Section 8.7. Avoid Dependency Hell with yum

8.7. Avoid Dependency Hell with yum

RPM packages include all the dependency information you need. Each RPM includes headers that list any other RPMs that are required to make it work. For example, when I tried to install the GNU C Compiler (gcc) on Fedora Core 3, I got the following message:

 # rpm -i gcc-3.4.2-6.fc3.i386.rpm error: Failed dependencies:      glibc-devel >= 2.2.90-12 is needed by gcc-3.4.2-6.fc3.i386 

Thanks to the information in the header of the gcc RPM package, I now know that I need a glibc-devel RPM of a certain version or greater. So I download the glibc-devel package from my favorite Fedora mirror, and try to install that:

 # rpm -i glibc-devel-2.3.3-74.i386.rpm error: Failed dependencies:      glibc-headers is needed by glibc-devel-2.3.3-74.i386      glibc-headers = 2.3.3 is needed by glibc-devel-2.3.3-74.i386 

I'm a patient person, so I try again. I download the glibc-headers RPM and try installing that. I get the following result:

 # rpm -i glibc-headers-2.3.3-74.i386.rpm error: Failed dependencies:      kernel-headers is needed by glibc-headers-2.3.3-74.i386      kernel-headers >= 2.2.1 is needed by glibc-headers-2.3.3-74.i386 

This process, which can go on for several levels, is also known as dependency hell.

With the yum command, you can take advantage of the headers to trace back through all dependencies automatically. Because yum repositories collect these headers in a database, you can install the GNU C Compiler and all dependent packages with the following command:

 yum install gcc 

This command searches through the repositories configured in your /etc/yum.repos.d directory. As you can see from the following excerpt, the command searches through the headers for each dependency and lists the packages to be downloaded and installed with the GNU C Compiler:

 --> Processing Conflict: glibc-common conflicts glibc > 2.3.3 --> Processing Dependency: kernel-headers >= 2.2.1 for package: glibc-headers --> Processing Dependency Resolution with new changes. --> Populating transaction set with selected packages. Please wait. ---> Package glibc-common.i386 0:2.3.5-0.fc3.1 set to be updated ---> Downloading header for glibc-kernheaders to pack into transaction set. glibc-kernheaders-2.4-9.1 100% |==================| 71kB  00:00 ---> Package glibc-kernheaders.i386 0:2.4-9.1.87 set to be updated --> Running transaction check Dependencies Resolved Transaction Listing:   Install: gcc.i386 0:3.4.3-22.fc3 - updates-released Performing the following to resolve dependencies:   Install: glibc-devel.i386 0:2.3.5-0.fc3.1 - updates-released   Install: glibc-headers.i386 0:2.3.5-0.fc3.1 - updates-released   Install: glibc-kernheaders.i386 0:2.4-9.1.87 - base   Install: cpp.i386 0:3.4.3-22.fc3 - updates-released   Install: glibc.i686 0:2.3.5-0.fc3.1 - updates-released   Install: glibc-common.i386 0:2.3.5-0.fc3.1 - updates-released   Install: libgcc.i386 0:3.4.3-22.fc3 - updates-released Total download size: 28M Is this ok [y/N]: 

But to optimize how yum works on your system, you should customize the related configuration files, understand different yum subcommands, configure a nightly update, and create your own yum repository. To customize the related configuration files, read the previous annoyance.

8.7.1. The yum Subcommands

There are a substantial number of yum subcommands. Run the yum command by itself to see the variety of options available. I've illustrated my output in Fedora Core 5:

 [michael@FedoraCore64 ~]$ yum Loading "installonlyn" plugin You need to give some command usage: yum [options] < update | install | info | remove | list |     clean | provides | search | check-update | groupinstall |     groupupdate | grouplist | groupinfo | groupremove |     makecache | localinstall | erase | upgrade | whatprovides |     localupdate | resolvedep | shell | deplist > options:   -h, --help            show this help message and exit  -t, --tolerant         be tolerant of errors   -C                    run entirely from cache, don't update cache   -c  [config file]     config file location   -R  [minutes]         maximum command wait time   -d  [debug level]     debugging output level   -e  [error level]     error output level   -y                    answer yes for all questions   --version             show Yum version and exit   --installroot=[path]  set install root   --enablerepo=[repo]   enable one or more repositories (wildcards allowed)   --disablerepo=[repo]  disable one or more repositories (wildcards allowed)  --exclude=[package]    exclude package(s) by name or glob   --obsoletes           enable obsoletes processing during updates   --noplugins           disable Yum plugins [michael@FedoraCore64 ~]$ 

