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Quick HOWTO : Ch07 : The Linux Boot ProcessFrom Linux Home Networking
IntroductionLearning how Linux boots up is critical. When you have this information you can use it to alter the type of login screen you get as well as which programs start up. Read on for the details. The Linux Boot SequenceYou might remember when you installed Linux that the installation process prompted you for a list of partitions and the sizes of each in which your filesystems would be placed. When allocating disk space for the partitions, the first sector, or data unit, for each partition is always reserved for programmable code used in booting. The very first sector of the hard disk is reserved for the same purpose and is called the master boot record (MBR). When booting from a hard disk, the PC system BIOS starts by loading and executing bootloader code. In many cases the complete bootloader code is larger than the space available in the MBR, so booting has to be done in stages. Fedora Linux uses the Grub staged bootloader system where the first stage bootloader code ( Grub Version 1In Stage 1.5 of the Grub version 1 process, additional code is loaded from the 30 kilobytes of hard disk immediately after the MBR. This 1.5 bootloader image file includes enough partition and filesystem drivers to allow the Stage 2 to be loaded from a known location in the boot filesystem, usually Grub Version 2Like version 1, version 2 reads an image file ( Grub After the Boot MenuThe familiar GRUB startup menu is now displayed, which will allow you to select an operating system to boot or to view and edit kernel startup parameters. Assuming you select a version of Linux to boot, the following steps will happen:
Let's discuss Grub in more detail. Grub Configuration FilesAs you would expect, the different versions of Grub have different configuration file names and syntaxes. These will be covered next. Grub Version 1 ConfigurationGrub v1 uses the Note: In some operating systems, such as Debian / Ubuntu, the /boot/grub/grub.conf file may also be referred to by the name Figure 7-1 shows a typical grub.conf file for a system that can boot both Fedora Linux and Windows 2000. Figure 7-1 Sample grub.conf filedefault=0
timeout=10
splashimage=(hd0,0)/grub/splash.xpm.gz
title Fedora Core (2.6.8-1.521)
root (hd0,0)
kernel /vmlinuz-2.6.8-1.521 ro root=LABEL=/
initrd /initrd-2.6.8-1.521.img
title Windows 2000
rootnoverify (hd0,1)
chainloader +1
In this configuration example, when default is set to 1, Windows boots. You can also see that the Fedora kernel file is The ( # # File: /boot/grub/device.map # (fd0) /dev/fd0 (hd0) /dev/hda So ( Grub Version 2 ConfigurationWhen Fedora installs new kernel versions, updated versions of the vmlinuz and initramfs files are deposited in the /etc/grub directory. After doing this, the script grub2-mkconfig is run to create an updated grub menu configuration file /boot/grub2/grub.cfg. This is then used to create the Grub boot menu. A very trimmed version of this file is shown below. #
# File: /boot/grub2/grub.cfg
#
...
...
...
menuentry 'Fedora (3.4.2-1.fc16.x86_64)' --class fedora --class gnu-linux --class gnu --class os {
echo 'Loading Fedora (3.4.2-1.fc16.x86_64)'
linux /vmlinuz-3.4.2-1.fc16.x86_64 ...
echo 'Loading initial ramdisk ...'
initrd /initramfs-3.4.2-1.fc16.x86_64.img
}
...
...
Note: The /boot/grub2/grub.cfg file has sections or stanzas called "menuentry" that define the entries to be shown in the boot menu and the parameters each requires. You don't edit /boot/grub2/grub.cfg as it is auto-generated. Customizations are done in the /etc/default/grub file. A sample is below. # # File: /etc/default/grub # GRUB_TIMEOUT=5 GRUB_DISTRIBUTOR="Fedora" GRUB_DEFAULT=saved GRUB_CMDLINE_LINUX="rd.md=0 rd.dm=0 rd.lvm.lv=vg_web003/LogVol01 rd.lvm.lv=vg_web003/LogVol00 \ KEYTABLE=us quiet SYSFONT=latarcyrheb-sun16 rhgb rd.luks=0 LANG=en_US.UTF-8" Here are what the keywords mean. If you need more information, the official Grub manual at http://www.gnu.org has more details.
