The initial process that runs on a Unix/Linux system is responsible for forking the rest of the processes that are needed in order to “boot” the system. This is the core function of an init-system. It is effectively the first process where execution is handed off from the kernel to “userspace”, where processes get PIDs, cpu time and network/memory/disk access via calls to the kernel or kernel modules (drivers).
The reason I put boot in quotes is because the actual boot process that happens on a modern computer involves many complex underlying systems handing off execution and initialization to the next.
The stages of booting generally go like this:
Power On -> UEFI/BIOS -> Bootloader -> Kernel -> init (systemd)
Pressing the Power On button starts with power and hardware and the first thing that happens is a POST (Power On Self Test), which ensures that the hardware is getting enough power, and everything is functioning as expected.
Special non-volatile memory (typically found on the Motherboard) has software and settings called either a legacy BIOS (Basic Input/Output System) ROM or the modern equivalent UEFI (Unified Extensible Firmware Interface). Modern UEFI implementations take over some of the roles of a bootloader, and have more features, like GUIs and network connectivity (but this all depends on the motherboard and firmware implementation).
The BIOS/Firmware looks for a boot drive, or more specifically, a special partition on the boot drive that has a program known as a bootloader. Different Operating Systems can be picky about partition schemes and filesystems, so it is best to do your research before messing with or creating boot partitions on systems and drives that you care about. The most popular bootloaders include GRUB 2, rEFInd and Clover.
The bootloader partition (usually mounted on
/booton Linux systems) includes a kernel (a compressed kernel image called vmlinuz) and a initial ram filesystem (a compressed filesystem that gets decompressed into memory to load kernel modules before init and the root
/filesystem get mounted) Hopefully the kernel, or a kernel module has the drivers needed to make the rest of your hardware (like your keyboard, mouse, graphics card, network card, etc.) accessible to userspace programs through higher layers of abstraction (like libraries) and userspace programs (like a GUI, shell or Web Interface).
These higher layers of abstraction are provided by pre-compiled libraries, which are similar to executables, but they are not run directly, they are loaded into memory by processes that link to them, which is something that happens at compile time, using something called dynamic linking – something that is facilitated through a combination of features provided by the compiler, linker, and the kernel. The idea is that linking executables to libraries that contain common code will save disk space, promote code re-use and generally make life easier. The alternative is called static linking and it basically pulls all the code into a single executable. This is up to the developer with C/C++ and the default in Go.
In Windows, libraries are called “Dynamic Linked Libraries” and end in a ‘.dll’ extension, and in Linux, they’re called a “Shared Object” or “Kernel Object” and the extension used is ‘.so’ and ‘.ko’ respectively.
In an OS model where processes are created with fork or clone, every process must have a parent, and can be traced back to an init system (another way of saying “the first program that is loaded”)
Where Booting Becomes Init
In other words the init system is the last stage of the booting process and relies entirely on data structures and software constructs provided by the kernel such as processes, threads, sockets, files and filesystems. PID 1 is a special process with lots of duties, but it is still just a process in the eyes of the kernel (for the most part).
Naming and Conventions
A daemon is the technical term for a process that runs in the background and forks into other processes – including other daemons. An example of this is systemd starting dockerd (the docker daemon). Another name for this type of process (in the context of an OS) is a service, although the term “service” can be ambiguous and highly dependent on the context, whereas a daemon generally means one thing.
For Unix, it is customary to name daemon processes something that ends in a
to identify itself as a daemon.
Brief History of Systemd
systemd follows in the tradition of naming, but diverges from the traditional
role of an “init system” in just about everything else that it does.
It started in 2010 as a reasonably sized project that sought to replace the old sequential init
systems that ran shell scripts with something that was capable of starting
processes in parallel. This sped up boot times substantially, and
was a necessary step in moving forward. But not all features have been received
with the same enthusiasm.
Anywhere systemd is mentioned online, there is always contention over the infamous feature creep which can only be described as a never-ending crusade through Linux userspace where systemd slowly consumes every aspect of system management – for better or worse.
This style of development is directly opposed to the Unix philosophy which is “do one thing and do it well”, and admins have mixed feelings about such widespread adoption of such a large chunk of software.
Supporting a single init system for all Linux distros is a big weight off developers, but many are worried that putting such trust in the hands of systemd maintainers is dangerous, as they don’t have the best history of respecting user’s feedback, and the scope of the project has not stopped expanding since it’s creation.
How Daemons Work
Daemons can be controlled and configured through a variety of
IPC (Inter Process Communication) mechanisms like sockets, pipes, and signals.
These are typically placed in the
There are configuration files that can change the default behavior of
a daemon as well.
systemctl for example, uses a private socket
/run/systemd/private for communication
with the daemon. It is also possible (but not recommended) to send signals
systemd with the
man systemd gives more info on what signals, sockets and pipes systemd uses.
systemctl is the one-stop-shop for interacting with systemd units and
The commands are pretty uniform and easy to understand.
systemctl list-units systemctl list-unit-files systemctl list-dependencies
systemctl status <unit> systemctl show <unit>
Changing unit states
systemctl enable <unit> systemctl disable <unit> systemctl start <unit> systemctl stop <unit>
Changing power states
systemctl poweroff systemctl reboot systemctl suspend systemctl hibernate
Other systemd daemon commands follow the same
ctl suffix such as
Note: though these commands may be present on a system, it does not necessarily
mean they are being managed by systemd.
The real complexity of systemd comes from underlying concepts, dependencies, and implementations of the various subsystems that have ballooned from the init system.
The concept of units is core to systemd. If you have ever heard someone say “everything is a file” when talking about how Linux works, just think “everything is a unit” when talking about systemd.
I’m going to be paraphrasing the
man systemctl.unit page, but a unit can be 11 different things.
It can be: a service, a socket, a device, a mount point, an automount point, a swap file or partition, a start-up target, a watched file system path, a timer
This may sound like lots of stuff to remember, and that’s because it is, but fortunately, this distinction is more relevant when creating units, so I will save this topic until then.
Another key concept is targets or runlevels. Targets are type of unit that hold other units and they are the way systemd organizes boot time dependencies.
This is another concept that becomes much more important when creating unit-files, because you must think about what your process depends or requires from the system before it can operate.
journalctl allows the viewing of logs managed by systemd.