ucf

ucf [options] new-package-file config-file
ucf [options] --install new-package-file config-file
ucf [options] --purge config-file
ucf [options] --remove-dead-links config-file

--install
    Installs or updates a configuration file, managing differences with the currently installed version.

--purge
    Purges (removes) a configuration file and its associated backup/tracking files.

--remove-dead-links
    Removes dead symbolic links associated with the configuration file.

--xattr
    Sets extended attributes on the configuration files.

--new-package
    Specifies the new configuration file provided by the package.

--config-file
    Specifies the path to the currently installed configuration file to be managed.

--verbose
    Enables verbose output, showing more details about the operations being performed.

--no-action, -n
    Performs a dry run; shows what would be done without making any actual changes.

--debconf-compatible
    Makes the output and interaction style more compatible for scripting with Debconf.

--help
    Displays a help message and exits.

--version
    Displays version information and exits.

The ucf (Update Configuration File) command is a powerful utility primarily used in Debian-based Linux distributions to handle configuration files during package installations and upgrades. Its main purpose is to prevent the loss of user-modified settings when a package provides a new version of a configuration file. ucf intelligently compares the new configuration file from the package with the currently installed version and the original version installed by the package.

Based on this comparison, it can prompt the user to decide whether to keep their local changes, install the new version, or view the differences. This interactive approach ensures that system administrators retain control over their configurations while benefiting from package updates. ucf is often invoked by package maintainer scripts (e.g., in `postinst` scripts) to manage configuration files like /etc/ssh/sshd_config or /etc/default/grub, making the upgrade process smoother and safer for system administrators.

ucf is primarily designed for and used within Debian-based Linux distributions. Its functionality heavily relies on `dpkg`'s file tracking mechanisms. In interactive mode, it may pause package upgrades to prompt for user input, which can be mitigated using `debconf`'s pre-seeding capabilities. Incorrect usage, particularly manual invocation, can lead to unintended configuration changes or loss of data.

When ucf detects differences between the new package file, the original installed file, and the user's current file, it typically prompts the user for a decision. The user can choose to install the maintainer's version, keep their local version, view the differences, or perform a three-way merge. This interaction is often mediated through debconf, which allows for pre-seeding answers to ensure unattended upgrades.

ucf is commonly invoked by maintainer scripts (e.g., `postinst` or `prerm`) within Debian packages. Package maintainers specify which configuration files ucf should manage, allowing for consistent and predictable behavior across the distribution. This integration ensures that package upgrades handle configuration files gracefully, balancing system stability with user customization.

ucf was developed specifically for the Debian project to address the complex challenge of configuration file management during package upgrades. Before utilities like ucf, package upgrades often involved overwriting user-modified configuration files or leaving `.dpkg-old` files, which could lead to manual merging nightmares.

ucf aims to automate and streamline this process by providing a robust mechanism for tracking and merging changes, giving users options without breaking their custom setups. It became an integral part of the Debian packaging ecosystem, significantly improving the user experience for system administrators maintaining their systems. Its design philosophy centers around preserving user choices while ensuring that systems benefit from updated configurations.

dpkg(1), debconf(7), ucfr(1)