{
    "mode": "man",
    "parameter": "boot",
    "section": "7",
    "url": "https://www.chedong.com/phpMan.php/man/boot/7/json",
    "generated": "2026-09-01T14:39:47Z",
    "sections": {
        "NAME": {
            "content": "boot - System bootup process based on UNIX System V Release 4\n",
            "subsections": []
        },
        "DESCRIPTION": {
            "content": "The  bootup  process (or \"boot sequence\") varies in details among systems, but can be roughly\ndivided into phases controlled by the following components:\n\n(1)  hardware\n\n(2)  operating system (OS) loader\n\n(3)  kernel\n\n(4)  root user-space process (init and inittab)\n\n(5)  boot scripts\n\nEach of these is described below in more detail.\n",
            "subsections": [
                {
                    "name": "Hardware",
                    "content": "After power-on or hard reset, control is given to a program stored in read-only memory  (nor‐\nmally  PROM);  for  historical reasons involving the personal computer, this program is often\ncalled \"the BIOS\".\n\nThis program normally performs a basic self-test of the machine and accesses nonvolatile mem‐\nory to read further parameters.  This memory in the PC is battery-backed CMOS memory, so most\npeople refer to it as \"the CMOS\"; outside of the PC world, it is usually called  \"the  NVRAM\"\n(nonvolatile RAM).\n\nThe  parameters stored in the NVRAM vary among systems, but as a minimum, they should specify\nwhich device can supply an OS loader, or at least which devices may be probed for one; such a\ndevice is known as \"the boot device\".  The hardware boot stage loads the  OS  loader  from  a\nfixed position on the boot device, and then transfers control to it.\n\nNote:  The  device  from  which the OS loader is read may be attached via a network, in which\ncase the details of booting are further specified by protocols  such  as  DHCP,  TFTP,\nPXE, Etherboot, etc.\n"
                },
                {
                    "name": "OS loader",
                    "content": "The  main  job  of the OS loader is to locate the kernel on some device, load it, and run it.\nMost OS loaders allow interactive use, in order to enable  specification  of  an  alternative\nkernel  (maybe a backup in case the one last compiled isn't functioning) and to pass optional\nparameters to the kernel.\n\nIn a traditional PC, the OS loader is located in the initial 512-byte block of the  boot  de‐\nvice; this block is known as \"the MBR\" (Master Boot Record).\n\nIn  most  systems,  the OS loader is very limited due to various constraints.  Even on non-PC\nsystems, there are some limitations on the size and complexity of this loader, but  the  size\nlimitation  of the PC MBR (512 bytes, including the partition table) makes it almost impossi‐\nble to squeeze much functionality into it.\n\nTherefore, most systems split the role of loading the OS between a primary OS  loader  and  a\nsecondary  OS loader; this secondary OS loader may be located within a larger portion of per‐\nsistent storage, such as a disk partition.\n\nIn Linux, the OS loader is often grub(8) (an alternative is lilo(8)).\n"
                },
                {
                    "name": "Kernel",
                    "content": "When the kernel is loaded, it initializes various components of the  computer  and  operating\nsystem;  each  portion of software responsible for such a task is usually consider \"a driver\"\nfor the applicable component.  The kernel starts the virtual memory swapper (it is  a  kernel\nprocess,  called  \"kswapd\"  in a modern Linux kernel), and mounts some filesystem at the root\npath, /.\n\nSome of the parameters that may be passed to the kernel relate to these activities (for exam‐\nple, the default root filesystem can be overridden); for further information on Linux  kernel\nparameters, read bootparam(7).\n\nOnly then does the kernel create the initial userland process, which is given the number 1 as\nits  PID (process ID).  Traditionally, this process executes the program /sbin/init, to which\nare passed the parameters that haven't already been handled by the kernel.\n"
                },
                {
                    "name": "Root user-space process",
                    "content": "Note:  The following description applies to an OS based on UNIX System V Release 4.  However,\na number of widely used systems have adopted a related but fundamentally different ap‐\nproach known as systemd(1), for which the bootup process is detailed in its associated\nbootup(7).\n\nWhen /sbin/init starts, it reads /etc/inittab for further instructions.   This  file  defines\nwhat should be run when the /sbin/init program is instructed to enter a particular run level,\ngiving  the  administrator  an  easy way to establish an environment for some usage; each run\nlevel is associated with a set of services (for example, run level S is single-user mode, and\nrun level 2 entails running most network services).\n\nThe administrator may change the current run level via init(1), and  query  the  current  run\nlevel via runlevel(8).\n\nHowever,  since  it  is  not  convenient  to manage individual services by editing this file,\n/etc/inittab only bootstraps a set of scripts that actually start/stop  the  individual  ser‐\nvices.\n"
