The First Process
When a Linux kernel finishes loading itself into memory, it has one final job: hand control over to user space by launching exactly one program. That program is Process ID 1 (PID 1). Every other process on the system — your web server, your SSH daemon, your shell — is a descendant of PID 1.
In modern Linux distributions (Ubuntu, Debian, CentOS, Fedora, RHEL, and most others), PID 1 is systemd.
Think of systemd as the conductor of an orchestra. The conductor doesn't play an instrument; their job is to ensure every musician (service) starts at the right time, waits for others when necessary, and gets back in rhythm if they stumble. The conductor also keeps a detailed log of everything that happened during the performance.
Before systemd, Linux used an older system called SysV init (pronounced "System Five init"), which started services one by one in a strict, sequential order defined by numbered shell scripts. If service 30 took 10 seconds to start, everything from service 31 onward had to wait — even if those services had no relationship to service 30 whatsoever. Systemd was designed to solve this by starting independent services in parallel, dramatically reducing boot times, and by actively managing services throughout the entire lifetime of the system — not just at startup.
Systemd is also controversial. It replaced many small, focused Unix tools with a single, large, integrated system. Critics argue this violates the Unix philosophy of "do one thing well." Supporters argue the integration enables features (like dependency tracking, structured logging, and socket activation) that are impossible with isolated tools. As a practitioner, you don't need to pick a side — you need to understand how it works, because it runs on virtually every Linux server you will ever touch.