How Linux Boots: From Kernel Load to User Login Explained

Linux Boot Overview

After the BIOS hands control to the hardware, the Linux kernel is read from the /boot directory. Typical files include vmlinuz-3.2.0‑amd64, initrd.img-3.2.0‑amd64, and various config-* files.

Loading the Kernel

The kernel image is loaded first, then the first user‑space program /sbin/init starts with PID 1. This init process is responsible for initializing the system environment and launching other services.

Init Process and Runlevels

Init reads /etc/inittab to determine the default runlevel (usually 2 on Debian). Each runlevel (0‑6) has a corresponding directory /etc/rcN.d containing scripts that start (prefix S) or stop (prefix K) services. The two‑digit numbers in script names define the order of execution.

$ ls /etc/rc2.d
README
S01motd
S13rpcbind
S14nfs-common
S16binfmt-support
S16rsyslog
S16sudo
S17apache2
S18acpid
...

All scripts in these directories are symbolic links to real startup scripts stored in /etc/init.d, simplifying management.

Managing Services

To restart a service, invoke its script in /etc/init.d, e.g.:

$ sudo /etc/init.d/apache2 restart

User Login Methods

After services start, the system presents three login options:

Console login: getty → login (and PAM modules) reads /etc/passwd to start the user's shell.

SSH login: sshd replaces getty and starts the shell.

Graphical login: a display manager (e.g., gdm) starts the session after authentication.

Login Shell Initialization

The login shell reads /etc/profile (system‑wide) and then one of the user files ~/.bash_profile, ~/.bash_login, or ~/.profile, stopping after the first one found.

Non‑Login Shells

When a user opens a new terminal after logging in, a non‑login shell is started. It does not read the login files but loads ~/.bashrc, where most user customizations reside.

Ensuring .bashrc Is Executed

Debian’s ~/.profile contains logic to source ~/.bashrc if Bash is the shell:

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

If a ~/.bash_profile exists, it may bypass ~/.profile. Adding the following to ~/.bash_profile forces ~/.profile to run:

if [ -f ~/.profile ]; then
  . ~/.profile
fi

Why Bash Has Multiple Config Files

Historically, loading all configuration at once was slow, so Bash split settings: system‑wide in /etc/profile, inherited user settings in ~/.profile, and non‑inherited settings in ~/.bashrc. macOS also uses Bash, loading only ~/.bash_profile which then sources ~/.bashrc.