A Linux User Reference

Search tips
  • search ignores words that are less than 4 characters in length
  • searches are case insensitve
  • if a search does not return anything try it in Boolean mode then Query expansion mode by checking the appropriate radio button e.g. searching for 'cron' in just the Administration category returns nothing - presumably because the 50% threshold is reached. Boolean mode ignores this threshold so a search for 'cron' returns several hits
  • in Boolean mode preceding a word with a '+' means the result must include that word, a '-' means it must not
  • in Boolean mode '+crontab -anacron' means match articles about crontab that DO NOT mention anacron
  • to match a phrase e.g. 'manage system' check the Boolean mode radio button and enclose the phrase in quotes "some phrase ..."
  • in Query expansion mode the search context is expanded beyond the keywords you entered - relevancy of hits may well be degraded


Boot process

  • System boot overview
    Step Action
    (1) Power on system
    (2) BIOS executes code stored in non-volatile ram (EEPROM, ROM, flash memory)
    (3) Code performs POST (Power ON Self Test)
    (4) Code loads MBR from first sector of boot drive
    (5) Control of boot process is passed to code read from MBR - the partition boot record and a stage 1 boot loader
    (6) Stage 1 boot loader locates and loads the Stage 2 (or Stage 1.5 which then loads Stage 2) boot loader
    (7) Stage 2 boot loader locates kernel image and loads
    (8) Root filesystem is mounted. A temporary ramdisk image can be loaded
    (9) The init process runs. The parent of all other Linux processes PID=1
    (10) init reads /etc/inittab (if it exists), determines configuration of the remaining boot steps. Default runlevel is set near top of file
  • System shutdown and restart
    'telinit' or 'init'

    can be used to stop multiuser activity, switch to single-user mode, halt the system etc..


    sends all running processes a SIGTERM signal. After 5 seconds, or a specified delay, 'init' then sends a SIGKILL signal to forcibly end each remaining process.


    sends a warning message to all logged on users, blocks further logins, signals init to switch runlevels.Brings the system down in a secure, organised manner, by default shutdown switches to runlevel 1 (single-user mode).

    a shudown can be cancelled if there is enough time remaining since it was initiated.

    Command usage

    shutdown [-t sec] [-arkhncfFHP]
             time [warning-message]
    Common options:
     -t sec                 Wait sec seconds between sending processes the warning and 
                            the kill signal
     -a                     Use /etc/shutdown.allow
     -k                     Do not really shutdown; only send the warning messages to 
     -r                     Reboot after shutdown
     -h                     Halt or poweroff after shutdown
     -f                     Skip fsck on reboot
     -F                     Force fsck on reboot
     -c                     Cancel an already running shutdown, no 'time' arg, can 
                            give a 'warning-message'
     time                   When to shutdown:
           hh:mm                 absolute time
           +m                    number of minutes to wait
           now                   alias for +0

    'Ctrl-c' may cancel depending on how far the process has progressed and if the command is running in the foreground.

  • Halt, shutdown and reboot a system
    /sbin/halt, reboot, poweroff

    Halt the system

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

    When used in runlevels 1-5, 'halt' calls 'shutdown' with '-h' or '-r' flags.

    Reboot - shutdown first then reboot

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

    Poweroff - shut down the system and power off the machine

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

    Some Options

     -n                    Do not sync before reboot or halt
     -w                    Do not reboot or halt but only write the wtmp record
     -d                    Do not write the wtmp record.  The -n flag implies -d.
     -f                    Force halt or reboot, do not call shutdown
     -i                    Shut down all network interfaces just before halt or reboot
     -h                    Put all hard drives on the system in standby mode just 
                           before halt or poweroff
     -p                    Do a poweroff on halt.  Default = poweroff on halt.

    If an option can be applied to one of the above commands then it has the same meaning as above.

  • Influencing the boot process

    There are four main areas where the booting of a system may be influenced:

    Area Method
    BIOS Change boot order, boot device. Modern BIOSes support booting from various devices e.g. floppy, USB disk, CD-Rom, DVD, network ...
    Boot Loader If applicable, select which OS and or OS version to boot
    Kernel Pass parameters to the kernel (via the boot loader) to influence what it does during loading
    /sbin/init Change the script or it's configuration file /etc/inittab to alter startup behavior
  • Influence the boot process using the LILO boot loader

    On startup LILO presents the user with the following prompt


    Can either pass parameters to the kernel

    LILO:  apci=force vga=0x317

    or hit the 'tab' key to select a kernel/OS to boot

      linux* vmlinuz-2.6.24-23-generic
      linux vmlinuz-2.6.24-21-custom

    (* denotes default image, unless overridden it is the first 'image=' entry in 'lilo.conf')

    If LILO is not configured to be interactive i.e. Tab key has no affect, press and hold the Alt or Shift key before the LILO message appears.

