Use the yum utility to modify the software on your system in four ways:

	Installing Software from a Package File
	The yum commands shown in this section use repositories as package sources. Refer to Section 9, “Manually Installing Software” for details of using yum to install software from an individual package file.

To use yum, specify a function and one or more packages or package groups. Each section below gives some examples.

For each operation, yum downloads the latest package information from the configured repositories. If your system uses a slow network connection yum may require several seconds to download the repository indexes and the header files for each package.

The yum utility searches these data files to determine the best set of actions to produce the required result, and displays the transaction for you to approve. The transaction may include the installation, update, or removal of additional packages, in order to resolve software dependencies.

=============================================================================
 Package                 Arch       Version          Repository        Size
=============================================================================
Installing:
 tsclient                i386       0.132-4          base              247 k
Installing for dependencies:
 rdesktop                i386       1.3.1-5          base              107 k

Transaction Summary
=============================================================================
Install      2 Package(s)
Update       0 Package(s)
Remove       0 Package(s)
Total download size: 355 k
Is this ok [y/N]:

Review the list of changes, and then press y to accept and begin the process. If you press N or Enter, yum does not download or change any packages.

	Package Versions
	The yum utility only displays and uses the newest version of each package, unless you specify an older version.

The yum utility also imports the repository public key if it is not already installed on the rpm keyring.

warning: rpmts_HdrFromFdno: Header V3 DSA signature: NOKEY, key ID 443E1821 
public key not available for tsclient-0.132-4.i386.rpm
Retrieving GPG key from http://mirror.centos.org/centos/RPM-GPG-KEY-CentOS-4 
Importing GPG key 0x443E1821 "CentOS-4 Key<centos-4key@centos.org>"
Is this ok [y/N]:

Check the public key, and then press y to import the key and authorize the key for use. If you press N or Enter , yum stops without installing any packages.

To ensure that downloaded packages are genuine, yum verifies the digital signature of each package against the public key of the provider. Once all of the packages required for the transaction are successfully downloaded and verified, yum applies them to your system.

	Downloads are Cached
	The yum utility keeps downloaded data files and packages for reuse. You may copy packages from the repository cache directories under /var/cache/yum/, and use them elsewhere if you wish. If you remove a package from the cache, you do not affect the copy of the software installed on your system.

4.1. Installing New Software with yum

To install the package tsclient, enter the command:

su -c 'yum install tsclient'

Enter the password for the root account when prompted.

To install the package group MySQL Database, enter the command:

su -c 'yum groupinstall "MySQL Database"'

Enter the password for the root account when prompted.

	New Services Require Activation
	When you install a service, CentOS does not activate or start it. To configure a new service to run on bootup, choose Desktop->System Settings->Server Settings->Services, or use the chkconfig and service command-line utilities.

4.2. Updating Software with yum

To update the tsclient package to the latest version, type:

su -c 'yum update tsclient'

Enter the password for the root account when prompted.

	New Software Versions Require Reloading
	If a piece of software is in use when you update it, the old version remains active until the application or service is restarted. Kernel updates take effect when you reboot the system.

To update all of the packages in the package group MySQL Database, enter the command:

su -c 'yum groupupdate "MySQL Database"'

Enter the password for the root account when prompted.

	Updating the Entire System
	To update all of the packages on your CentOS system, use the commands described in Section 6, “Updating Your System with yum”.

4.3. Removing Software with yum

To remove software, yum examines your system for both the specified software, and any software which claims it as a dependency. The transaction to remove the software deletes both the software and the dependencies.

To remove the tsclient package from your system, use the command:

su -c 'yum remove tsclient'

Enter the password for the root account when prompted.

To remove all of the packages in the package group MySQL Database, enter the command:

su -c 'yum groupremove "MySQL Database"'

Enter the password for the root account when prompted.

	Data and Configuration File Retention
	The removal process leaves user data in place but may remove configuration files in some cases. If a package removal does not include the configuration file, and you reinstall the package later, it may reuse the old configuration file.