systemd服務(wù)和目標(biāo)是Linux系統(tǒng)管理的核心，服務(wù)通過(guò).service文件管理進(jìn)程，使用systemctl命令控制啟停、啟用禁用，如systemctl start sshd；目標(biāo)代表系統(tǒng)運(yùn)行狀態(tài)，如multi-user.target或graphical.target，通過(guò)systemctl set-default和isolate切換，默認(rèn)目標(biāo)可設(shè)置，服務(wù)文件包含[Unit]、[Service]、[Install]三部分，WantedBy指定目標(biāo)依賴，自定義服務(wù)需創(chuàng)建.service文件並執(zhí)行systemctl daemon-reload使配置生效。

Linux systemd is the init system and service manager used by most modern Linux distributions. It controls how services start, stop, and behave during system boot and runtime. Understanding services and targets in systemd is essential for managing system behavior effectively.

What Are Systemd Services?

A service in systemd represents a unit that manages a specific process or daemon—like a web server ( httpd ), SSH server ( sshd ), or database ( mariadb ). Services define how and when a program should run.

Service files (ending in .service ) are usually located in:

• /usr/lib/systemd/system/ – for default system-provided services
• /etc/systemd/system/ – for user-defined or overridden services

Common Service Management Commands

You can control services using the systemctl command:

 # Start a service immediately
sudo systemctl start sshd.service

# Stop a running service
sudo systemctl stop sshd.service

# Enable a service to start at boot
sudo systemctl enable sshd.service

# Disable a service from starting at boot
sudo systemctl disable sshd.service

# Check the status of a service
systemctl status sshd.service

# Restart or reload a service
sudo systemctl restart sshd.service
sudo systemctl reload sshd.service # if supported

Note: The .service suffix is optional. You can just type sshd instead of sshd.service .

Anatomy of a Service File

A typical .service file looks like this:

 [Unit]
Description=OpenSSH Server
After=network.target

[Service]
Type=notify
ExecStart=/usr/sbin/sshd -D $SSHD_OPTS
ExecReload=/bin/kill -HUP $MAINPID
Restart=on-failure

[Install]
WantedBy=multi-user.target

Key sections:

• [Unit] : Defines metadata and dependencies (eg, what must start before this).
• [Service] : Specifies how the service should run (start command, restart policy, etc.).
• [Install] : Controls how the service is enabled/disabled (used with enable / disable ).

What Are Systemd Targets?

Targets in systemd are like runlevels in the old SysV init system, but more flexible. They represent a state or mode the system should be in (eg, multi-user, graphical, rescue).

Common targets include:

Managing and Switching Targets

To see the current target:

 systemctl get-default

To change the default boot target:

 # Set system to boot into text mode by default
sudo systemctl set-default multi-user.target

# Set system to boot into GUI by default
sudo systemctl set-default graphical.target

To switch to a target immediately (without rebooting):

 # Switch to single-user/rescue mode
sudo systemctl isolate rescue.target

# Switch to multi-user mode
sudo systemctl isolate multi-user.target

isolate stops services not part of the target and starts those that are—similar to switching runlevels.

Understanding Target Dependencies

Targets use .target files to group other units. For example, graphical.target typically pulls in multi-user.target and display manager services.

You can see what's included in a target:

 systemctl list-dependencies graphical.target

And check which target a service belongs to via the [Install] section ( WantedBy= ).

Practical Example: Creating a Custom Service

Let's say you want a script to run at boot.

1. Create a script:

    sudo nano /opt/myscript.sh

    #!/bin/bash
   echo "Custom service running at $(date)" >> /var/log/myscript.log

2. Make it executable:

    sudo chmod x /opt/myscript.sh

3. Create a service file:

    sudo nano /etc/systemd/system/myscript.service

    [Unit]
   Description=My Custom Script
   After=network.target
   
   [Service]
   Type=oneshot
   ExecStart=/opt/myscript.sh
   RemainAfterExit=yes
   
   [Install]
   WantedBy=multi-user.target

4. Reload systemd and enable the service:

    sudo systemctl daemon-reexec
   sudo systemctl enable myscript.service

   Now it will run at every boot.

   ---

   Key Takeaways

   • Services manage daemons and processes; use systemctl to control them.
   • Targets define system states, replacing old runlevels.
   • Use WantedBy= in service files to tie services to specific targets.
   • Custom services can be created easily in /etc/systemd/system/ .
   • Always run systemctl daemon-reload after creating or editing service files.

   Basically, systemd makes service and boot management more consistent and dependency-aware than older init systems. Once you understand services and targets, you're well on your way to mastering Linux system administration.

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