Bootstrapping processes

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1. What is bootstrapping and what processes are involved in it?

In simple words, "bootstrapping" means starting up your computer. It involves all those stages, from the moment you power on your machine till the system is ready to log you in.

In operating systems, the term refers to the process in which a part of the operating system is brought into the Main Memory, with the processor executing it. The internal data structures of the Linux Kernel are also initialized, values are set to the constituent variable(s), and processes are created (that usually spawn other significant processes later). Computer bootstrapping is a long and complicated task, cause when the computer is switched on, all the hardware devices are in a unpredictable state, while the RAM is inactive and in a random state. Thus, the thing to be kept in mind is, the process called "bootstrapping" is highly dependent on the computer architecture.

The Boot Process

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The Boot process involves several different stages that the system undergoes while it is being booted. If any of these stages fail, then the system cannot start itself.

  • The BIOS
  • Kernel Initialization
  • Hardware Configuration
  • System Processes
  • Startup Scripts
  • Multiuser Mode

In a Linux system what are the important files checked by the system when it is (i.e. system) is re-started after failure?

the /etc/rc.d directory

contains the system startup scripts. This is a good directory to backup

often. It will definitely save you a lot of re-configuration later if you

re-install or lose your current installation.

What are boot loaders? Identify any two boot loaders available under Linux systems

The BIOS invokes (note: NOT executes) a special program whose main (rather only) task is to load the image of an operating system Kernel into RAM. This program is called the Boot Loader. Before we proceed any further, let's take a brief look in the different ways a system can be booted:

  • Booting Linux from a Floppy disk m
  • Booting Linux from a Hard disk
  • Booting Linux from a Floppy disk : When booting from a floppy disk, the instructions stored in the first sector of the floppy disk is loaded in RAM and executed. These instructions then copy all the remaining sectors containing the Kernel image into RAM.
  • Booting Linux from a Hard disk : When booting from the hard disk, the booting procedure is different. The first sector of the hard disk, called the Mater Boot Record (MBR) includes the partition table and a small program. This program loads the first sector of the partition containing the operating system to be started. Linux is highly flexible and sophisticated piece of software, thus it replaces this small program in the MBR with a sophisticated program called LILO (LInux boot LOader). LILO allows users to select the operating system to be booted.

The installation program provides two boot loaders for you to choose from, GRUB and LILO.

GRUB (GRand Unified Bootloader), which is installed by default, is a very powerful boot loader. GRUB can load a variety of free operating systems, as well as proprietary operating systems with chain-loading (the mechanism for loading unsupported operating systems, such as DOS or Windows, by loading another boot loader).

LILO (LInux LOader) is a versatile boot loader for Linux. It does not depend on a specific file system, can boot Linux kernel images from floppy diskettes and hard disks, and can even boot other operating systems.

Explain the following commands or terms used in Linux administration:

shutdown –h now

halt

reboot

telinit

power off

vmliuz

fsck

SHUTDOWN

Name

shutdown - bring the system down

Synopsis

/sbin/shutdown [-t sec] [-arkhncfFHP] time

-h

Halt or poweroff after shutdown

-r

Reboot after shutdown.

-h

Halt or poweroff after shutdown.

-H

Halt action is to halt or drop into boot monitor on systems that support it.

-P

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Halt action is to turn off the power.

-n

[DEPRECATED] Don't call init(8) to do the shutdown but do it ourself. The use of this option is discouraged, and its results are not always what you'd expect.

-f

Skip fsck on reboot.

-F

Force fsck on reboot.

-c

Cancel an already running shutdown. With this option it is of course not possible to give the time argument, but you can enter a explanatory message on the command line that will be sent to all users.

time

when to shutdown

HALT, REBOOT, POWEROFF:

Name

halt, reboot, poweroff - stop the system.

Synopsis

/sbin/halt [-n] [-w] [-d] [-f] [-i] [-p] [-h]

/sbin/reboot [-n] [-w] [-d] [-f] [-i]

/sbin/poweroff [-n] [-w] [-d] [-f] [-i] [-h]

Description

Halt notes that the system is being brought down in the file /var/log/wtmp, and then either tells the kernel to halt, reboot or poweroff the system.

If halt or reboot is called when the system is not in runlevel 0 or 6, in other words when it's running normally, shutdown will be invoked instead (with the -h or -r flag). For more info see the shutdown(8) manpage.

