to leave the build root in place. Therefore, it's a good idea to include the necessary commands in the spec file's %clean section. Here's an example:
%clean rm -rf $RPM_BUILD_ROOT
Since RPM executes the %clean section after the binary package has been created, it's the perfect place to delete the build root tree. In the example above, that's exactly what we're doing. We're also doing the right thing by using the RPM_BUILD_ROOT, instead of a hard-coded path.
The last issue to keep in mind revolves around the %clean section we just created. At the start of the chapter, we mentioned that it's not a good idea to define a build root of "/". The %clean section is why: If the build root was set to "/", the %clean section would blow away your root filesystem! Keep in mind that this can bite you, even if the package's spec file doesn't specify "/" as a build root. It's possible to use the --buildroot option to specify a dangerous build root, too:
# rpm -ba --buildroot / cdplayer-1.0.spec
But for all the possible hazards using build roots can pose for the careless, it's the only way to prevent a build from disrupting the operation of certain packages on the build system. And for the person wanting to build packages without root access, it's the first of three steps necessary to accomplish the task. The next step is to direct RPM to build the software in a directory other than RPM's default one.
Keep in mind that the build root can be overridden at build-time using the --buildroot option or the buildroot rpmrc file entry. See Chapter 12 for more details.