This document describes how to build custom Drush commands to update Drupal aliases across multiple development machines. It discusses using Composer, Symfony YAML, and Drush hooks like hook_drush_help() and hook_drush_command() to create a command called "aliasup" that downloads an aliases file from a remote server. The process of creating the command is explained, including installing dependencies with Composer, parsing the YAML file, and providing help and executing the command.