What is the first step that occurs when turning on a Linux server?

The first step that occurs when turning on a Linux server is the Power-On Self-Test (POST). This process is critical as it initializes the system hardware and ensures that all components, such as memory, CPU, and storage devices, are functioning correctly before the operating system takes control.

During POST, the BIOS or firmware conducts a series of diagnostic tests to confirm that the hardware is operational. If any issues are detected, the system may provide error codes or alerts, preventing the boot process from proceeding. This step is fundamental because it lays the groundwork for a successful boot, ensuring that any faults in the hardware are identified before the operating system is loaded.

Once the POST is successfully completed, the next stages—such as bootloader activation, kernel initialization, and runlevel or target activation—can occur in sequence to prepare the Linux server for normal operation. However, without a successful POST, the server cannot move to these later stages. This illustrates the importance of the initial hardware check in the boot-up process.