Of course, the control exerted by the excludexxx and exclusivexxx tags over package building is often too coarse. There may be packages, for example, that would build just fine on another platform, if only you could substitute a platform-specific patch file or change some paths in the %files list.
The key to exerting this kind of platform-specific control in the spec file is to use RPM's conditionals. The conditionals provide a general-purpose means of constructing a platform-specific version of the spec file during the actual build process.
There are a few things that are common to each conditional, so let's discuss them first. The first thing is that conditionals are block-structured. The second is that conditionals can be nested. Finally, conditionals can span any part of the spec file.
Every conditional is block-structured - in other words, the conditional begins at a certain point within the spec file and continues some number of lines until it is ended. This forms a block that will be used or ignored, depending on the platform the conditional is checking for, as well as the build platform itself.
Every conditional starts with a line beginning with the characters %if and is followed by one of four platform-related conditions. Every conditional ends with a line containing the characters %endif.
Ignoring the platform-related conditions for a moment, here's an example of a conditional block:
%ifos Linux Summary: This is a package for the Linux operating system %endif
It's a one-line block, but a block nonetheless.
There's also another style of conditional block. As before, it starts with a %if, and ends with a %endif. But there's something new in the middle:
%ifos Linux Summary: This is a package for the Linux operating system %else Summary: This is a package for some other operating system %endif
Here we've replaced one summary tag with another.
Conditionals can be nested -- That is, the block formed by one conditional can enclose another conditional. Here's an example:
echo "This is an i386"
echo "This is a Linux system"
echo "This is not a Linux system"
echo "This is not an i386"
In this example, the first conditional block formed by the %ifarch i386 line contains a complete %ifos -- %else -- %endif conditional. Therefore, if the build system was Intel-based, the %ifos conditional would be tested. If the build system was not Intel-based, the %ifos conditional would not be tested.
The next thing each conditional has in common is that there is no limit to the number of lines a conditional block can contain. You could enclose the entire spec file within a conditional, if you like. But it's much better to use conditionals to insert only the appropriate platform-specific contents.
Now that we have the basics out of the way, let's take a look at each of the conditionals and see how they work.
The %ifxxx conditionals are used to control the inclusion of a block, as long as the platform-dependent information is true. Here are two examples:
%ifarch i386 alpha
In this case, the block following the conditional would be included only if the build architecture was i386 or alpha.
This example would include the block following the conditional only if the operating system was Linux.
The %ifnxxx conditionals are used to control the inclusion of a block, as long as the platform-dependent information is not true. Here are two examples:
%ifnarch i386 alpha
In this case, the block following the conditional would be included only if the build architecture was not i386 or alpha.
This example would include the block following the conditional only if the operating system was not Linux.