1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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3 | "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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4 | <!ENTITY % general-entities SYSTEM "../general.ent">
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5 | %general-entities;
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6 | ]>
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7 |
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8 | <sect1 id="ch-scripts-introduction">
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9 | <?dbhtml filename="introduction.html"?>
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10 |
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11 | <title>Introduction</title>
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12 |
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13 | <para>Booting a Linux system involves several tasks. The process must
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14 | mount both virtual and real file systems, initialize devices, activate swap,
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15 | check file systems for integrity, mount any swap partitions or files, set
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16 | the system clock, bring up networking, start any daemons required by the
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17 | system, and accomplish any other custom tasks needed by the user. This
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18 | process must be organized to ensure the tasks are performed in the correct
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19 | order but, at the same time, be executed as fast as possible.</para>
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20 |
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21 | <!-- <para>In the packages that were installed in Chapter 6, there were two
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22 | different boot systems installed. LFS provides the ability to easily
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23 | select which system the user wants to use and to compare and contrast the
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24 | two systems by actually running each system on the local computer. The
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25 | advantages and disadvantages of these systems is presented below.</para>-->
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26 |
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27 | <sect2 id='sysv-desc'>
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28 | <title>System V</title>
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29 |
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30 | <para>System V is the classic boot process that has been used in Unix and
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31 | Unix-like systems such as Linux since about 1983. It consists of a small
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32 | program, <command>init</command>, that sets up basic programs such as
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33 | <command>login</command> (via getty) and runs a script. This script,
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34 | usually named <command>rc</command>, controls the execution of a set of
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35 | additional scripts that perform the tasks required to initialize the
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36 | system.</para>
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37 |
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38 | <para>The <command>init</command> program is controlled by the
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39 | <filename>/etc/inittab</filename> file and is organized into run levels that
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40 | can be run by the user:</para>
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41 |
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42 | <literallayout>
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43 | 0 — halt
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44 | 1 — Single user mode
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45 | 2 — Multiuser, without networking
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46 | 3 — Full multiuser mode
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47 | 4 — User definable
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48 | 5 — Full multiuser mode with display manager
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49 | 6 — reboot
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50 | </literallayout>
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51 |
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52 | <para>The usual default run level is 3 or 5.</para>
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53 |
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54 | <bridgehead renderas="sect3">Advantages</bridgehead>
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55 |
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56 | <itemizedlist>
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57 | <listitem>
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58 | <para>Established, well understood system.</para>
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59 | </listitem>
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60 |
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61 | <listitem>
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62 | <para>Easy to customize.</para>
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63 | </listitem>
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64 |
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65 | </itemizedlist>
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66 |
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67 |
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68 | <bridgehead renderas="sect3">Disadvantages</bridgehead>
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69 |
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70 | <itemizedlist>
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71 | <listitem>
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72 | <para>Slower to boot. A medium speed base LFS system
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73 | takes 8-12 seconds where the boot time is measured from the
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74 | first kernel message to the login prompt. Network
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75 | connectivity is typically established about 2 seconds
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76 | after the login prompt.</para>
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77 | </listitem>
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78 |
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79 | <listitem>
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80 | <para>Serial processing of boot tasks. This is related to the previous
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81 | point. A delay in any process such as a file system check, will
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82 | delay the entire boot process.</para>
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83 | </listitem>
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84 |
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85 | <listitem>
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86 | <para>Does not directly support advanced features like
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87 | control groups (cgroups), and per-user fair share scheduling.</para>
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88 | </listitem>
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89 |
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90 | <listitem>
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91 | <para>Adding scripts requires manual, static sequencing decisions.</para>
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92 | </listitem>
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93 |
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94 | </itemizedlist>
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95 |
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96 | </sect2>
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97 | <!--
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98 | <sect2 id='sysd-desc'>
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99 | <title>Systemd</title>
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100 |
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101 | <para>Systemd is a group of interconnected programs that handles system and
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102 | individual process requests. It provides a dependency system between
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103 | various entities called "units". It automatically addresses dependencies
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104 | between units and can execute several startup tasks in parallel. It
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105 | provides login, inetd, logging, time, and networking services. </para>
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106 |
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107 | <bridgehead renderas="sect3">Advantages</bridgehead>
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108 |
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109 | <itemizedlist>
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110 | <listitem>
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111 | <para>Used on many established distributions by default.</para>
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112 | </listitem>
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113 |
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114 | <listitem>
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115 | <para>There is extensive documentation.