Now we'll discuss booting runlevels and targets. Boot Runlevels and TargetsThe final post booting state of Linux systems is configured by the systems administrator. With Fedora / RedHat systems there are seven of these states as defined in Table 7-1 "Runlevel and Target Descriptions". Table 7-1 Runlevel and Target Descriptions
Depending on your version of Linux there will be one of two main types of runlevel management systems. These are systemd which uses the "target" terminology and init which uses the "runlevel" terminology. For example, when the system is set for runlevel zero, it halts, and when it's set to runlevel six, it reboots. The default level is usually 3, for booting without a GUI interface, or 5 for booting with one. Single user mode is used for maintenance or emergency repair purposes as it loads a very limited set of drivers and often has network services disabled. Some differences between the init and systemd systems include:
Both systems will be discussed next. SystemdSymbolic links to the systemd startup configuration files are located in the /etc/systemd/system directory. Each target will have its own subdirectory as shown in Table 7-2. Table 7-2 Systemd Target File Locations
When the system boots under systemd, it follows these basic steps. 1. First, systemd reads all the .target files in the /lib/systemd/system/ directory. Each target file contains a list of services that need to be run during the target activation; a list of pre-requisite targets that have to be completed and the target which must be completed immediately beforehand. In some cases the file will include targets that must be completed immediately afterwards. In this sample target file we see that the target expects the steps in sysinit.target and sockets.target to be completed as pre-requisites and that the target will also run immediately after they are completed # # File: /lib/systemd/system/basic.target # [Unit] Description=Basic System Requires=sysinit.target sockets.target After=sysinit.target sockets.target RefuseManualStart=yes 2. Using this information, systemd creates a master list of services and the order in which they should be started. The system will boot and systemd will stop starting daemons in the list after it executes the services in the default.target file found in the /etc/systemd/system directory. 3. When all this is completed without errors, the system has booted successfully. Table 7-3 provides a summary of some important systemd commands that will be helpful to you with systemd. These are then covered in more detail. Table 7-3 Important Systemd Boot Related Commands
Determine the current default target groupAs stated before the target control files are located in the /etc/systemd directory tree. The file that sets the default target is /etc/systemd/system/default.target. In this case doing a directory listing of this file shows that when the system boots next, it will be in target 3. [root@bigboy tmp]# ll /etc/systemd/system/default.target lrwxrwxrwx. 1 root root 36 Jan 1 2012 /etc/systemd/system/default.target -> /lib/systemd/system/runlevel3.target [root@bigboy tmp]# The currently running target can be determined using the runlevel command. Here we see that it is set to 3 also. [root@bigboy tmp]# runlevel N 3 [root@bigboy tmp]# If you need to see all the various targets that are active then use the systemctl list-units --type=target command as shown here. [root@bigboy tmp]# systemctl list-units --type=target UNIT LOAD ACTIVE SUB JOB DESCRIPTION basic.target loaded active active Basic System cryptsetup.target loaded active active Encrypted Volumes getty.target loaded active active Login Prompts local-fs-pre.target loaded active active Local File Systems (Pre) local-fs.target loaded active active Local File Systems multi-user.target loaded active active Multi-User network.target loaded active active Network remote-fs.target loaded active active Remote File Systems sockets.target loaded active active Sockets sound.target loaded active active Sound Card swap.target loaded active active Swap sysinit.target loaded active active System Initialization syslog.target loaded active active Syslog LOAD = Reflects whether the unit definition was properly loaded. ACTIVE = The high-level unit activation state, i.e. generalization of SUB. SUB = The low-level unit activation state, values depend on unit type. JOB = Pending job for the unit. 13 units listed. Pass --all to see inactive units, too. [root@bigboy tmp]# Set the default target groupTo set the default target use either the systemctl enable x.target command or the ln -sf command to link the /lib/systemd/system/*.target file to /etc/systemd/system/default.target. In these cases we set the default target to 3 and 5. [root@bigboy tmp]# systemctl enable multi-user.target [root@bigboy tmp]# systemctl enable graphical.target [root@bigboy tmp]# ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target [root@bigboy tmp]# ln -sf /lib/systemd/system/graphical.target /etc/systemd/system/default.target Next we'll discuss systems that use init. SysV InitWhen a Linux system running SysV Init begins to boot with its kernel, it first runs the /sbin/init program, which does some system checks, such as verifying the integrity of the file systems, and starts vital programs needed for the operating system to function properly. It then inspects the /etc/inittab file to determine Linux's overall mode of operation or runlevel. A listing of valid runlevels can be seen in Table 7-4. Table 7-4 "Init Runlevel File Locations" 7-4 Init Runlevel File Locations
