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Smart Upgrades

The rpm command really shines in the upgrade process. Not only can you upgrade a package with one simple command, but the rpm command has some built-in smarts that really help.
When upgrading, the rpm command checks each file in the package. It actually compares checksums of the files. An MD5 checksum is a small value computed from the data in a file. Any change to a file results in a different checksum value.
The rpm command compares the checksums of three versions of each file: the version of the file from the old package, the version of the file in the new package, and the version of the file on disk.
The rpm command looks at all three versions of the files to handle the common case where you may have edited a configuration file on disk. This is where the rpm command starts working with some upgrade rules. Note that this special processing only applies to files marked as configuration files within the RPM. Chapter 9, Working with Spec Files covers how to mark files as configuration files.
If the file on disk is identical to the file in the original package, meaning you have not changed the file, the rpm command simply installs the file from the new version of the package on top of the old file.
If the original package configuration file and the new package file are the same, that is, the file has not changed between the package versions, but the configuration file has been changed on disk, the rpm command leaves that file alone. The rpm command makes the assumption that if the file hasn’t changed between versions of the package, and you have modified the file, chances are your file will work with the new package. This takes care of the common case where you have edited an application’s configuration files.
But if the file on disk is different from the original version of the file, and the file on disk is different from the new version of the file, the rpm command installs the new version of the file on top of your changed version. This is because the new version of the file is known to work with the new package. The rpm command saves your work by backing up your modified configuration file, renaming the file with a new .rpmsave extension. The rpm command also warns you that it is backing up your modified file with a message that tells you the old file name and the new file name.
If the RPM package uses the option %config(noreplace) with the file, then the rpm command will store the new file with a rpmnew extension if you have modified the file on disk.