In the age of systemd, many Linux users have never seen a real init script. They know systemctl and journalctl, but have no clear picture of what actually happens when the kernel hands control over to userspace.

Slackware, one of the oldest living Linux distributions, still uses a traditional init system. The entire boot process is readable in an afternoon.

If something breaks, you can reason about it because it is just shell scripts running top to bottom. If you want something to happen at boot, you put it in a file and make it executable.

This document walks through the Slackware boot process from the moment the kernel starts to the moment you see a login prompt. Scripts covered here are from Slackware -current.

Manual page references appear like this: init(8), where the number in parentheses is the manual section. Read them with:

$ man 8 init

kernel -> /sbin/init -> /etc/inittab -> rc.S -> rc.M -> login
                                                    |
                                          rc.0 / rc.6 (halt/reboot)

For users coming from Ubuntu or Fedora, this table maps familiar concepts to their Slackware equivalents.

Once the kernel has initialized memory management and attached drivers to hardware, it starts exactly one userspace process:

/sbin/init

Everything else descends from this single process.

init runs as PID 1. It must never die. If it crashes, the kernel panics. It is the ancestor of every process on the system.

On Slackware, /sbin/init is the sysvinit(8) binary — a small C program, not a shell script. The kernel requires a real executable as PID 1 because it must:

• read /etc/inittab before any shell is available
• manage runlevel transitions reliably
• respawn login processes
• catch kernel signals (e.g., Ctrl+Alt+Del)
• reap orphaned child processes

A shell script exits when it reaches the end. init never exits. That is why PID 1 must be a compiled program.

After starting, init(8) reads /etc/inittab(5). This file defines the entire system behavior.

Format:

id:runlevels:action:process

id:3:initdefault:

Sets the default runlevel to 3 (multiuser mode).

Slackware runlevels:

• 0 = halt
• 1 = single-user
• 3 = multiuser (default)
• 4 = graphical (X11/Wayland via display manager)
• 6 = reboot

si:S:sysinit:/etc/rc.d/rc.S

Runs once at boot, before entering any runlevel. rc.S performs low-level system initialization.

rc:2345:wait:/etc/rc.d/rc.M

For runlevels 2–5, rc.M is executed and init waits for it to finish. This brings the system to full multiuser state.

ca::ctrlaltdel:/sbin/shutdown -t5 -r now

Handles Ctrl+Alt+Del cleanly.

c1:12345:respawn:/sbin/agetty --noclear 38400 tty1 linux

Keeps login prompts alive. When you log out, init respawns agetty.

x1:4:respawn:/etc/rc.d/rc.4

In runlevel 4, starts the display manager and restarts it if needed.

That is the entire boot configuration — in one readable file.

rc.S runs once, very early. At this point:

• root is read-only
• /proc and /sys are not mounted
• /dev is mostly empty
• no network exists

/sbin/mount -v proc /proc -t proc
/sbin/mount -v sysfs /sys -t sysfs

These expose kernel and hardware state to userspace.

/etc/rc.d/rc.udev start

udev dynamically creates device nodes in /dev.

/sbin/fsck $FORCEFSCK -C -a /

Fixes inconsistencies after crashes.

• /etc/fastboot — skip check once
• /etc/forcefsck — force check

/sbin/mount -w -v -o remount /

Before this, nothing can write to disk.

/sbin/mount -a -t nonfs,nosmbfs,nocifs,noproc,nosysfs

/usr/sbin/seedrng

Ensures secure randomness early in boot.

At this point, the system is stable enough to continue.

rc.M builds the full system.

/bin/hostname $(cat /etc/HOSTNAME)

/etc/rc.d/rc.inet1
/etc/rc.d/rc.inet2

Services follow this pattern throughout rc.M:

if [ -x /etc/rc.d/rc.crond ]; then
    /etc/rc.d/rc.crond start
fi

This applies to all services: syslog, NTP, SSH, cron, Apache, MariaDB, Samba, and others. Everything is optional and controlled the same way.

if [ -x /etc/rc.d/rc.local ]; then
    /etc/rc.d/rc.local
fi

Place any custom startup commands here.

There is no service database. There is only the filesystem.

• Enabled = executable
• Disabled = not executable

$ ls -l /etc/rc.d/

# chmod -x /etc/rc.d/rc.httpd   # disable
# chmod +x /etc/rc.d/rc.httpd   # enable

A directory listing shows the entire system state at a glance.

Slackware's init system is not simple because it lacks features. It is simple because it solves the problem directly:

• run scripts
• keep processes alive
• shut down cleanly

The entire boot process lives in four files:

/etc/inittab
/etc/rc.d/rc.S
/etc/rc.d/rc.M
/etc/rc.d/rc.local

Read them, and you understand your system. That kind of transparency has real value — especially when something breaks and you need to reason about what is actually happening.

• init(8) manual page
• inittab(5) manual page
• agetty(8) manual page
• fsck(8) manual page
• udev(7) manual page

• Using runit on Slackware
• Using OpenRC on Slackware

* Originally written by r1w1s1