101. System Architecture

This is a note I made while I was reading a book about Linux

@: Command, +: File, #: Example

• Change Runlevels and Shut Down or Reboot System
  • – runlevel 0
    • – shutdown
  • – runlevel 1
    • – single user mode
  • – runlevel 2
    • – multiuser on Debian
    • – multiuser without NFS file sharing or X Window System (Red hat)
  • – runlevel 3
    • – Defulat multiuser (Red hat)
      • – usually except X Window System
  • – runlevel 4
    • – typically unused
  • – runlevel 5
    • – Full multiuser with GUI (Red hat)
  • – runlevel 6
    • – reboot
  • – init starts all other processes and place the system in default runlevel

• Single User Mode
  • – used for system configuration tasks
    • – to correct problems with corrupt fs and cannot be auto-correct
  • – must be performed with no user activity
  • – done by changing bootloader parameter
    • – add 1 or single at the end of the line
    • – the argument not interpreted by kernel, but pass long to the init process
  • – or # init 1

• Overview of the /etc Directory Tree and the init Process
  • + init and /etc
    • – actions for each runlevel are derived from the style of initialization in Unit System V
      • – and are specified in a series of directories and script files under the /etc directory
  • – When Linux Systems starts
    • – runs scripts in /etc to initial configuration
    • – and switch among runlevels
    • – may differ among distributions
  • – /etc/rc.sysinit or /etc/init.d/rcS
    • – rc.sysinit
      • – redhat
      • – monolithic init script
    • – rcS
      • – running several small scripts
    • – lunched by init process
    • – esential preparation for system
      • – run before any system daemons are started
  • + /etc/rc.local
    • – called after all other init files
      • – (after all system daemon started)
    • – local customizations
      • – affecting system startup
      • – provides an alternative to modifying the other init scripts
    • – modifying rc.sysinit might lost
      • – but modifying rc.local does not lost
  • + /etc/rc
    • – used to changed between runlevels
    • – no Debian
  • – starting and stopping daemons is handled by scrips and symbolic links in /etc/init.d
    • – and by runlevel-specific directories
      • /etc/rc0.d to /etc/rc6.d
    • + /etc/init.d
      • – contains startup/shutdown scripts for each services
        • + ex) /etc/init.d/http
          • – take a single argument
            • – start and stop
            • @ /etc/init.d/http restart
        • – admin can use scripts to start and stop sercies
        • – add new services’ init files could be installed automatically
          • – if not, need to create one
            • – or place startup commands in rc.local
        • – the scripts are not directly executed by the init process
      • + /etc/rc0.d to /etc/rc6.d
        • – contains soft symbolic links to the scripts in /etc/init.d
        • – When init process enter runlevel N,
          • – look throughs all links in rcN.d
        • – KNNname and SNNname
        • – K and S prefixes
          • – kill and start
          • – defines which sercies should run or not
          • – K
            • – mark files for all services should not run
          • – S
            • – mark files for all services are to be running
        • – NN
          • – sequence number
          • – relative order for services to be started or stopped
          • – lower run first
        • – name
        • – since K comes before S,
          • – K services are stopped before S

• Setting the Default Runlevel
  • + /etc/inittab
    • – to determine, init proc reads configuration file in the directory
      • – looking for line containint word initdefault
        • – id:N:initdefault
          • – N is valid runlevel

• Determining Your System’s Runlevel
  • @ runlevel
    • – display previous and current runlevel
      • – if no, print N

• Changing runlevels with init and telinit
  • @ init N
    • – send signals to execute init process
      • – and instructing it to change runlevel
  • @ telinit
    • – link to init
  • – System shutdown with shutdown
    • – when shutdown initiated
      • – all logged on users are notified
      • – further logins are blockes
    • @ shutdown [options] time [warning_message]
      • – shutdown planned matter
      • – default to single user mode
      • – initally use init
      • – time
        • – hh:m
        • – +n
          • – minutes to wait
        • – now
      • – options
        • -f -h -k -r -F
        • -f
          • -fast boot (skips filesystem checks on next boot)
        • -k
          • – just send warning message

• On LPIC
  • – familiar with default runlevels steps for init process