Chapter 1. Yum

Yum is the Red Hat package manager that is able to query for information about packages, fetch packages from repositories, install and uninstall packages using automatic dependency resolution, and update an entire system to the latest available packages. Yum performs automatic dependency resolution on packages you are updating, installing or removing, and thus is able to automatically determine, fetch and install all available dependent packages. Yum can be configured with new, additional repositories, or package sources, and also provides many plugins which enhance and extend its capabilities. Yum is able to perform many of the same tasks that RPM can; additionally, many of the command line options are similar. Yum enables easy and simple package management on a single machine or on groups of them.

Secure Package Management with GPG-Signed Packages

Yum provides secure package management by enabling GPG (Gnu Privacy Guard; also known as GnuPG) signature verification on GPG-signed packages to be turned on for all package repositories (i.e. package sources), or for individual repositories. When signature verification is enabled, Yum will refuse to install any packages not GPG-signed with the correct key for that repository. This means that you can trust that the RPM packages you download and install on your system are from a trusted source, such as Red Hat, and were not modified during transfer. Refer to Section 1.3, “Configuring Yum and Yum Repositories” for details on enabling signature-checking with Yum, or Section 3.3, “Checking a Package's Signature” for information on working with and verifying GPG-signed RPM packages in general.

Yum also enables you to easily set up your own repositories of RPM packages for download and installation on other machines.

Learning Yum is a worthwhile investment because it is often the fastest way to perform system administration tasks, and it provides capabilities beyond those provided by the PackageKit graphical package management tools. Refer to Chapter 2, PackageKit for details on using PackageKit.

1.1. Checking For and Updating Packages

1.1.1. Checking For Updates

You can use the yum check-update command to see which installed packages on your system have updates available.

Note: Yum and Superuser Privileges

You must have superuser privileges in order to use yum to install, update or remove packages on your system. All examples in this chapter assume that you have already obtained superuser privileges by using either the su or sudo command.

~]# yum check-update
Loaded plugins: presto, refresh-packagekit, security
PackageKit.x86_64                  0.5.3-0.1.20090915git.fc12  fedora
PackageKit-glib.x86_64             0.5.3-0.1.20090915git.fc12  fedora
PackageKit-yum.x86_64              0.5.3-0.1.20090915git.fc12  fedora
PackageKit-yum-plugin.x86_64       0.5.3-0.1.20090915git.fc12  fedora
glibc.x86_64                       2.10.90-22                  fedora
glibc-common.x86_64                2.10.90-22                  fedora
kernel.x86_64                      2.6.31-14.fc12              fedora
kernel-firmware.noarch             2.6.31-14.fc12              fedora
rpm.x86_64                         4.7.1-5.fc12                fedora
rpm-libs.x86_64                    4.7.1-5.fc12                fedora
rpm-python.x86_64                  4.7.1-5.fc12                fedora
yum.noarch                         3.2.24-4.fc12               fedora

These packages are listed as having updates available. The first package in the list is PackageKit, the graphical package manager. The first line of the above output tells us:

PackageKit — the name of the package
x86_64 — the CPU architecture the package was built for
0.5.3-0.1.20090915git.fc12 — the version of the updated package to be installed
fedora — the repository in which the updated package is located

The output also shows us that we can update the kernel (the kernel package), Yum and RPM themselves (the yum and rpm packages), as well as their dependencies (such as the kernel-firmware, rpm-libs and rpm-python packages), all using yum.

1.1.2. Updating Packages

You can choose to update a single package, multiple packages, or all packages at once. If any dependencies of the package (or packages) you update have updates available themselves, then they are updated too. To update a single package, enter yum update <package_name>:

~]# yum update glibc
Loaded plugins: presto, refresh-packagekit, security
Setting up Install Process
Resolving Dependencies
--> Running transaction check
--> Processing Dependency: glibc = 2.10.90-21 for package: glibc-common-2.10.90-21.x86_64
---> Package glibc.x86_64 0:2.10.90-22 set to be updated
--> Running transaction check
---> Package glibc-common.x86_64 0:2.10.90-22 set to be updated
--> Finished Dependency Resolution
Dependencies Resolved
======================================================================
 Package            Arch         Version          Repository     Size
======================================================================
Updating:
 glibc              x86_64       2.10.90-22       fedora       2.7 M
Updating for dependencies:
 glibc-common       x86_64       2.10.90-22       fedora       6.0 M
Transaction Summary
======================================================================
Install       0 Package(s)
Upgrade       2 Package(s)
Total download size: 8.7 M
Is this ok [y/N]:

This output contains several items of interest:

Loaded plugins: presto, refresh-packagekit, security — yum always informs you which Yum plugins are installed and enabled. Here, yum is using the presto, refresh-packagekit and security plugins. Refer to Section 1.4, “Yum Plugins” for general information on Yum plugins, or to Section 1.4.3, “Plugin Descriptions” for descriptions of specific plugins.
kernel.x86_64 — you can download and install new kernels safely with yum.
Important: Updating and Installing Kernels with Yum
yum always installs a new kernel in the same sense that RPM installs a new kernel when you use the command rpm -i kernel. Therefore, you do not need to worry about the distinction between installing and upgrading a kernel package when you use yum: it will do the right thing, regardless of whether you are using the yum update or yum install command.
When using RPM, on the other hand, it is important to use the rpm -i kernel command (which installs a new kernel) instead of rpm -u kernel (which replaces the current kernel). Refer to Section 3.2.2, “Installing” for more information on installing/updating kernels with RPM.
yum presents the update information and then prompts you as to whether you want it to perform the update; yum runs interactively by default. If you already know which transactions yum plans to perform, you can use the -y option to automatically answer yes to any questions yum may ask (in which case it runs non-interactively). However, you should always examine which changes yum plans to make to the system so that you can easily troubleshoot any problems that might arise.
If a transaction does go awry, you can view Yum's log of transactions by entering cat /var/log/yum.log at the shell prompt. The most recent transactions are listed at the end of the log file.

Updating All Packages and Their Dependencies

To update all packages and their dependencies, simply enter yum update (without any arguments):

~]# yum update

Example 1.1. Updating all packages at once

1.1.3. Updating Security-Related Packages

Discovering which packages have security updates available and then updating those packages quickly and easily is important. Yum provides the security plugin for this purpose. The security plugin extends the yum command with a set of highly-useful security-centric commands, subcommands and options. Refer to Section 1.4.3, “security (yum-plugin-security)” for specific information.

1.1.4. Preserving Configuration File Changes

You will inevitably make changes to the configuration files installed by packages as you use your Fedora system. RPM, which Yum uses to perform changes to the system, provides a mechanism for ensuring their integrity. Refer to Section 3.2.4, “Upgrading” for details on how to manage changes to configuration files across package upgrades.