                },
                {
                    "name": "Boot scripts",
                    "content": "Note:  The following description applies to an OS based on UNIX System V Release 4.  However,\na  number  of  widely used systems (Slackware Linux, FreeBSD, OpenBSD) have a somewhat\ndifferent scheme for boot scripts.\n\nFor each managed service (mail, nfs server, cron, etc.), there is a single startup script lo‐\ncated in a specific directory (/etc/init.d in most versions of Linux).  Each of these scripts\naccepts as a single argument the word \"start\" (causing it to start the service) or  the  word\n\"stop\"  (causing  it  to  stop  the service).  The script may optionally accept other \"conve‐\nnience\" parameters (e.g., \"restart\" to stop and then start, \"status\" to display  the  service\nstatus, etc.).  Running the script without parameters displays the possible arguments.\n"
                },
                {
                    "name": "Sequencing directories",
                    "content": "To make specific scripts start/stop at specific run levels and in a specific order, there are\nsequencing directories, normally of the form  /etc/rc[0-6S].d.  In each of these directories,\nthere are links (usually symbolic) to the scripts in the /etc/init.d directory.\n\nA  primary script (usually /etc/rc) is called from inittab(5); this primary script calls each\nservice's script via a link in the relevant sequencing directory.  Each link whose  name  be‐\ngins  with 'S' is called with the argument \"start\" (thereby starting the service).  Each link\nwhose name begins with 'K' is called with the argument \"stop\" (thereby stopping the service).\n\nTo define the starting or stopping order within the same run level, the name of a  link  con‐\ntains  an  order-number.  Also, for clarity, the name of a link usually ends with the name of\nthe service to which it refers.  For example,  the  link  /etc/rc2.d/S80sendmail  starts  the\nsendmail(8)  service  on run level 2.  This happens after /etc/rc2.d/S12syslog is run but be‐\nfore /etc/rc2.d/S90xfs is run.\n\nTo manage these links is to manage the boot order and run levels; under many  systems,  there\nare tools to help with this task (e.g., chkconfig(8)).\n"
                },
                {
                    "name": "Boot configuration",
                    "content": "A  program that provides a service is often called a \"daemon\".  Usually, a daemon may receive\nvarious command-line options and parameters.  To allow a system administrator to change these\ninputs without editing an entire boot script, some separate configuration file is  used,  and\nis  located in a specific directory where an associated boot script may find it (/etc/syscon‐\nfig on older Red Hat systems).\n\nIn older UNIX systems, such a file contained the actual command line options  for  a  daemon,\nbut  in  modern  Linux systems (and also in HP-UX), it just contains shell variables.  A boot\nscript in /etc/init.d reads and includes its configuration file (that is,  it  \"sources\"  its\nconfiguration file) and then uses the variable values.\n"
                }
            ]
        },
        "FILES": {
            "content": "/etc/init.d/, /etc/rc[S0-6].d/, /etc/sysconfig/\n",
            "subsections": []
        },
        "SEE ALSO": {
            "content": "init(1), systemd(1), inittab(5), bootparam(7), bootup(7), runlevel(8), shutdown(8)\n\nLinux man-pages 6.7                          2023-10-31                                      boot(7)",
            "subsections": []
        }
    },
    "summary": "boot - System bootup process based on UNIX System V Release 4",
    "flags": [],
    "examples": [],
    "see_also": [
        {
            "name": "init",
            "section": "1",
            "url": "https://www.chedong.com/phpMan.php/man/init/1/json"
        },
        {
            "name": "systemd",
            "section": "1",
            "url": "https://www.chedong.com/phpMan.php/man/systemd/1/json"
        },
        {
            "name": "inittab",
            "section": "5",
            "url": "https://www.chedong.com/phpMan.php/man/inittab/5/json"
        },
        {
            "name": "bootparam",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/bootparam/7/json"
        },
        {
            "name": "bootup",
            "section": "7",
            "url": "https://www.chedong.com/phpMan.php/man/bootup/7/json"
        },
        {
            "name": "runlevel",
            "section": "8",
            "url": "https://www.chedong.com/phpMan.php/man/runlevel/8/json"
        },
        {
            "name": "shutdown",
            "section": "8",
            "url": "https://www.chedong.com/phpMan.php/man/shutdown/8/json"
        }
    ]
}