systemd單元文件是定義系統(tǒng)資源行為的配置文件，用于控制啟動(dòng)和運(yùn)行時(shí)的進(jìn)程。它分為[Unit]、[Service]、[Install]三個(gè)主要部分，分別描述元數(shù)據(jù)與依賴關(guān)系、服務(wù)執(zhí)行方式、啟用方式。常見位置包括/usr/lib/systemd/system/、/etc/systemd/system/、/run/systemd/system/。查看和編輯可通過文本工具或systemctl edit命令實(shí)現(xiàn)，并需執(zhí)行systemctl daemon-reload以生效更改。當(dāng)需要自定義服務(wù)或腳本自動(dòng)運(yùn)行時(shí)，應(yīng)創(chuàng)建新的單元文件。

A systemd unit file is a configuration file that defines how a service, socket, device, or other system resource should behave when managed by systemd. These files are essential for controlling processes during system startup and runtime. They specify dependencies, execution methods, and conditions under which the unit should run.

Understanding What a Unit File Contains

Unit files are plain text files with a structured format divided into sections. The most common sections are:

• [Unit]: Describes metadata and dependencies (like description, after which services it should start).
• [Service]: Defines how the service should be run (start command, restart behavior, user, etc.).
• [Install]: Specifies how the unit should be enabled or started automatically at boot.

For example, a simple service unit file might include a line like ExecStart=/usr/bin/myprogram to tell systemd what command to run when starting the service.

These files help systemd understand not just what to run, but also when and how to manage it.

Common Locations for Unit Files

Systemd looks for unit files in specific directories. The main ones you’ll encounter are:

• /usr/lib/systemd/system/ – This is where most default unit files from installed packages live.
• /etc/systemd/system/ – User-created or customized unit files go here. Changes made in this directory take precedence over the ones in /usr/lib.
• /run/systemd/system/ – Temporary unit files stored in memory; they disappear after reboot.

If you're modifying or creating your own unit file, /etc/systemd/system/ is usually the right place to put it.

How to View and Edit Unit Files

You can look at existing unit files using standard tools like cat, less, or any text editor. For example:

cat /usr/lib/systemd/system/sshd.service

This shows how the SSH daemon is configured.

If you want to override part of a unit file without replacing the whole thing, use systemctl edit myservice.service. That creates a drop-in file in /etc/systemd/system/ that only changes what you specify.

After editing, always reload systemd:

systemctl daemon-reload

When You Might Need to Create One

You don’t always need to write a unit file — most services come with them. But if you're running a custom script or application that needs to start on boot or restart automatically, writing a unit file makes sense.

For example, if you have a background Python script you want to run as a service, you’d create a .service file in /etc/systemd/system/ that tells systemd how to handle it.

It doesn’t have to be complex — just enough to define the basics like what to run, under which user, and whether to restart if it fails.

基本上就這些.

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