The rest of this manpage describes the behaviour in runlevels 0 and 6, that is when the systems shutdown scripts are being run.

Options

-n

Don't sync before reboot or halt. Note that the kernel and storage drivers may still sync.

-w

Don't actually reboot or halt but only write the wtmp record (in the /var/log/wtmp file).

-d

Don't write the wtmp record. The -n flag implies -d.

-f

Force halt or reboot, don't call shutdown(8).

-i

Shut down all network interfaces just before halt or reboot.

-h

Put all harddrives on the system in standby mode just before halt or poweroff.

-p

When halting the system, do a poweroff. This is the default when halt is called as poweroff.

TELINIT:

Name

init, telinit - process control initialization

Synopsis

/sbin/init [ -a ] [ -s ] [ -b ] [ -z xxx ] [ 0123456Ss ]

/sbin/telinit [ -t sec ] [ 0123456sSQqabcUu ]

Description

Init

Init is the parent of all processes. Its primary role is to create processes from a script stored in the file /etc/inittab (see inittab(5)). This file usually has entries which cause init to spawn gettys on each line that users can log in. It also controls autonomous processes required by any particular system.

Telinit

/sbin/telinit is linked to /sbin/init. It takes a one-character argument and signals init to perform the appropriate action. The following arguments serve as directives to telinit:

0,1,2,3,4,5 or 6

tell init to switch to the specified run level.

a,b,c

tell init to process only those /etc/inittab file entries having runlevel a,b or c.

Q or q

tell init to re-examine the /etc/inittab file.

S or s

tell init to switch to single user mode.

U or u

tell init to re-execute itself (preserving the state). No re-examining of /etc/inittab file happens. Run level should be one of Ss12345, otherwise request would be silently ignored.

telinit can also tell init how long it should wait between sending processes the SIGTERM and SIGKILL signals. The default is 5 seconds, but this can be changed with the -t sec option.

telinit can be invoked only by users with appropriate privileges.

The init binary checks if it is init or telinit by looking at its process id; the real init's process id is always 1. From this it follows that instead of calling telinit one can also just use init instead as a shortcut.

FSCK:

Name

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fsck - check and repair a Linux file system

Synopsis

fsck [ -sAVRTNP ] [ -C [ fd ] ] [ -t fstype ] [filesys ... ] [--] [ fs-specific-options ]

Description

fsck is used to check and optionally repair one or more Linux file systems. filesys can be a device name (e.g. /dev/hdc1, /dev/sdb2), a mount point (e.g. /, /usr, /home), or an ext2 label or UUID specifier (e.g. UUID=8868abf6-88c5-4a83-98b8-bfc24057f7bd or LABEL=root). Normally, the fsck program will try to handle filesystems on different physical disk drives in parallel to reduce the total amount of time needed to check all of the filesystems.

If no filesystems are specified on the command line, and the -A option is not specified, fsck will default to checking filesystems in /etc/fstab serially. This is equivalent to the -As options.

The exit code returned by fsck is the sum of the following conditions:

0 - No errors

1 - File system errors corrected

2 - System should be rebooted

4 - File system errors left uncorrected

8 - Operational error

16 - Usage or syntax error

32 - Fsck canceled by user request

128 - Shared library error

The exit code returned when multiple file systems are checked is the bit-wise OR of the exit codes for each file system that is checked.

In actuality, fsck is simply a front-end for the various file system checkers (fsck.fstype) available under Linux. The file system-specific checker is searched for in /sbin first, then in /etc/fs and /etc, and finally in the directories listed in the PATH environment variable. Please see the file system-specific checker manual pages for further details.

Options

-s

Serialize fsck operations. This is a good idea if you are checking multiple filesystems and the checkers are in an interactive mode. (Note: e2fsck(8) runs in an interactive mode by default. To make e2fsck(8) run in a non-interactive mode, you must either specify the -p or -a option, if you wish for errors to be corrected automatically, or the -n option if you do not.)

-t fslist

Specifies the type(s) of file system to be checked. When the -A flag is specified, only filesystems that match fslist are checked. The fslist parameter is a comma-separated list of filesystems and options specifiers. All of the filesystems in this comma-separated list may be prefixed by a negation operator 'no' or '!', which requests that only those filesystems not listed in fslist will be checked. If all of the filesystems in fslist are not prefixed by a negation operator, then only those filesystems listed in fslist will be checked.