I'll explain some of the more important options here. For more information, see the yum HOWTO at http://www.phy.duke.edu/~rgb/General/yum_HOWTO/:

yum update

When run by itself, the yum update command checks all current packages against the listed repositories for later versions. If the packages are found (and you confirm you want installation), yum proceeds to upgrade them. If there is a new version of the kernel, this command proceeds to install the kernel. If you aren't ready to install a specific package, you can rerun the command with the --exclude option. For example, the following command excludes kernel-related packages from the update:

 yum --exclude kernel* update 

Alternatively, you could run the command with the package name of your choice. One common option is yum update yum, which ensures that you have the latest version of this utility before proceeding with updates. It may be best to first inspect all packages that can be updated with the yum check-update command.

yum install package

You can install one or more packages with this command. If the package is already installed, it is upgraded. You can use wildcards; for example, new versions of the X Server are often associated with several different packages, which you could install with the yum install xorg* command.

yum info package

You can get more information about a package with this command. The output includes the version number and repository as specified in the appropriate /etc/yum.repos.d directory.

yum remove packages

You can remove the packages you specify, along with dependencies, with this command.

yum list repository

You can list the packages available in the repositories of your choice with this command. If you do not specify a repository, this command lists all packages available from every repository configured in the /etc/yum.repos.d directory.

yum clean sub-option

Repository data can become corrupt. If you're seeing unexpected errors from your yum commands, you may try refreshing downloaded repository data; it's similar to refreshing the cache on a browser. There are five sub-options:

packages

Cleans RPMs from the /var/cache/yum subdirectory

headers

Cleans the downloaded header information related to each repository

cache

Cleans download information files with a .pickle extension that keeps yum start times to a minimum

metadata

Cleans the compressed XML files associated with some yum repositories

all

Cleans all of the preceding items

yum provides filename

If you're looking for the RPM that provides a specific file, this command can help. It's most useful when you don't already know the name of the RPM that you need.

yum search searchterm

If you're looking for a package associated with a specific topic, this command can help. For example, the yum search madwifi command can help you find all packages associated with the project of the same name.

yum check-update

This checks all repositories listed in /etc/yum.repos.d for later versions of currently installed packages. If there are packages such as kernel* or firefox* that you're not ready to update, you can use the --exclude packagename switch the next time you run the yum update command.

There are several yum commands related to groups, which are listed in the comps.xml file in the /usr/share/comps/`uname -i` directory. You can install or remove all of the mandatory and default packages in a group, along with dependencies. The details are beyond the scope of this annoyance.

8.7.2. Configuring a Nightly yum Update

The yum RPM includes its own cron job, which you can modify and activate to keep your system updated on a nightly basis. The job is located in the yum.cron file in the /etc/cron.daily directory. As defined in /etc/crontab, jobs in this directory are automatically run at 4:02 A.M. The default version of yum.cron includes some simple commands:

 if [ -f /var/lock/subsys/yum ]; then     /usr/bin/yum -R 120 -e 0 -d 0 -y update yum     /usr/bin/yum -R 10 -e 0 -d 0 -y shell /etc/yum/yum-daily.yum fi 

In other words, as long as no other yum command is already running at the noted time, this job first checks for an update of the yum package within the preceding two hours (-R 120). The second command, within the first 10 minutes (-R 10), updates the repository, as directed in yum-daily.yum. It proceeds to show only critical errors (-e 0), disable debugging (-d 0), and answer all prompts (including those that download and install) with a yes (-y).

The default directives in /etc/yum/yum-daily.yum (update, ts run, and exit) download all updates from your configured update repositories, use those updates to upgrade appropriate RPMs, and exit the yum shell.

If you want to exercise more control over certain updates, you may want to add an --exclude switch. For example, if you substitute the following for the first yum command, kernel-related packages are excluded from the update:

 /usr/bin/yum -R 120 -e 0 -d 0 -y --exclude=kernel* update 

Once you're satisfied with the cron job, you can let it run the next time your system is active at 4:02 A.M.; all you need to do is activate the yum daemon with the following command:

 chkconfig yum on 

8.7.3. Preparing Your Repository for Yum

In "Too Many Computers to Update over the Internet" in Chapter 11, I'll show how you can create your own Fedora repository from an available mirror. If you already have the packages you need, you can make it yum-aware; in other words, you can prepare your repository for yum with the right command.

Until recently, the standard for yum mifying a repository has been the yum-arch command. It's already a part of current yum RPMs. Starting with Fedora Core 3, Red Hat has adapted the createrepo command, which is part of an RPM of the same name.

The commands are simple; all you need to do is navigate to the directory with the RPMs that you want to configure as a yum repository and then run one of the following commands:

 yum-arch . createrepo . 

The yum-arch command separates the headers from the RPMs and collects them in a headers/ subdirectory. The createrepo command takes the headers, collects them in XML format, and then compresses them in the repodata/ subdirectory.

Once complete, you can share this directory using NFS, HTTP, or FTP, and connect to it from the clients of your choice.