What is systemd?
systemd is the init system and service manager used in most Linux distributions (Fedora, Ubuntu, Arch, etc). It is the first process (PID 1) that runs after the kernel boots and is responsible for bringing up the rest of the system.
It is designed to provide a robust framework for managing the system’s lifecycle, starting from early boot and continuing until system shutdown.
Role and Functionality of systemd?
PID 1 Process
systemd runs as the first process during the boot sequence, orchestrated by the Linux kernel. All the other user-space processes are started directly by systemd or it’s child processes.
Service Management
systemd manages the startup, shutdown, monitoring, and lifecycle of system services. It replaces older init systems like SysV init and offer backward compatibility with traditional init scripts.
Parallelization
Unlike traditional init systems that starts services one second after another, systemd can start services in parallel, significantly reducing boot times.
Dependency Management
systemd handles dependencies between services and other system resources, ensuring that services start in the correct order and only when required.
Resource Management
It uses Linux control groups (cgroups) to track and manage system resources, providing better control and isolation of processes.
Logging
systemd includes a built-in logging mechanism called journalctl, which collects and manages logs from all system services, making troubleshooting easier.
List of Commands
| Action | Command Example | Description / Usage | Extra Commands & Tips for Beginners |
|---|---|---|---|
| Start a service | sudo systemctl start | Starts the specified service immediately | systemctl status (check if running) |
| Stop a service | sudo systemctl stop | Stops the specified service immediately | systemctl is-active (check if stopped) |
| Restart a service | sudo systemctl restart | Restarts the specified service | |
| Reload a service | sudo systemctl reload | Reloads configuration without full restart | |
| Enable a service | sudo systemctl enable | Enables the service to start at boot | systemctl is-enabled (check enabled) |
| Disable a service | sudo systemctl disable | Disables the service from starting at boot | |
| Check service status | systemctl status | Shows detailed status of the service | |
| Check if active | systemctl is-active | Returns “active” or “inactive” for the service | |
| Check if enabled | systemctl is-enabled | Returns “enabled” or “disabled” for the service | |
| Mask a service | sudo systemctl mask | Prevents a service from starting, even manually | sudo systemctl unmask (to undo) |
| Show service info | systemctl show | Displays detailed properties of the service | systemctl show -p |
| List dependencies | systemctl list-dependencies | Lists units that depend on or are required by this service | --before and --after flags for order[1][2] |
| List all services | systemctl list-units --type=service | Lists all active service units | |
| Reload systemd | sudo systemctl daemon-reload | Reloads all unit files and updates systemd after changes |
What is journalctl?
journalctl is the primary command-line tool for viewing and managing logs collected by systemd’s journald service. Instead of traditional plain-text log files are created around the system, systemd centralizes logs in structured, binary format, making it easier to search, filter and analyze system and application events.
1. Viewing and Following Logs
| Command Example | Description / Purpose |
|---|---|
journalctl | View all logs |
journalctl -f | Follow (tail) logs in real-time |
2. Filtering Logs by Service
| Command Example | Description / Purpose |
|---|---|
journalctl -u | View logs for a specific service |
3. Filtering Logs by Time and Boot
| Command Example | Description / Purpose |
|---|---|
journalctl -n | Show the last N log entries |
journalctl --since "" | Show logs since a specific time |
journalctl -b | Show logs from the current boot |
journalctl -b -1 | Show logs from the previous boot |
4. Filtering Logs by Priority and Content
| Command Example | Description / Purpose |
|---|---|
journalctl -r | Show logs in reverse order (newest first) |
journalctl -p | Filter by log priority (e.g., -p err, -p info) |
journalctl -g "" | Search logs for a pattern |
journalctl _PID= | Show logs for a specific process ID |
5. Output Formatting
| Command Example | Description / Purpose |
|---|---|
journalctl -o json | Output logs in JSON format |
journalctl -o cat | Output only the message content (no metadata) |
journalctl --no-pager | Dump logs directly to terminal (no pager) |
6. Maintenance and Disk Usage
| Command Example | Description / Purpose |
|---|---|
journalctl --disk-usage | Show journal disk usage |
sudo journalctl --vacuum-time= | Remove logs older than specified time |
Units, Services & Target
What is Units?
A unit is any resource that systemd can manage. Each unit is described by a configuration file(aka unit file) with a specific extension indicating its type.
Common Units
.service.target.socket.device.mount.automount,.swap,.path,.timer,.slice, and.scope
Example
A unit file can be,
etc/systemd/system/example.service(service unit),etc/systemd/system/my-mount.mount(mount unit),etc/systemd/system/daily-backup.time(time unit)
What is Services?
A service unit (.service) is used to manage applications or daemons running on the system. It defines how to start, stop, restart, or reload a service, and it’s dependencies.
Example
A simple service unit file for a custom script (/etc/systemd/system/myscript.service):
[Unit]
Description= My Custom Script Service
[Service]
ExecStart=/usr/loca/bin/myscript.sh
[Install]
WantedBy=multi-user.target
What is Targets?
A target unit (.target) is a group of units used to synchronize the system’s state. Targets are used to bring the system to a specific state, such as booting to a graphical or multi-user environment.
Common Targets
- multi-user.target: System is ready for multiple users, but without a graphical interface.
- graphical.target: System is ready for multiple users with a graphical interface.
- rescue.target: System is in single-user rescue mode.
Example:
The multi-user.target is a standard target for non-graphical, multi-user system. When the system boots, it tries to reach this target, starting all services that are required for this state.
Summary of Units, Services & Target
| Unit Type | Example Name | Purpose/Example Use |
|---|---|---|
| service | myscript.service | Manages a custom script or application |
| target | multi-user.target | Groups services for multi-user, non-graphical boot |
| socket | sshd.socket | Activates a service when a network connection arrives |
| device | dev-sda5.device | Manages a hardware device |
| mount | mnt-data.mount | Manages a filesystem mount point |
| timer | daily-backup.timer | Schedules tasks (like cron jobs) |