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116 | See <ulink url="http://www.freedesktop.org/wiki/Software/systemd/"/>.</para>
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117 | </listitem>
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118 |
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119 | <listitem>
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120 | <para>Parallel execution of boot processes. A medium speed
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121 | base LFS system takes 6-10 seconds from kernel start to a
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122 | login prompt. Network connectivity is typically established
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123 | about 2 seconds after the login prompt. More complex startup
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124 | procedures may show a greater speedup when compared to System V.</para>
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125 | </listitem>
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126 |
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127 | <listitem>
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128 | <para>Implements advanced features such as control groups to
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129 | manage related processes.</para>
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130 | </listitem>
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131 |
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132 | <listitem>
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133 | <para>Maintains backward compatibility with System V programs
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134 | and scripts.</para>
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135 | </listitem>
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136 | </itemizedlist>
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137 |
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138 | <bridgehead renderas="sect3">Disadvantages</bridgehead>
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139 |
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140 | <itemizedlist>
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141 | <listitem>
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142 | <para>There is a substantial learning curve.</para>
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143 | </listitem>
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144 |
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145 | <listitem>
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146 | <para>Some advanced features such as dbus or cgroups cannot be
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147 | disabled if they are not otherwise needed.</para>
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148 | </listitem>
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149 |
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150 | <listitem>
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151 | <para>Although implemented as several executable programs
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152 | the user cannot choose to implement only the portions desired.</para>
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153 | </listitem>
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154 |
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155 | <listitem>
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156 | <para>Due to the nature of using compiled programs, systemd is
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157 | more difficult to debug.</para>
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158 | </listitem>
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159 |
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160 | <listitem>
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161 | <para>Logging is done in a binary format. Extra tools must
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162 | be used to process logs or additional processes must be implemented
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163 | to duplicate traditional logging programs.</para>
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164 | </listitem>
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165 |
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166 | </itemizedlist>
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167 |
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168 | </sect2>
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169 | -->
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170 | <!--
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171 | <sect2 id='sysv'>
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172 | <title>Selecting a Boot Method</title>
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173 |
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174 | <para>Selecting a boot method in LFS is relatively easy.
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175 | Both systems are installed side-by-side. The only task needed is to
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176 | ensure the files that are needed by the system have the correct names.
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177 | The following scripts do that.</para>
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178 |
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179 | <screen><userinput remap="install">cat > /usr/sbin/set-systemd << "EOF"
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180 | #! /bin/bash
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181 |
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182 | ln -svfn init-systemd /sbin/init
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183 | ln -svfn init.d-systemd /etc/init.d
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184 |
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185 | for tool in halt poweroff reboot runlevel shutdown telinit; do
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186 | ln -sfvn ${tool}-systemd /sbin/${tool}
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187 | ln -svfn ${tool}-systemd.8 /usr/share/man/man8/${tool}.8
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188 | done
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189 |
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190 | echo "Now reboot with /sbin/reboot-sysv"
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191 | EOF
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192 |
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193 | chmod 0744 /usr/sbin/set-systemd
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194 |
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195 | cat > /usr/sbin/set-sysv << "EOF"
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196 | #! /bin/bash
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197 |
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198 | ln -sfvn init-sysv /sbin/init
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199 | ln -svfn init.d-sysv /etc/init.d
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200 |
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201 | for tool in halt poweroff reboot runlevel shutdown telinit; do
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202 | ln -sfvn ${tool}-sysv /sbin/${tool}
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203 | ln -svfn ${tool}-sysv.8 /usr/share/man/man8/${tool}.8
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204 | done
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205 |
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206 | echo "Now reboot with /sbin/reboot-systemd"
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207 | EOF
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208 |
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209 | chmod 0744 /usr/sbin/set-sysv</userinput></screen>
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210 |
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211 | <note><para>The comment about the correct command to reboot in the
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212 | above scripts is correct. The reboot command for the current boot
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213 | system must be used after the script changes the default reboot command.
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214 | </para></note>
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215 |
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216 | <para>Now set the desired boot system. The default is System V:</para>
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217 |
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218 | <screen><userinput remap="install">/usr/sbin/set-sysv</userinput></screen>
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219 |
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220 | <para>Changing the boot system can be done at any time by running the
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221 | appropriate script above and rebooting.</para>
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222 |
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223 | </sect2>
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224 | -->
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225 | </sect1>
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