Linux Boot Process ¶

The boot process with the system's firmware initializes the hardware and loads the bootloader. 🧳

The bootloader will load the Linux Kernel. It's the menu we have to select which OS we are booting on. 🪧

The kernel will initialize some hardware components, mount the filesystem and delegate the initialization process which will start processes and services until the system becomes fully operational. 🗃️

The initialization process is commonly handled by systemd daemon, which is a modern replacement for the traditional init system. This daemon process is the parent of every process and has the PID 1. 🪴

Some of the well-known ones are:

• System firmware 🧳: BIOS or UEFI
• Bootloaders 🪧: GRUB or U-Boot
• Initialization 🪴: systemd or launchd

Systemd is a system and service manager for Linux operating systems. It is responsible for starting, stopping, and managing system services.

$ ls -la /etc/systemd/system/   # target + enabled services
$ ls -la /lib/systemd/system/   # services

systemctl command ¶

The systemctl allows us to interact with the systemd daemon. Note that you can create all files manually without the systemctl command.

$ systemctl list-units --type=service # list all services
$ systemctl show xxx      # show information, use -p to filter
$ systemctl status xxx    # status of the service
$ systemctl start xxx     # start a service
$ systemctl reload xxx    # reload the configuration
$ systemctl daemon-reload # reload systemd configuration
$ systemctl restart xxx   # stop+start a service
$ systemctl stop xxx      # stop a service
$ systemctl enable xxx    # enable a service at boot
$ systemctl enable --now xxx # enable it now
$ systemctl disable xxx   # disable a service at boot
$ systemctl link xxx      # create a link in systemd files

➡️ You can use -f to force an operation.

journalctl command ¶

We can use journalctl to access the system logs.

$ journalctl            # all logs
$ journalctl -b         # since last boot
$ journalctl -f         # real-time
$ journalctl -u xxx     # for a specific service

Systemd Targets ¶

Systemd has a concept of targets to group services. The goal of the initialization often is to reach the multi-user.target which means the user can interact with the system and most services are active.

$ ls /lib/systemd/system/*.target
$ cat my_target.target
[Unit]
Description=Some Description
Requires=multi-user.target
Conflicts=rescue.service rescue.target
After=multi-user.target rescue.service rescue.target
AllowIsolate=yes

➡️ See also: How to create a systemd target?

Systemd service ¶

Services support a lot of options, especially when we need to define dependencies between services or targets.

$ ls /lib/systemd/system/*.service
$ cat my_service.service
[Service]
Type=oneshot
ExecStart=/bin/echo "Hello, World!"

[Install]
WantedBy=my_target.target

➡️ See also: WorkingDirectory, Restart, RestartSec, StandardOutput, StandardError, SyslogIdentifier, Environment, User, etc.

GRUB (GRand Unified Bootloader) is a widely used bootloader on Linux. It allow users to select which operating system or kernel to boot. It automatically boots using the first one after 5 seconds.

📚 It worth noting that a file system is just a file. If we have access to an image such as a .vmdk, we can read its content without booting it.

If the GRUB configuration is not password protected and insecurely configured, we may be able to edit it during boot ⚠️.

• Press Esc to cancel the automatic selection
• Press E to edit GRUB configuration
• Look for a line linux [...]
• Edit it to your liking

You may remove quiet and add init=/bin/bash to start the system with a bash as root with no password.

• Press CTRL+X or F10

• Systemd Timers: using systemd for task scheduling. xxx.timer files.

[Unit]
Description=XXX

[Service]
Type=forking
ExecStart=/bin/bash -c "cat /root/root.txt > /tmp/output && chmod 777 /tmp/output"

[Install]
WantedBy=multi-user.target