Startup scripts

Contents

This part covers the steps involved in getting a Linux system booted, and the various customisations possible.

Booting

• The BIOS code is called from firmware to check basic devices and call into the O/S code
• The grub boot manager is installed by Linux as O/S entry point
• Grub uses a file to choose which disk/partition/code to boot:
  • the /boot/grub/grub.cfg or /boot/grub/menu.lst (Ubuntu and most sytems)
  • the /etc/grub2.cfg (Fedora)
  • UEFI versions are coming
• Linux boots either from the process init (many older Linux's), the upstart process (Ubuntu 11.10 - 14.04) or systemd (Fedora 16+). Note: Debian is moving to systemd and Ubuntu will drop upstart in favour of systemd. Very contentious!

init boot (old style)

• During booting, Linux starts the process init
• This reads the file /etc/inittab
• The script starts up the Linux services. e.g. the Western Digital World Book NAS disk has

  # Startup the system
  ::sysinit:/bin/mount /proc
  ::sysinit:/bin/mount -a
  ::sysinit:/bin/mount -o remount,rw,noatime /
  ::sysinit:/bin/ln -s /proc/mounts /etc/mtab
  ::sysinit:/sbin/swapon -a
  ::sysinit:/usr/sbin/sshd
  
  # Run any rc scripts
  ::sysinit:/etc/init.d/rcS
        

• The script loads services and modules and then calls the script /etc/init.d/rcS

upstart boot

This is the replacement method used by Ubuntu upto 14.04 http://upstart.ubuntu.com/)

• The init boot method was developed for static computer systems. Now with plug-n-play, devices and services need to be handled dynamically
• The upstart system is an event based system and responds to events in /etc/init
• Grub calls the script /etc/init/rc.conf
• This calls the script /etc/init.d/rc[0-6] for the appropriate runlevel
• Other events are handled e.g. /etc/init/networking.conf contains

  	
  ifup -a
  	
        

  to start the network devices

Systemd

This is the method now used by Fedora 15+ (see http://fedoraproject.org/wiki/Systemd) and future versions of Debian and Ubuntu

• Systemd uses services and targets listed in /etc/systemd/system
• The first target called is /etc/systemd/system/default.target
• This is usually a link to /lib/systemd/system/runlevel5.target for runlevel 5 (gui interface) or to /lib/systemd/system/runlevel3.target (command line interface)
• Change the link to change the default runlevel at boot time
• To start a service at boot time use systemctl enable foo.service
• To stop a service at any time use systemctl stop foo.service

Boot layers

Linux initialisation: setting the hostname

• In Ubuntu 14.04 /etc/init/hostname.conf calls

  	
  exec hostname -b -F /etc/hostname
  	
        

  while /etc/hostname in VirtualBox contains "newmarch-VirtualBox"
• Under systemd, all of the shell scripts are vanishing to be replaced by binary executables. Not impressed

Service intialisation: Network file systems

• The script to start the NFS under Fedora 16 booting into runlevel 3 is in /etc/rc.d/rc3.d/S25netfs:

  [ -f /etc/sysconfig/network ] || exit 0
  . /etc/init.d/functions
  . /etc/sysconfig/network
        

User startup scripts

• The shells read some scripts when they start. All shells read some scripts (e.g /etc/profile), some shells read extra scripts (e.g. bash reads /etc/bashrc)
• When bash is invoked as an interactive login shell, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable
• When an interactive shell that is not a login shell is started, bash reads and executes commands from /etc/bash.bashrc and ~/.bashrc, if these files exist

/etc/profile

• This file is run at login time to set shell variables for all users
• e.g. Ubuntu

  if [ -d /etc/profile.d ]; then
    for i in /etc/profile.d/*.sh; do
      if [ -r $i ]; then
        . $i
      fi
    done
    unset i
  fi
  
  if [ "$PS1" ]; then
    if [ "$BASH" ]; then
      PS1='\u@\h:\w\$ '
      if [ -f /etc/bash.bashrc ]; then
          . /etc/bash.bashrc
      fi
    else
      if [ "`id -u`" -eq 0 ]; then
        PS1='# '
      else
        PS1='$ '
      fi
    fi
  fi
  
  umask 022
        

• Note: the login shell does not read /etc/bash.bashrc by default, so one line in the script does that

~/.bash_profile

• This file can be modified by each user to change their environment
• It is used at login time to set/change shell variables for all of your own sessions
• e.g. Ubuntu

  # Get the aliases and functions
  if [ -f ~/.bashrc ]; then
          . ~/.bashrc
  fi
  
  # User specific environment and startup programs
  
  PATH=$PATH:$HOME/bin
  
  export PATH
  unset USERNAME
        

• Note: the login shell does not read ~/.bashrc by default, so one line in the script does that

/etc/bash.bashrc

• This script is read by each non-login shell (i.e. nearly all of them) (Not in Fedora 16)
• It is used to set shell functions and shell aliases for all users
• e.g. if you want "dir" to mean "ls -l" for all users you would add

  alias dir="ls -l"
        

~/.bashrc

• This script is read by each non-login shell (i.e. nearly all of them)
• It is used to set shell functions and shell aliases for yourself
• e.g. if you want "dir" to mean "ls -l" for yourself you would add

  alias dir="ls -l"
        

Examples

• If all users run Java, you need to set JAVA_HOME for all users. Add this to /etc/profile

  export JAVA_HOME=/usr/jdk
        

• If only you run Java, then instead add this to ~/bash_profile

  export JAVA_HOME=/usr/jdk
        

• To add the command "cleanup" to all users, add to /etc/bash.bashrc

  cleanup() {
      rm *~
  }
        

• To add the command "cleanup" to your shells, add to ~/.bashrc

  cleanup() {
      rm *~
  }
        

Changing runlevel

• Use who -r to show the current runlevel
• Use telinit 3 to change to runlevel three, etc

Conclusion

Booting up a Linux system requires starting hardware and software to manage this hardware. Logging in a user requires setting various parameters both from system files and from user overrides. Fedora, Debian and other systems accomplish the same results but have minor variations in how they do it.