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Booting process of Linux
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<p>The Linux <a href="/facts/Booting/j6N1kRpc">booting</a> process involves multiple stages and is in many ways similar to the <a href="/facts/BSD/XQKcURrh">BSD</a> and other <a href="/facts/Unix/0pNL2seI">Unix</a>-style boot processes, from which it derives. Although the Linux booting process depends very much on the computer architecture, those architectures share similar stages and software components, including system startup, <a href="/facts/Bootloader/6sNpcLMK">bootloader</a> execution, loading and startup of a <a href="/facts/Linux_kernel/dlkduHuA">Linux kernel</a> image, and execution of various <a href="/facts/Startup_scripts/0NuCuc1g">startup scripts</a> and <a href="/facts/Daemon_(computing)/bzuEVGGX">daemons</a>. Those are grouped into 4 steps: system startup, bootloader stage, kernel stage, and init process. </p><p>When a Linux system is powered up or reset, its processor will execute a specific firmware/program for system initialization, such as the <a href="/facts/Power-on_self-test/zwTA1bhH">power-on self-test</a>, invoking the <a href="/facts/Reset_vector/rfAcxhw9">reset vector</a> to start a program at a known address in flash/ROM (in embedded Linux devices), then load the bootloader into RAM for later execution. In <a href="/facts/IBM_PC%E2%80%93compatible/zGUpl7Sh">IBM PC–compatible</a> <a href="/facts/Personal_computers/koaYfNce">personal computers</a> (PCs), this firmware/program is either a <a href="/facts/BIOS/AYYPUogS">BIOS</a> or a <a href="/facts/UEFI/w86VGmy2">UEFI</a> monitor, and is stored in the mainboard. In embedded Linux systems, this firmware/program is called <a href="/facts/Boot_ROM/j6N1kRpc">boot ROM</a>. After being loaded into RAM, the bootloader (also called first-stage bootloader or primary bootloader) will execute to load the second-stage bootloader (also called secondary bootloader). The second-stage bootloader will load the kernel image into memory, decompress and initialize it, and then pass control to this kernel image. The second-stage bootloader also performs several operation on the system such as system hardware check, mounting the root device, loading the necessary kernel modules, etc. Finally, the first user-space process (init process) starts, and other high-level system initializations are performed (which involve with startup scripts). </p><p>For each of these stages and components, there are different variations and approaches; for example, <a href="/facts/GNU_GRUB/pbwqDCh1">GRUB</a>, <a href="/facts/Systemd-boot/xXak8bcS">systemd-boot</a>, <a href="/facts/Coreboot/GIeazmzw">coreboot</a> or <a href="/facts/Das_U-Boot/ePTYfLk1">Das U-Boot</a> can be used as bootloaders (historical examples are <a href="/facts/LILO_(boot_loader)/CSlDBvwD">LILO</a>, <a href="/facts/SYSLINUX/cV2DANPk">SYSLINUX</a> or <a href="/facts/Loadlin/r5lfclUY">Loadlin</a>), while the startup scripts can be either traditional <a href="/facts/Init/0NuCuc1g">init</a>-style, or the system configuration can be performed through modern alternatives such as <a href="/facts/Systemd/zls5HoVh">systemd</a> or <a href="/facts/Upstart_(software)/Kt2ozI2a">Upstart</a>. </p>