prerm

prerm, short for pre-removal, is a Debian/Ubuntu package maintainer script that runs before a package is removed from the system. It is part of the set of dpkg maintainer scripts, which also include preinst, postinst, and postrm. The prerm script is invoked when a package is being removed, upgraded, or otherwise uninstalled, and its main purpose is to perform cleanup and release resources that cannot be safely removed merely by deleting files.

During its execution, prerm is given an argument that indicates the action being performed (for example, removal or upgrade). The script should handle these states gracefully, stopping services, unregistering from init or service managers, closing or freeing resources, and performing any other necessary cleanup. Typical tasks include ensuring daemons are stopped, listeners or hooks are detached, and temporary or runtime resources created by the package are cleaned up.

Best practices for writing a prerm script include making it idempotent and non-interactive, checking for the existence of resources before acting, and avoiding operations that assume the presence of files that will be removed as part of the uninstall. The script should exit with status 0 on success and a nonzero status to signal a failure condition, which can influence whether the removal proceeds.

Prerm is complemented by preinst (before installation), postinst (after installation), and postrm (after removal). Together, these scripts manage the lifecycle of a package and its interaction with the system.