|Maximum RPM: Taking the RPM Package Manager to the Limit|
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This example is a bit of a fairy tale, in that it went perfectly the first time. In real life, it often takes several tries to get it right.
As we alluded to earlier in the chapter, RPM can stop at various points in the build process. This allows package builders to look through the build directory and make sure everything is proceeding properly. If there are problems, stopping during the build process permits them to see exactly what is going wrong, and where. Here is a list of points RPM can be stopped at during the build:
After the %prep section.
After doing some cursory checks on the %files list.
After the %build section.
After the %install section.
After the binary package has been created.
In addition, there is also a method that permits the package builder to "short circuit" the build process and direct RPM to skip over the initial steps. This is handy when the application is not yet ready for packaging and needs some fine tuning. This way, once the package builds, installs, and operates properly, the required patches to the original sources can be created, and plugged into the package's spec file.
Of course, the fact that an application has been packaged successfully doesn't necessarily mean that it will operate correctly when the package is actually installed. Testing is required. In the case of our example, it's perfect and doesn't need such testing.  But here is how testing would proceed:
The first step is to find a test system. If you thought of simply using the build system, bzzzzt, try again! Think about it — in the course of building the package, the build system actually had the application installed on it. That is how RPM gets the files that are to be packaged: by building the software, installing it, and grabbing copies of the installed files, which are found using the %files list.
Some of you dissenters that have read the first half of the book might be thinking, "Why not just install the package on the build system using the --replacefiles option? That way, it'll just blow away the files installed by the build process and replace them with the packaged files." Well, you folks get a bzzzzt, too! Here's why.
Say, for example, that the software you're packaging installs a bunch of files — maybe a hundred. What does this mean? Well for one thing, it means that the package's %files list is going to be quite large. For another thing, the sheer number of files makes it likely that you'll miss one or two. What would happen then?
When RPM builds the software, there's no problem: the software builds, and the application's makefile merrily installs all the files. The next step in RPM's build process is to collect the files by reading the %files list, and to add each file listed to a cpio archive. What happens to the files you've missed? Nothing — they aren't added to the package file, but they are on your build system, installed just where they should be.
Next, when the package is installed using --replacefiles, RPM dutifully installs each of the packaged files, replacing the ones originally installed on the build system. The missed files? They aren't overwritten by RPM since they weren't in the package. But they're still on disk, right where the application expects them to be! If you go to test the application then, it will find every file it needs. But not every file came from the package. Bad news! Using a different system on which the application had never been built is one sure way to test for missing files.
That wraps up our fictional build. Now that we have some experience with RPM's build process, we can take a more in-depth look at RPM's build command.
Like we said, it's a fairy tale!