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Systemd stands as the most widely used init system in modern Linux distributions, overseeing the management of system processes upon the booting of the operating system. Its primary strengths lie in speed, flexibility, and ease of configuration. In this article, we delve into the advanced aspects of working with systemd, including unit management, targets, and dependency management.

Units

Units are the fundamental building blocks in systemd, representing system services, mount points, devices, or even grouped services that are intended to run together. Each unit is defined in a configuration file, specifying its behavior and dependencies. Unit files are typically located in /etc/systemd/system or /lib/systemd/system.

Types of Units include services (.service), mount points (.mount), devices (.device), and many others. For advanced work, understanding the differences between these types and knowing when to use them is crucial.

Targets

Targets in systemd serve to group units into logical sets, akin to runlevels in traditional SysV init. They facilitate easy control over complex unit groups, such as launching desktop environments, network services, or achieving specific system states. Targets can be dependent on other targets, allowing for flexible and efficient system boot management.

Dependency Management

Dependency management is a key feature of systemd, enabling the definition of relationships between units. This includes specifying which services must be started before others, which services cannot run concurrently, and what happens when a service fails.

Directives like Requires, Wants, Before, and After in unit files allow administrators to precisely define when and how units should be started or stopped. For example, After=network.target ensures that a given service will not be launched until the network infrastructure is active.

Advanced Techniques

1. Configuration Overrides enable administrators to modify unit behavior without altering the original unit files. This is achieved by creating drop-in files in /etc/systemd/system/<unit>.service.d/.

2. Unit Templates are template files that allow the creation of service instances with slightly different configurations using a single configuration file.

3. System Targets for Specific Purposes, such as rescue.target or emergency.target, can be used for diagnostic and repair purposes in case of failure.

4. Unit Isolation is another advanced technique that allows administrators to run a unit isolated from other system processes, useful for testing and debugging.

5. Controlling systemd via the D-Bus API offers a programmatic approach to systemd, allowing dynamic management of services, units, and targets without direct usage of system tools like systemctl. This is particularly useful for applications needing low-level system interaction.

Advanced work with systemd opens doors to more efficient and flexible management of Linux systems. Understanding and utilizing units, targets, and dependency management enables administrators and developers to have finer control over how services and applications are started and managed throughout the system's lifecycle. With the increasing complexity and demands on systems, it's imperative for IT professionals to continually acquaint themselves with the latest options and best practices in systemd-based service management.