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|Maximum RPM: Taking the Red Hat Package Manager to the Limit|
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The way we direct RPM in the build process is to create a spec file. As
we saw in the previous chapter, the spec file contains eight different
sections, most of which are required. Let's go through each section and
cdplayer's spec file as we go.
In general, the preamble consists of entries, one per line, that start with a tag followed by a colon, and then some information. For example, the line starting with "Summary:" gives a short description of the packaged software that can be displayed by RPM. The order of the lines is not important, as long as they appear in the preamble.
Let's take a look at each line and see what function it performs:
The name line defines what the package will actually be called. In general, it's a good idea to use the name of the software. The name will also be included in the package label, and the package filename.
The version line should be set to the version of the software being packaged. The version will also be included in the package label, and the package filename.
The release is a number that is used to represent the number of times the software, at the present version, has been packaged. You can think of it as the package's version number. The release is also part of the package label and package filename.
The copyright line is used to hold the packaged
software's copyright information. This makes it easy to determine
which packages can be freely redistributed, and which cannot. In
cdplayer is made available under the
terms of the GNU General Public License, so we've put
GPL on the line.
The group line is used to hold a string that
defines how the packaged software should be grouped with other
packages. The string consists of a series of words separated by
slashes. From left to right, the words describe the packaged
software more explicitly. We grouped
Applications, because it is an
application, and then under
Sound, since it is
an application that is sound-related.
The source line serves two purposes:
To document where the packaged software's sources can be found.
To give the name of the source file as it exists in the
In our example, the
cdplayer sources are
contained in the file
is available from
ftp.gnomovision.com, in the
/pub/cdplayer. RPM actually ignores
everything prior to the last filename in the source line, so the
first part of the source string could be anything you'd like.
Traditionally, the source line usually contains a
Uniform Resource Locator, or URL.
The URL line is used to contain a URL, like the source line. How are they different? While the source line is used to provide the source filename to RPM, the URL line points to documentation for the software being packaged.
The distribution line contains the name of the product which the packaged software is a part of. In the Linux world, the operating system is often packaged together into a "distribution", hence the name. Since we're using a fictional application in this example, we've filled in the distribution line with the name of a fictional distribution. There's no requirement that the spec file contain a distribution line, so individuals will probably omit this.
The vendor line identifies the organization that distributes the software. Maintaining our fictional motif, we've invented fictional company, White Socks Software, to add to our spec file. Individuals will probably omit this as well.
The packager line is used to identify the organization that actually packaged the software, as opposed to the author of the software. In our example, we've chosen the greatest packager of them all, Santa Claus, to work at White Socks Software. Note that we've included contact information, in the form of an e-mail address.
The description line is a bit different, in that it starts with a percent sign. It is also different because the information can take up more than one line. It is used to provide a more detailed description of the packaged software than the summary line.
At the top of the spec file, there are three lines, each starting with a pound sign. These are comments and can be sprinkled throughout the spec file to make it more easily understood.
With the preamble, we provided a wealth of information. The majority of this information is meant for human consumption. Only the name, version, release, and source lines have a direct bearing on the package building process. However, in the %prep section, the focus is entirely on directing RPM through the process of preparing the software for building.
If the %prep section looks like a script, that's
because it is. Any sh constructs can be used here,
including expansion of environment variables (Like the
$RPM_BUILD_DIR variable defined by RPM), and piping the
output of zcat through tar.
In this case, we perform a recursive delete in the build directory to remove any old builds. We then uncompress the gzipped tar file, and extract its contents into the build directory.
Quite often, the sources may require patching in order to build properly. The %prep section is the appropriate place to patch the sources, but in this example, no patching is required. Fear not, however, as we'll explore patching in all its glory in Chapter 20, when we build a more complex package.
While the %prep section as we've described it isn't that difficult to understand, RPM provides macros to make life even easier. In this simple example, there's precious little that can be made easier, but macros will prevent a wealth of headaches when it's time to build more complex packages. The macro we'll introduce here is the %setup macro.
That is simpler than our %prep section, so let's use the %setup macro instead. The %setup macro has a number of options to handle many different situations. For more information on this and other macros, please see the section called Macros: Helpful Shorthand for Package Builders in Chapter 13.
In our example here, the %prep section is complete. Next comes the actual build.
Thanks to the make utility, only one command is
necessary to build the
cdplayer application. In
the case of an application with more esoteric build requirements, the
%build section could get a bit more interesting.
cdplayerapplication is a good example of this:
If the application doesn't have a means of automatically installing itself, it will be necessary to create a script to do so, and place it in the %install section.
The line starting with %doc is an example of RPM's
handling of different file types. As you might guess,
%doc stands for documentation. The
%doc directive is used to mark files as being
documentation. In the example above, the
file will be placed in a package-specific directory, located in
/usr/doc, and called
cdplayer-1.0-1. It's also possible to mark files
as documentation and have them installed in other directories. This
is covered in more detail in the section called The %doc Directive in Chapter 13.
The rest of the files in the example are shown with complete paths. This is necessary as the files will actually be installed in those directories by the application's makefile. Since RPM needs to be able to find the files prior to packaging them, complete paths are required.
Since RPM automates so many aspects of software installation, it's easy to fall into the trap of assuming that RPM does everything for you. Not so! One task that is still a manual process is creating the file list. While it may seem at first glance, that it could be automated somehow, it's actually a more difficult problem than it seems.
Since the majority of an application's files are installed by its makefile, RPM has no control over that part of the build process, and therefore, cannot automatically determine which files should be part of the package. Some people have attempted to use a modified version of install that logs the name of every file it installs. But not every makefile uses install, or if it does, uses it sporadically.
Another approach tried was to obtain a list of every file on the
build system, immediately before and after a build, and use the
differences as the file list. While this approach will certainly
find every file that the application installed, it can also pick up
extraneous files, such as system logs, files in
/tmp, and the like. The only way to begin to
make this approach workable would be to do nothing else on the build
system, which is highly inconvenient. This approach also precludes
building more than one package on the system at any given time.
At present, the best way to create the file list is to read the makefile to see what files it installs, verify this against the files installed on the build system, and create the list.
Since our example spec file is somewhat simplistic, it's missing two sections that might be used in more complex situations. We'll go over each one briefly here. More complete information on these sections will be covered at various points in the book.
One missing section to our spec file is the section that would define one or more of four possible scripts. The scripts are executed at various times when a package is installed or erased.
The scripts can be executed:
Before a package is installed.
After a package is installed.
Before a package is erased.
After a package is erased.
We'll see how these scripts are used in Chapter 20.
The other missing section has the rather descriptive title of
%clean. This section can be used to clean up any
files that are not part of the application's normal build area. For
example, if the application creates a directory structure in
/tmp as part of its build, it will not be
removed. By adding a sh script to the
%clean section, such situations can be handled
gracefully, right after the binary package is created.
For more information on the environment variables used in the build-time scripts, please refer to the section called Build-time Scripts in Chapter 13.
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