systemd-services

这段内容是审核时保存的快照。外部来源才是完整且最新的版本。

---
name: systemd-services
description: Create and manage systemd services and timers. Configure service dependencies and resource limits. Use when managing system services.
license: MIT
metadata:
  author: devops-skills
  version: "1.0"
---

# Systemd Services

Create, manage, and monitor systemd services and timers. Covers unit file authoring, dependency management, socket activation, resource limits, journalctl log analysis, and production hardening.

## When to Use

- Deploying an application as a managed background service
- Replacing cron jobs with systemd timers for better logging and dependency control
- Setting up socket activation for on-demand service startup
- Configuring resource limits (CPU, memory, I/O) for services
- Debugging service startup failures and runtime crashes
- Managing service dependencies and ordering

## Prerequisites

- Linux system running systemd (most modern distributions)
- Root or sudo access for creating system-level unit files
- Application binary or script to run as a service
- Understanding of the application's start/stop lifecycle

## Service Unit File -- Complete Example

```ini
# /etc/systemd/system/myapp.service
[Unit]
Description=MyApp Production Server
Documentation=https://docs.example.com/myapp
After=network-online.target postgresql.service
Wants=network-online.target
Requires=postgresql.service

[Service]
Type=notify
User=myapp
Group=myapp
WorkingDirectory=/opt/myapp

# Environment configuration
EnvironmentFile=/etc/myapp/env
Environment=NODE_ENV=production
Environment=PORT=8080

# Execution
ExecStartPre=/opt/myapp/bin/migrate --check
ExecStart=/opt/myapp/bin/server --config /etc/myapp/config.yaml
ExecStartPost=/opt/myapp/bin/healthcheck.sh
ExecReload=/bin/kill -HUP $MAINPID
ExecStop=/opt/myapp/bin/graceful-stop.sh

# Restart behavior
Restart=on-failure
RestartSec=5
StartLimitIntervalSec=300
StartLimitBurst=5

# Timeouts
TimeoutStartSec=30
TimeoutStopSec=30
WatchdogSec=60

# Security hardening
NoNewPrivileges=true
ProtectSystem=strict
ProtectHome=true
PrivateTmp=true
ReadWritePaths=/var/lib/myapp /var/log/myapp
CapabilityBoundingSet=
AmbientCapabilities=

# Logging
StandardOutput=journal
StandardError=journal
SyslogIdentifier=myapp

[Install]
WantedBy=multi-user.target
```

## Service Management Commands

```bash
# Reload systemd after creating or modifying unit files
systemctl daemon-reload

# Start, stop, restart a service
systemctl start myapp
systemctl stop myapp
systemctl restart myapp

# Reload service configuration without restart (if supported)
systemctl reload myapp

# Enable service to start on boot
systemctl enable myapp

# Enable and start in one command
systemctl enable --now myapp

# Disable and stop
systemctl disable --now myapp

# Check service status
systemctl status myapp

# Check if a service is active, enabled, or failed
systemctl is-active myapp
systemctl is-enabled myapp
systemctl is-failed myapp

# List all running services
systemctl list-units --type=service --state=running

# List all failed services
systemctl list-units --type=service --state=failed

# Show all properties of a service
systemctl show myapp

# Show specific property values
systemctl show myapp -p MainPID,MemoryCurrent,CPUUsageNSec

# Mask a service (prevent it from being started at all)
systemctl mask myapp

# Unmask
systemctl unmask myapp

# Reset a failed service state
systemctl reset-failed myapp
```

## Timer Units (Cron Replacement)

### Timer File

```ini
# /etc/systemd/system/backup.timer
[Unit]
Description=Daily backup timer

[Timer]
# Run daily at 2:30 AM
OnCalendar=*-*-* 02:30:00
# If the system was off at the scheduled time, run when it boots
Persistent=true
# Add random delay up to 15 minutes to avoid thundering herd
RandomizedDelaySec=900
# Associate with a specific service (defaults to same name .service)
Unit=backup.service

[Install]
WantedBy=timers.target
```

### Corresponding Service File

```ini
# /etc/systemd/system/backup.service
[Unit]
Description=Daily backup job