What must be done after adding a new repository for updates to take effect?

When a new repository is added to a Linux system that uses the Advanced Package Tool (APT), it is essential to update the local package index to include the information from the newly added repository. This ensures that the package manager is aware of the available packages and versions that can be installed from that repository.

Running the command to update the package index retrieves the latest list of available packages and their versions from the repositories configured on the system. It does not upgrade any packages or install new software but prepares the system to be able to do so in the future.

The other commands serve different purposes. For instance, upgrading packages is performed with the upgrade command but requires the index to be updated first to ensure it has the latest information. The remove command deletes installed packages, and the install command is used to add specific packages to the system. However, before using these commands effectively after adding a repository, running the update command is a necessary step to synchronize the package database with the new repository.