  • Influence the boot process using the GRUB boot loader

    Grub provides a menu as opposed to a prompt. The menu consists of available OSs to boot.

    Grub menu commands

     e                  On an entry to edit it
     c                  To switch to a GRUB command line - alter or append kernel parameters
     b                  To boot the system
     Enter              Boots
     p                  To enter a password
     esc                To return to the menu or to the previous step. 

    Run a grub simulated shell, can test configuration or run commands

    # grub
    Probing devices to guess BIOS drives.  This may take a long time.
    grub> help
    blocklist FILE      boot
    cat FILE            chainloader [--force] FILE

    *** you are probably using grub2 now ...

  • Influence the boot process via kernel parameters

    To control kernel modules use /etc/modprobe.conf (/etc/modules.conf or /etc/conf.modules for earlier versions).

    Kernel parameters can be supplied via the boot loaders cmd-line or set in the boot loader config files. They control the kernel ONLY.

    Partial list of kernel parameters

    console=ttyn                    Use virtual console device n.
    debug                           Set kernel logging to debug level KERN_DEBUG (7).
    quiet                           Set log level to (4), suppress all messages except really
                                    serious ones.
    loglevel=0  (KERN_EMERG)        System is unusable.
    loglevel=1  (KERN_ALERT)        Actions that must be taken immediately.
    loglevel=2  (KERN_CRIT)         Critical conditions.
    loglevel=3  (KERN_ERR)          Non-critical error conditions.
    loglevel=4  (KERN_WARNING)      Warning conditions that should be addressed.
    loglevel=5  (KERN_NORMAL)       Normal but significant events.
    loglevel=6  (KERN_INFO)         Informational messages that require no action.
    loglevel=7  (KERN_DEBUG)        Debug info if kernel debugging is enabled in config.
    apic=[quiet|verbose|debug]      Intel chip Advanced Programmable Interrupt Controller. 
    noapic                          Do not use any IOAPICs that may be in the system.
    irqfixup                        Search all known interrupt handlers to find one for an 
                                    interrupt that is not handled.  Intended for systems with
                                    badly broken firmware.
    irqpoll                         Search all known interrupt handlers and check all handlers
                                    on each timer interrupt for an interrupt that is not 
                                    handled.  Intended for systems with badly broken firmware.
    resume=suspend_device           Which disk contains the suspended kernel image.  This is
                                    the kernel device name not userspace device name.
    noresume                        Disable.  Any swap space used to hold suspended system
                                    images will revert back to available swap space.
    cachesize=n (bytes)             Override level 2 CPU cache size detection.  Sometimes CPU 
                                    hw bugs may report this size incorrectly.  Kernel attempts
                                    to suss it out but cannot always do so.  This option 
                                    provides an override for this situation.
    nosmp                           Tell SMP kernel to act as a uniprocessor even on a 
                                    multiprocessor machine.
    max_cpus                        Max number of CPUs to use even if there are more.
    SCHEDULER OPTIONS:              There are some.
    initrd=filename                 Location of ramdisk image.
    noinitrd                        Do not use one even if configured elsewhere to be used.
    ro                              Mount root device as read-only at boot.
    rw                              Read-write.
    root=device                     Specify root filesystem to be used.
    rootfstype=type                 Try to mount root fs as this type e.g. rootfstype=ext3.
    init=filename                   Run this binary as the init process instead of default is
    rdinit=full_path_name           Run init process from ramdisk instead of root fs
    S|s|single|1                    Run init in single-user-mode.  Default is multi-user-mode
    number                          Run level to boot into
  • Print or manage the kernel ring buffer

    Command usage

    dmesg [ -c ] [ -n level ] [ -s bufsize ]
     -c                         Clear the ring buffer contents after printing.
     -sbufsize                  Use buffer of size bufsize to query the kernel ring buffer.
                                16392 by default
     -nlevel                    Set the level at which logging of messages is done to the console
                                e.g. -n 1 prints only panic messages to console

    View boot messages

    $ dmesg | head -n 6
    [    0.000000] BIOS EBDA/lowmem at: 0009fc00/0009fc00
    [    0.000000] Initializing cgroup subsys cpuset
    [    0.000000] Initializing cgroup subsys cpu
    [    0.000000] Linux version 2.6.28-16-generic (buildd@yellow) (gcc version 4.3.3 \
                   (Ubuntu 4.3.3-5ubuntu4) ) #57-Ubuntu SMP Wed Nov 11 09:49:32 UTC   \
                   2009 (Ubuntu 2.6.28-16.57-generic)
    [    0.000000] Command line: root=UUID=155af2b2-9107-43c0-a034-67ce99db1ebb ro vga=0x317
    [    0.000000] KERNEL supported cpus:
  • Boot log files

    These are configurable in terms of logfile names and what messages get logged to each via 'syslogd' i.e. /etc/syslog.conf. Will vary from distribution to distribution.

    Common boot log files