Based on the selected runlevel, the init process then executes startup scripts located in subdirectories of the Here is a directory listing of the scripts in the
[root@bigboy tmp]# ls /etc/rc.d/rc3.d ... ... K75netfs K96pcmcia ... ... ... ... K86nfslock S05kudzu ... ... ... ... K87portmap S09wlan ... ... ... ... K91isdn S10network ... ... ... ... K92iptables S12syslog ... ... ... ... K95firstboot S17keytable ... ... [root@bigboy tmp]# As you can see, each filename in these directories either starts with an "S" which signifies the script should be run at startup, or a K, which means the script should be run when the system is shutting down. If a script isn't there, it won't be run. Most Linux packages place their startup script in the The number that follows the K or S specifies the position in which the scripts should be run in ascending order. In our example, Determining and Setting the Default Boot runlevelThe default boot runlevel is set in the file /etc/inittab with the initdefault variable. When set to 3, the system boots up with the text interface on the VGA console; when set to 5, you get the GUI. Here is a snippet of the file (delete the initdefault line you don't need): # Default runlevel. The runlevels used by RHS are: # 0 - halt (Do NOT set initdefault to this) # 1 - Single user mode # 2 - Multiuser, without NFS (The same as 3, if you do not have networking) # 3 - Full multiuser mode # 4 - unused # 5 - X11 # 6 - reboot (Do NOT set initdefault to this) # id:3:initdefault: # Console Text Mode id:5:initdefault: # Console GUI Mode Note the following:
System Shutdown and RebootingIt is usually not a good idea to immediately power off your system when you are finished using it. This can cause files that are being updated to become corrupted, or worse, you could corrupt the filesystem directory structure. Linux has a number of ways to gracefully shut down and reboot your system which will be outlined in this section.
Halt/Shut Down The SystemThe init command will allow you to change the current runlevel, and for a shutdown, that value is 0. Here is an example: [root@bigboy tmp]# init 0 Fedora also has a shutdown command which can also be used to the same effect. It often prompts you as to whether you are sure you want to execute the command, which can be avoided with the -y switch. The -h switch forces the system to halt, and the first argument tells it how long to wait before starting the procedure, in this case 0 minutes. You can also specify shutting down at a specific time of the day; please refer to the man pages for details. Another advantage of the shutdown command is that it warns people that the shutdown is going to occur. [root@bigboy tmp]# shutdown -hy 0 Broadcast message from root (pts/0) (Sat Nov 6 13:15:27 2004): The system is going down for system halt NOW! [root@bigboy tmp]# Reboot The SystemYou can also use the init command to reboot the system immediately by entering runlevel 6. [root@bigboy tmp]# init 6 The "reboot" command has the same effect, but it also sends a warning message to all users. [root@bigboy tmp]# reboot Broadcast message from root (pts/0) (Sat Nov 6 12:39:31 2004): The system is going down for reboot NOW! [root@bigboy tmp]# More graceful reboots can be done with the shutdown command using the -r switch and specifying a delay, which in this case is 10 minutes. [root@bigboy root]# shutdown -ry 10 Broadcast message from root (pts/0) (Sat Nov 6 13:26:39 2004): The system is going DOWN for reboot in 10 minutes! Broadcast message from root (pts/0) (Sat Nov 6 13:27:39 2004): The system is going DOWN for reboot in 9 minutes! ... ... ... Broadcast message from root (pts/0) (Sat Nov 6 13:36:39 2004): The system is going down for reboot NOW! Entering Single-user ModeSome activities require you to force the system to log off all users, third-party applications and networking so that only the systems administrator has access to the system from the VGA console. A typical scenario is the addition of a new hard disk, as mentioned in Chapter 27, "Expanding Disk Capacity", or the troubleshooting of a failed boot process. Another reason is the recovery of your root password. Switching to Single-user ModeWhen the system is running normally, this can be done by using the init command to enter runlevel 1. It is best to do this from the console, because if you do it from a remote terminal session you'll be logged out. [root@bigboy root]# init 1 ... ... bash-2.05b# Unfortunately, this gives no prior warning to users, and the shutdown command doesn't have a single-user mode option. This can be overcome by running the shutdown command with a delay in minutes as the only argument. [root@bigboy tmp]# shutdown 1 Broadcast message from root (pts/0) (Sat Nov 6 13:44:59 2004): The system is going DOWN to maintenance mode in 1 minute! Broadcast message from root (pts/0) (Sat Nov 6 13:45:59 2004): The system is going down to maintenance mode NOW! ... ... bash-2.05b# Entering Single-user Mode At The Grub Splash ScreenYou can enter single user mode directly after turning on the power to your system. The steps to do this are listed below. 