Understanding Linux Boot Process: From Kernel Loading to User Login

Loading the Kernel

After the BIOS hands control to the operating system, the kernel image located in /boot is read. Typical Debian /boot files include vmlinuz-3.2.0‑amd64, initrd.img-3.2.0‑amd64, and related configuration files.

$ ls /boot
config-3.2.0-amd64
initrd.img-3.2.0-amd64
vmlinuz-3.2.0-amd64
...

Starting the Init Process

The loaded kernel launches the first user‑space program /sbin/init, which becomes process ID 1. All other processes are descendants of this init process.

Determining the Runlevel

Init reads /etc/inittab to set the default runlevel (e.g., id:2:initdefault: means runlevel 2). Each runlevel (0‑6) has a corresponding directory /etc/rcN.d that lists the programs to start.

/etc/rc0.d
/etc/rc1.d
/etc/rc2.d
/etc/rc3.d
/etc/rc4.d
/etc/rc5.d
/etc/rc6.d

Files in these directories follow the naming pattern S<NN>program (start) or K<NN>program (kill), where NN determines execution order.

$ ls /etc/rc2.d
README
S01motd -> ../init.d/motd
S13rpcbind -> ../init.d/rpcbind
S14nfs-common -> ../init.d/nfs-common
S16rsyslog -> ../init.d/rsyslog
S16sudo -> ../init.d/sudo
S17apache2 -> ../init.d/apache2
S18acpid -> ../init.d/acpid
...

All entries are symbolic links to scripts in /etc/init.d, allowing a single copy of each script to serve multiple runlevels.

User Login

Once startup programs finish, the system presents a login interface. Three common login methods are:

Console login via getty and login (or PAM modules).

SSH login using sshd.

Graphical login via a display manager such as GDM.

Entering the Login Shell

After successful authentication, the user’s login shell (default bash on Debian) is started. Bash reads configuration files in a specific order depending on the login method.

~/.bash_profile
~/.bash_login
~/.profile

Only the first existing file among the three is sourced; it typically loads /etc/profile and then the user‑specific ~/.profile.

Opening a Non‑Login Shell

When a user opens a terminal after login, a non‑login shell is created. This shell does not read the login files but does source ~/.bashrc. Debian ensures ~/.bashrc is executed for all shells by adding the following snippet to ~/.profile (or ~/.bash_profile if it exists):

if [ -n "$BASH_VERSION" ]; then
  if [ -f "$HOME/.bashrc" ]; then
    . "$HOME/.bashrc"
  fi
fi

Thus, user customizations placed in ~/.bashrc are applied consistently, regardless of how the shell was started.

Additional Notes

Mac OS X also uses Bash as its default shell; it loads .bash_profile which in turn sources .bashrc, ensuring similar behavior for both SSH and graphical terminal sessions.