Typical Linux Bootup

From Power Up to Bash Prompt

[..in a nutshell..
loading vmlinuz
loading initrd.gz
booting kernel
]


1. POST

2. LILO
- This reads the configuration file /etc/lilo.conf. The main thing in lilo.conf is one entry for each thing that lilo is set up to boot. For a Linux entry, this includes where the kernel is, and what disk partition to mount as the root filesystem.

3. Kernel -
Config:
--------
Most of the configuration of the kernel is done when you build it, using make menuconfig, or make xconfig in /usr/src/linux/ (or wherever your Linux kernel source is). You can reset the default video mode, root filesystem, swap device and RAM disk size using rdev. These parameters and more can also be passed to the kernel from lilo. You can give lilo parameters to pass to the kernel either in lilo.conf, or at the lilo prompt. For example if you wanted to use hda3 as your root file system instead of hda2, you might type
LILO: linux root=/dev/hda3

4. init
The next thing that happens as your computer starts up is that init is loaded and run. However, init, like almost all programs, uses functions from libraries.
In fact, you can put any program you like in /sbin/init, and the kernel will run it when it has finished loading.

It is init's job to get everthing running the way it should be. It checks that the file systems are ok and mounts them. It starts up 'daemons' to log system messages, do networking, serve web pages, listen to your mouse and so on. It also starts the getty processes that put the login prompts on your virtual terminals.

Init reads the file /etc/inittab, which tells it what to do. Typically, the first thing it is told to do is to run an initialisation script. The program that executes (or interprets) this script is bash, the same program that gives you a command prompt. In Debian systems, the initialisation script is /etc/init.d/rcS, on Red Hat, /etc/rc.d/rc.sysinit. This is where the filesystems get checked and mounted, the clock set, swap space enabled, hostname gets set etc.

Next, another script is called to take us into the default run-level. This just means a set of subsystems to start up. There is a set of directories /etc/rc.d/rc0.d, /etc/rc.d/rc1.d, ..., /etc/rc.d/rc6.d in Red Hat, or /etc/rc0.d, /etc/rc1.d, ..., /etc/rc6.d in Debian, which correspond to the run-levels. If we are going into runlevel 3 on a Debian system, then the script runs all the scripts in /etc/rc3.d that start with `S' (for start). These scripts are really just links to scripts in another directory usually called init.d.

So our run-level script was called by init, and it is looking in a directory for scripts starting with `S'. It might find S10syslog first. The numbers tell the run-level script which order to run them in. So in this case S10syslog gets run first, since there were no scripts starting with S00 ... S09. But S10syslog is really a link to /etc/init.d/syslog which is a script to start and stop the system logger. Because the link starts with an `S', the run-level script knows to execute the syslog script with a ``start'' parameter. There are corresponding links starting with `K' (for kill), which specify what to shut down and in what order when leaving the run-level.

To change what subsystems start up by default, you must set up these links in the rcN.d directory, where N is the default runlevel set in your inittab.

/etc/inittab is the top level configuration file for init.

The rcN.d directories, where N = 0, 1, ..., 6 determine what subsystems are started.

Somewhere in one of the scripts invoked by init, the mount -a command will be issued. This means mount all the file systems that are supposed to be mounted. The file /etc/fstab defines what is supposed to be mounted. If you want to change what gets mounted where when your system starts up, this is the file you will need to edit. There is a man page for fstab.

5. File system
In Linux, you 'mount' a disk filesystem onto the system's filesystem.

In the previous section I mentioned that init scripts check and mount the filesystems. The commands that do this are fsck and mount respectively.

6. Kernel Daemons
The information for ps aux (or top) command comes from the /proc filesystem that I mentioned in the previous section. Note that init is process number one. Processes 2, 3, 4 and 5 are kflushd, kupdate, kpiod and kswapd.
The kernel daemons are started after init, so they get process numbers like normal processes do. But their code and data lives in the kernel's part of the memory.

There are brackets around the entries in the command column because the /proc filesystem does not contain command line information for these processes.

7. System Logger
Init starts the syslogd and klogd daemons. They write messages to logs. The kernel's messages are handled by klogd, while syslogd handles log messages from other processes. The main log is /var/log/messages. This is a good place to look if something is going wrong with your system.

The file /etc/syslog.conf tells the loggers what messages to put where. Messages are identified by which service they come from, and what priority level they are. This configuration file consists of lines that say messages from service x with priority y go to z, where z is a file, tty, printer, remote host or whatever.

NOTE: Syslog requires the /etc/services file to be present. The services file allocates ports

8. Getty and Login
Getty is the program that enables you to log in through a serial device such as a virtual terminal, a text terminal, or a modem. It displays the login prompt. Once you enter your username, getty hands this over to login which asks for a password, checks it out and gives you a shell.

There are many getty's available. Some distributions, including Red Hat use a very small one called mingetty that only works with virtual terminals.

The login program is part of the util-linux package, which also contains a getty called agetty, which works fine. This package also contains mkswap, fdisk, passwd, kill, setterm, mount, swapon, rdev, renice, more (the program) and more (ie more programs).

9. Bash
If you give login a valid username and password combination, it will check in /etc/passwd to see which shell to give you. In most cases on a Linux system this will be bash. It is bash's job to read your commands and see that they are acted on. It is simultaneously a user interface, and a programming language interpreter.

The file /etc/profile controls the system-wide behaviour of bash. What you put in here will affect everybody who uses bash on your system. It will do things like add directories to the PATH, set your MAIL directory variable.

Once bash has read the system-wide configuration file, it looks for your personal configuration file. It checks in your home directory for .bash_profile, .bash_login and .profile. It runs the first one of these it finds. If you want to change the way bash behaves for you, without changing the way it works for others, do it here.