1. Power on your system. Wait for the "Grub loading" message to appear and, depending on your Linux distribution, get ready to hit either any key or the ESC key to enter the grub boot menu. Grub loading, please wait ... Press ESC to enter the menu or Grub loading, please wait ... Press any key to enter the menu 2. You will then get grub's main menu which will display a list of available kernels. Use the arrow keys to scroll to your desired version of the kernel and then press e for "edit". Fedora Core (2.6.18-1.2239.fc5smp) Fedora Core (2.6.18-1.2200.fc5smp) 3. The kernel's boot menu will appear. Use the arrow keys to scroll to the "kernel" line and then press e for "edit". root (hd0,0) kernel /vmlinuz-2.6.18-1.2239.fc5smp ro root=LABEL=/ initrd /initrd-2.6.18-1.2239.fc5smp.img 4. A grub edit prompt will appear. Use the arrow keys to move to the end of the line and add the word "single" to the end, separated by a space. Change grub edit> kernel /vmlinuz-2.6.18-1.2239.fc5smp ro root=LABEL=/ to grub edit> kernel /vmlinuz-2.6.18-1.2239.fc5smp ro root=LABEL=/ single 5. Press enter to save your changes, and then b for "boot". 6. The system will continue to boot, but will go straight to the root # prompt without first asking for a username and password. Reverting To Your Default runlevel From Single User ModeThe exit command forces the system to exit runlevel 1 and revert to the default runlevel for the system. You can also use the init command (for example init 3 and init 5) to alter this default behavior: bash-2.05b# exit INIT: Entering runlevel: 3 ... ... ... Fedora Core release 2 (Tettnang) Kernel 2.6.8-1.521 on an i686 bigboy login: Root Password RecoverySometimes you might forget the root password, or the previous systems administrator may move on to a new job without giving it to you. To do this, follow these steps:
The Linux ConsoleA good knowledge of some basic Linux console commands is always helpful immediately after booting. Here are some examples. Getting a GUI ConsoleManual Method: You can start the X terminal GUI application each time you need it by running the startx command at the VGA console. Remember that when you log out you will get the regular text-based console again. [root@bigboy tmp]# startx Automatic Method: You can have Linux automatically start the X terminal GUI console for every login attempt until your next reboot by using the init command. You will need to edit your initdefault variable in your /etc/inittab file, as mentioned in the preceding section to keep this functionality even after you reboot. [root@bigboy tmp]# init 5 When the CPU capacity or available memory on your server is low or you want to maximize all system resources, you might want to operate in text mode runlevel 3 most of the time, using the GUI only as necessary with the startx command. Servers that double as personal workstations, or servers that might have to be operated for an extended period of time by relatively nontechnical staff, may need to be run at runlevel 5 all the time through the init 5 command. Remember you can make runlevel 5 permanent even after a reboot by editing the /etc/inittab file. Get a Basic Text Terminal Without Exiting the GUIThere are a number of ways for you to get a command prompt when running a Linux GUI. This can be important if you need quick access to commands or you are not familiar with the GUI menu option layout. Using a GUI Terminal WindowYou can open a GUI-based window with a command prompt inside by doing the following:
Using Virtual ConsolesBy default, Linux runs six virtual console or TTY sessions running on the VGA console. These are defined by the mingetty statements in the /etc/inittab file. The X terminal GUI console creates its own virtual console using the first available TTY that is not controlled by mingetty. This makes the GUI run as number 7:
ConclusionThe topics discussed in this chapter might seem simple, but like syslog, which was covered in Chapter 5, "Troubleshooting Linux with syslog", they are an essential part of Linux administration that gets frequently overlooked especially when new software is installed. Whenever possible, always try to reboot your system to make sure all the newly installed applications start up correctly. Sometimes they start but give errors listed only in the /var/log directory. Taking the time to configure and test your startup scripts could prevent you from being awakened in the middle of the night while you are on vacation! It is really important. 
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