How Does Linux Boot? A Step‑by‑Step Guide to the Linux Startup Process

1. Load Kernel

After the operating system takes control of the hardware, it reads the kernel file from the /boot directory.

Example /boot directory listing on a typical machine:

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

2. Start Init Process

Once the kernel is loaded, it runs the first program /sbin/init, which initializes the system environment. The init process receives PID 1, and all other processes are its descendants.

3. Determine Runlevel

Programs that start automatically at boot are called daemons. Init runs them according to the system’s runlevel. Linux defines seven runlevels (0‑6); Debian’s default runlevel is 2, which corresponds to multi‑user mode.

Init reads /etc/inittab. The line id:2:initdefault: sets the default runlevel to 2.

Each runlevel has a directory /etc/rcN.d that contains scripts to start (prefix S) or stop (prefix K) services. The numeric suffix determines the order; lower numbers run earlier.

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

4. Load Startup Programs

All entries in /etc/rcN.d are symbolic links to real scripts in /etc/init.d. Init executes these scripts in order.

Example of the links in /etc/rc2.d:

$ ls -l /etc/rc2.d
  README
  S01motd -> ../init.d/motd
  S13rpcbind -> ../init.d/rpcbind
  S14nfs-common -> ../init.d/nfs-common
  S16binfmt-support -> ../init.d/binfmt-support
  S16rsyslog -> ../init.d/rsyslog
  S16sudo -> ../init.d/sudo
  S17apache2 -> ../init.d/apache2
  S18acpid -> ../init.d/acpid
  ...

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

$ sudo /etc/init.d/apache2 restart

5. User Login

After startup programs finish, the system presents a login prompt. Three login methods are supported:

Console (getty → login)

SSH (sshd)

Graphical login (display manager, e.g., gdm)

6. Enter Login Shell

Debian’s default shell is Bash. For console and SSH login, Bash reads /etc/profile and then the first existing file among ~/.bash_profile, ~/.bash_login, or ~/.profile. For graphical login, only /etc/profile and ~/.profile are read.

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

7. Open Non‑Login Shell

When a user opens a new terminal, a non‑login shell starts; it does not read /etc/profile or ~/.profile but reads ~/.bashrc. Debian ensures ~/.bashrc is sourced from ~/.profile:

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

Similarly, adding the following to ~/.bash_profile guarantees ~/.bashrc runs even when ~/.bash_profile exists:

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

Thus, regardless of the login method, user customizations placed in ~/.bashrc are applied.