1BOOT(7) Linux Programmer's Manual BOOT(7)
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6 boot-scripts - General description of boot sequence
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9 The boot sequence varies in details among systems but can be roughly
10 divided to the following steps: (i) hardware boot, (ii) OS loader,
11 (iii) kernel startup, (iv) init and inittab, (v) boot scripts. We will
12 describe each of these in more detail below.
13 Hardware-boot
16 After power-on or hard reset, control is given to a program stored on
17 read only memory (normally PROM). In PC we usually call this program
18 the BIOS.
19 This program normally makes a basic self-test of the machine and
21 accesses non-volatile memory to read further parameters. This memory in
22 the PC is battery-backed CMOS memory, so most people refer to it as the
23 CMOS, although outside of the PC world, it is usually called nvram
24 (non-volatile ram).
25 The parameters stored in the nvram vary between systems, but as a mini‐
27 mum, the hardware boot program should know what is the boot device, or
28 which devices to probe as possible boot devices.
29 Then the hardware boot stage accesses the boot device, loads the OS
31 Loader, which is located on a fixed position on the boot device, and
32 transfers control to it.
33 Note: We do not cover here booting from network. Those who want to
36 investigate this subject may want to research: DHCP, TFTP, PXE,
37 Etherboot.
38 OS Loader
41 In PC, the OS Loader is located in the first sector of the boot device
42 - this is the MBR (Master Boot Record).
43 In most systems, this primary loader is very limited due to various
45 constraints. Even on non-PC systems there are some limitations to the
46 size and complexity of this loader, but the size limitation of the PC
47 MBR (512 bytes including the partition table) makes it almost impossi‐
48 ble to squeeze a full OS Loader into it.
49 Therefore, most operating systems make the primary loader call a sec‐
51 ondary OS loader which may be located on a specified disk partition.
52 In Linux the OS loader is normally lilo(8) or grub(8). Both of them
54 may install either as secondary loaders (where the DOS installed MBR
55 points to them), or as a two part loader where they provide special MBR
56 containing the bootstrap code to load the second part of the loader
57 from the root partition.
58 The main job of the OS Loader is to locate the kernel on the disk, load
60 it and run it. Most OS loaders allow interactive use, to enable speci‐
61 fication of alternative kernel (maybe a backup in case the last com‐
62 piled one isn't functioning) and to pass optional parameters to the
63 kernel.
64 Kernel Startup
67 When the kernel is loaded, it initializes the devices (via their driv‐
68 ers), starts the swapper (it is a "kernel process", called kswapd in
69 modern Linux kernels), and mounts the root file system (/).
70 Some of the parameters that may be passed to the kernel relate to these
72 activities (e.g: You can override the default root file system). For
73 further information on Linux kernel parameters read bootparam(7).
74 Only then the kernel creates the first (user land) process which is
76 numbered 1. This process executes the program /sbin/init, passing any
77 parameters that weren't handled by the kernel already.
78 init and inittab
81 When init starts it reads /etc/inittab for further instructions. This
82 file defines what should be run in different run-levels.
83 This gives the system administrator an easy management scheme, where
85 each run-level is associated with a set of services (e.g: S is sin‐
86 gle-user, on 2 most network services start, etc.). The administrator
87 may change the current run-level via init(8) and query the current run-
88 level via runlevel(8).
89 However, since it is not convenient to manage individual services by
91 editing this file, inittab only bootstraps a set of scripts that actu‐
92 ally start/stop the individual services.
93 Boot Scripts
96 Note: The following description applies to System V release 4 based
97 system, which currently covers most commercial Unix systems
98 (Solaris, HP-UX, Irix, Tru64) as well as the major Linux distri‐
99 butions (RedHat, Debian, Mandrake, Suse, Caldera). Some systems
100 (Slackware Linux, FreeBSD, OpenBSD) have a somewhat different
101 scheme of boot scripts.
102 For each managed service (mail, nfs server, cron, etc.) there is a sin‐
104 gle startup script located in a specific directory (/etc/init.d in most
105 versions of Linux). Each of these scripts accepts as a single argument
106 the word 'start' -- causing it to start the service, or the word 'stop'
107 -- causing it to stop the service. The script may optionally accept
108 other "convenience" parameters (e.g: 'restart', to stop and then start,
109 'status' do display the service status). Running the script without
110 parameters displays the possible arguments.
111 Sequencing Directories
114 To make specific scripts start/stop at specific run-levels and in spe‐
115 cific order, there are sequencing directories. These are normally in
116 /etc/rc[0-6S].d. In each of these directories there are links (usually
117 symbolic) to the scripts in the init.d directory.
118 A primary script (usually /etc/rc) is called from inittab(5) and calls
120 the services scripts via the links in the sequencing directories. All
121 links with names that begin with 'S' are being called with the argument
122 'start' (thereby starting the service). All links with names that begin
123 with 'K' are being called with the argument 'stop' (thereby stopping
124 the service).
125 To define the starting or stopping order within the same run-level, the
127 names of the links contain order-numbers. Also, to make the names
128 clearer, they usually end with the name of the service they refer to.
129 Example: the link /etc/rc2.d/S80sendmail starts the sendmail service on
130 runlevel 2. This happens after /etc/rc2.d/S12syslog is run but before
131 /etc/rc2.d/S90xfs is run.
132 To manage the boot order and run-levels, we have to manage these links.
134 However, on many versions of Linux, there are tools to help with this
135 task (e.g: chkconfig(8)).
136 Boot Configuration
139 Usually the daemons started may optionally receive command line options
140 and parameters. To allow system administrators to change these parame‐
141 ters without editing the boot scripts themselves, configuration files
142 are used. These are located in a specific directory (/etc/sysconfig on
143 RedHat systems) and are used by the boot scripts.
144 In older Unix systems, these files contained the actual command line
146 options for the daemons, but in modern Linux systems (and also in HP-
147 UX), these files just contain shell variables. The boot scripts in
148 /etc/init.d source the configuration files, and then use the variable
149 values.
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152 /etc/init.d/, /etc/rc[S0-6].d/. /etc/sysconfig/
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156 inittab(5), bootparam(7), init(8), runlevel(8), shutdown(8)
157 2002-06-07 BOOT(7)