Yes, there are several ways to handle dynamic environment variables in systemd service files. Here are the most common approaches:
## Option 1: Using ExecStartPre with a script
Create a script that generates the environment file:
**Create `/usr/local/bin/set-local-ip.sh`:**
```bash
#!/bin/bash
LOCAL_IP=$(ip route get 1.1.1.1 | awk '{print $7}' | head -n1)
echo "EXTERNAL_LOCALWEB_URL=http://${LOCAL_IP}:8443" > /run/myservice/env
```
**In your service file:**
```ini
[Unit]
Description=My Service
After=network.target
[Service]
Type=simple
ExecStartPre=/bin/mkdir -p /run/myservice
ExecStartPre=/usr/local/bin/set-local-ip.sh
EnvironmentFile=/run/myservice/env
ExecStart=/path/to/your/application
User=myuser
Group=mygroup
[Install]
WantedBy=multi-user.target
```
## Option 2: Inline in ExecStartPre
**In your service file:**
```ini
[Unit]
Description=My Service
After=network.target
[Service]
Type=simple
ExecStartPre=/bin/bash -c 'mkdir -p /run/myservice && echo "EXTERNAL_LOCALWEB_URL=http://$(ip route get 1.1.1.1 | awk "{print \\$7}" | head -n1):8443" > /run/myservice/env'
EnvironmentFile=/run/myservice/env
ExecStart=/path/to/your/application
User=myuser
Group=mygroup
[Install]
WantedBy=multi-user.target
```
## Option 3: Using a wrapper script
**Create `/usr/local/bin/myservice-wrapper.sh`:**
```bash
#!/bin/bash
LOCAL_IP=$(ip route get 1.1.1.1 | awk '{print $7}' | head -n1)
export EXTERNAL_LOCALWEB_URL="http://${LOCAL_IP}:8443"
exec /path/to/your/actual/application "$@"
```
**In your service file:**
```ini
[Unit]
Description=My Service
After=network.target
[Service]
Type=simple
ExecStart=/usr/local/bin/myservice-wrapper.sh
User=myuser
Group=mygroup
[Install]
WantedBy=multi-user.target
```
## Alternative IP detection methods
Depending on your system configuration, you might want to use different methods to get the local IP:
```bash
# Method 1: Default route (most reliable)
LOCAL_IP=$(ip route get 1.1.1.1 | awk '{print $7}' | head -n1)
# Method 2: Specific interface
LOCAL_IP=$(ip addr show eth0 | grep 'inet ' | awk '{print $2}' | cut -d/ -f1)
# Method 3: Hostname resolution
LOCAL_IP=$(hostname -I | awk '{print $1}')
```
## Important considerations:
1. **Permissions**: Make sure your scripts are executable (`chmod +x`)
2. **Directory permissions**: The service user needs write access to `/run/myservice/`
3. **Network timing**: Use `After=network-online.target` if you need to ensure network connectivity
4. **Security**: Consider the security implications of the commands you're running
The first option (using ExecStartPre with a separate script) is generally the cleanest and most maintainable approach.