Options specifiers may be included in the comma-separated fslist. They must have the format opts=fs-option. If an options specifier is present, then only filesystems which contain fs-option in their mount options field of /etc/fstab will be checked. If the options specifier is prefixed by a negation operator, then only those filesystems that do not have fs-option in their mount options field of /etc/fstab will be checked.

For example, if opts=ro appears in fslist, then only filesystems listed in /etc/fstab with the ro option will be checked.

For compatibility with Mandrake distributions whose boot scripts depend upon an unauthorized UI change to the fsck program, if a filesystem type of loop is found in fslist, it is treated as if opts=loop were specified as an argument to the -t option.

Normally, the filesystem type is deduced by searching for filesys in the /etc/fstab file and using the corresponding entry. If the type can not be deduced, and there is only a single filesystem given as an argument to the -t option, fsck will use the specified filesystem type. If this type is not available, then the default file system type (currently ext2) is used.

-A

Walk through the /etc/fstab file and try to check all file systems in one run. This option is typically used from the /etc/rc system initialization file, instead of multiple commands for checking a single file system.

The root filesystem will be checked first unless the -P option is specified (see below). After that, filesystems will be checked in the order specified by the fs_passno (the sixth) field in the /etc/fstab file. Filesystems with a fs_passno value of 0 are skipped and are not checked at all. Filesystems with a fs_passno value of greater than zero will be checked in order, with filesystems with the lowest fs_passno number being checked first. If there are multiple filesystems with the same pass number, fsck will attempt to check them in parallel, although it will avoid running multiple filesystem checks on the same physical disk.

Hence, a very common configuration in /etc/fstab files is to set the root filesystem to have a fs_passno value of 1 and to set all other filesystems to have a fs_passno value of 2. This will allow fsck to automatically run filesystem checkers in parallel if it is advantageous to do so. System administrators might choose not to use this configuration if they need to avoid multiple filesystem checks running in parallel for some reason --- for example, if the machine in question is short on memory so that excessive paging is a concern.

-C [ "fd" ]

Display completion/progress bars for those filesystem checkers (currently only for ext2 and ext3) which support them. Fsck will manage the filesystem checkers so that only one of them will display a progress bar at a time. GUI front-ends may specify a file descriptor fd, in which case the progress bar information will be sent to that file descriptor.

-N

Don't execute, just show what would be done.

-P

When the -A flag is set, check the root filesystem in parallel with the other filesystems. This is not the safest thing in the world to do, since if the root filesystem is in doubt things like the e2fsck(8) executable might be corrupted! This option is mainly provided for those sysadmins who don't want to repartition the root filesystem to be small and compact (which is really the right solution).

-R

When checking all file systems with the -A flag, skip the root file system (in case it's already mounted read-write).

-T

Don't show the title on startup.

-V

Produce verbose output, including all file system-specific commands that are executed.

fs-specific-options

Options which are not understood by fsck are passed to the filesystem-specific checker. These arguments must not take arguments, as there is no way for fsck to be able to properly guess which arguments take options and which don't.

Options and arguments which follow the

-- are treated as file system-specific options to be passed to the file system-specific checker.

Please note that fsck is not

designed to pass arbitrarily complicated options to filesystem-specific checkers. If you're doing something complicated, please just execute the filesystem-specific checker directly. If you pass fsck some horribly complicated option and arguments, and it doesn't do what you expect, don't bother reporting it as a bug. You're almost certainly doing something that you shouldn't be doing with fsck.

Options to different filesystem-specific fsck's are not standardized. If in doubt, please consult the man pages of the filesystem-specific checker. Although not guaranteed, the following options are supported by most file system checkers:

-a

Automatically repair the file system without any questions (use this option with caution). Note that e2fsck(8) supports -a for backwards compatibility only. This option is mapped to e2fsck's -p option which is safe to use, unlike the -a option that some file system checkers support.

-n

For some filesystem-specific checkers, the -n option will cause the fs-specific fsck to avoid attempting to repair any problems, but simply report such problems to stdout. This is however not true for all filesystem-specific checkers. In particular, fsck.reiserfs(8) will not report any corruption if given this option. fsck.minix(8) does not support the -n option at all.

-r

Interactively repair the filesystem (ask for confirmations). Note: It is generally a bad idea to use this option if multiple fsck's are being run in parallel. Also note that this is e2fsck's default behavior; it supports this option for backwards compatibility reasons only.

