Paranoid Penguin - Using Yum for RPM Updates

where http://your.distro.homepage/GPGsignature should be replaced with a real URL.

This may seem like a hassle, but it's worth it. Several intrusions on Linux distributors' sites over the years have resulted in trojaned or otherwise compromised software packages being downloaded by unsuspecting users. Taking advantage of RPM's support for GnuPG signatures is the best defense against such skulduggery.

The other notable change in Listing 2 is that I've specified failovermethod=priority, which tells Yum to try the URLs on this list in order, starting with the one at the top. The default behavior, failovermethod=roundrobin, is for Yum to choose one of the listed URLs at random. Personally, I prefer the priority method because it lets me prioritize faster, closer mirrors over my distribution's primary site.

Now we come to the easy part, using the yum command. There are two ways to run Yum, manually from a command prompt or automatically by way of the /etc/init.d/yum startup script. If enabled, which you must do manually by issuing a chkconfig --add yum command, this script simply touches a runfile, /var/lock/subsys/yum, which the cron.daily job yum.cron checks for. If the script is enabled, that is, if the runfile exists, this cron job runs the Yum command first to check for and install an updated Yum package and then to check for and install updates for all other system packages. In so doing, Yum automatically and transparently resolves any relevant dependencies. If an updated package depends on another package, even if it didn't previously, Yum retrieves and installs the other package.

For most users, this script is powerful and useful stuff. If your environment demands meticulous change-control procedures, however, and you don't want any new software installed automatically, you should run Yum manually.

To see a list of available updates without installing anything, use yum check-update (Listing 3).

[root@iwazaru-fedora etc]# yum check-update
Gathering header information file(s) from server(s)
Server: Fedora Core 1 - i386 - Base
Server: Fedora Core 1 - i386 - Released Updates
Finding updated packages
Downloading needed headers
getting /var/cache/yum/updates-released/headers/
↪coreutils-0-5.0-34.1.i386.hdr
coreutils-0-5.0-34.1.i386 100% |=================|
↪13 kB  00:01
Name                 Arch  Version   Repo
-----------------------------------------------------
XFree86              i386  4.3.0-55  updates-released
XFree86-100dpi-fonts i386  4.3.0-55  updates-released
XFree86-75dpi-fonts  i386  4.3.0-55  updates-released
XFree86-Mesa-libGL   i386  4.3.0-55  updates-released

To install a single update, plus any other updates necessary to resolve dependencies, use yum update packagename, for example: yum update yum.

This example actually updates Yum itself. If indeed an updated version of the package Yum is available, you are prompted to go ahead and install it. If you're invoking Yum from a script and you want all such prompts to be answered y automatically, use the -y option:

yum -y update yum

The check-update command, the Yum command that is, isn't mandatory before installing updates. If you prefer, you can use yum update directly—it performs the same checks as yum check-update.

In the last sample command, we specified a single package to update, yum. To initiate a complete update session for all installed packages on your system, you simply can omit the last argument, the package specification: yum update.

After Yum checks for all available updates and calculates dependencies, it presents you with a list of all updates it intends to download. Unless you used the -y option, it asks you whether to download and install each of them.

For the sake of completeness here's a bonus tip: you also can install new packages with Yum—you probably figured that out already. For any package contained in the sources you've defined in /etc/yum.conf, you can use the command yum install packagename to install the latest version of that package, plus anything it depends on. For example, to install the FTP server package vsftpd, you'd issue this command: yum install vsftpd.