-y

For some filesystem-specific checkers, the -y option will cause the fs-specific fsck to always attempt to fix any detected filesystem corruption automatically. Sometimes an expert may be able to do better driving the fsck manually. Note that not all filesystem-specific checkers implement this option. In particular fsck.minix(8) and fsck.cramfs(8) does not support the -y option as of this writing.

Author

 

Files

/etc/fstab.

Environment Variables

The fsck program's behavior is affected by the following environment variables:

FSCK_FORCE_ALL_PARALLEL

If this environment variable is set, fsck will attempt to run all of the specified filesystems in parallel, regardless of whether the filesystems appear to be on the same device. (This is useful for RAID systems or high-end storage systems such as those sold by companies such as IBM or EMC.)

FSCK_MAX_INST

This environment variable will limit the maximum number of file system checkers that can be running at one time. This allows configurations which have a large number of disks to avoid fsck starting too many file system checkers at once, which might overload CPU and memory resources available on the system. If this value is zero, then an unlimited number of processes can be spawned. This is currently the default, but future versions of fsck may attempt to automatically determine how many file system checks can be run based on gathering accounting data from the operating system.

PATH

The PATH environment variable is used to find file system checkers. A set of system directories are searched first: /sbin, /sbin/fs.d, /sbin/fs, /etc/fs, and /etc. Then the set of directories found in the PATH environment are searched.

FSTAB_FILE

This environment variable allows the system administrator to override the standard location of the /etc/fstab file. It is also useful for developers who are testing fsck.

What will you do when you are stuck and unable to know the command to use as well as where to find configuration files? Explain the commands you will use to get you out of this mess.

System and Network Configuration

  • linuxconf - A GUI interactive interface available on Redhat 6.0 or later which includes netconf configuration.
  • netconf - A GUI interactive interface available on Redhat 6.0 and later.
  • kbdconf - A Redhat Linux tool which configures the /etc/sysconfig/keyboard file which specifies the location of the keyboard map file. This is a GUI based tool.
  • mouseconfig - A Redhat Linux tool used to configure the /etc/sysconfig.mouse file. This is a GUI tool.
  • timeconfig - A Redhat Linux tool used to configure the /etc/sysconfig/clock file. This is a GUI tool used to set timezone and whether or not the clock is set to GMT time.
  • kernelcfg - A Redhat kernel configuration utility to be started from X.
  • stty - Used to configure and print the console devices.
  • setterm - Set terminal attributes.
  • vmstat - Report statistics on virtual memory.

X Configuration

  • XF86Setup - A newer X configuration program with a GUI interface which modifies the "/etc/X11/XF86Config" configuration file.
  • xf86config - An older X configuration program with a text based interface. It also modifies the "/etc/X11/XF86Config" configuration file.
  • Xconfigurator - The Redhat tool used during system setup to configure X.
  • SuperProbe - A program that probes the video card to determine its type for use with setting up X.
  • xvidtune - This program will test video modes on the fly without modification to your X configuration. Read the usr/X11R6/lib/X11/doc/VideoModes.doc file before running this program.

Library and kernel Dependency Management

Library management:

  • ldd - Used to determine shared libraries used by binary files. Type "ldd /bin/ls" to see the shared libraries used by the "ls" command.
  • ldconfig - Used to update links and cache for system use of the most recent runtime shared libraries.

Kernel Management:

  • lsmod - List currently installed kernel modules.
  • depmod - Creates a dependency file, "modules.dep" in the directory "/lib/modules/x.x.x", later used by modprobe to automatically load the relevant modules.
  • insmod - Installs a loadable kernel module into the running kernel.
  • rmmod - Unloads modules, Ex: rmmod ftape
  • modprobe - Used to load a module or set of modules. Loads all modules specified in the file "modules.dep".

General Diagnostic

System resources

  • free - Show system memory availability and usage
  • df - Show the amount of disk free space on each mounted filesystem.
  • du - Show disk usage
  • lspci - List PCI devices
  • pnpdump - Lists ISA PNP device resource information.
  • vmstat - Reports virtual memory statistics.

Other:

  • env - List the current environment variables.
  • printenv - Print a copy of the environment.
  • set - Shows how the environment is set up. This command can be very useful when debugging the environment.
  • runlevel - List the current and previous runlevel.
  • uname - Print system information. In my case, it prints "Linux".
  • dmesg - Show the last kernel messages printed during the last boot.