PM2 Useful Commands

PM2 commands for managing and monitoring Node.js applications:

1. Starting and Stopping Applications

Start an application:
```
pm2 start app.js
```
Starts a Node.js app (app.js), creating a managed process.
Start with a specific name:
```
pm2 start app.js --name "myApp"
```
Stop an application:
```
pm2 stop <app_name or id>
```
Restart an application:
```
pm2 restart <app_name or id>
```
Delete an application:
```
pm2 delete <app_name or id>
```

2. Application Management

Seeing all running applications:
```
pm2 status
```
List all running applications:
```
pm2 list
```
View application logs:
```
pm2 logs
```
View logs for a specific app:
```
pm2 logs <app_name or id>
```
Display details of a specific app:
```
pm2 show <app_name or id>
```
Monitor all running apps in real-time:
```
pm2 monit
```

3. Process Scaling

Scale an application:
```
pm2 scale <app_name or id> <number>
```
This command allows you to increase or decrease the number of instances.

4. Environment Variables and JSON Config Files

Start with environment variables:
```
pm2 start app.js --env production
```
Start an application with a JSON config file:
```
pm2 start ecosystem.config.js
```

5. Saving and Loading Processes

Save the current process list:
```
pm2 save
```
This will allow PM2 to resurrect the saved processes on server reboot.
Reload saved process list on startup:
```
pm2 resurrect
```

6. Automatic Restart on Code Changes

Enable auto-restart on file changes:
```
pm2 start app.js --watch
```

Specify files or folders to watch:

pm2 start app.js --watch --ignore-watch="node_modules"

7. Startup Script for Reboot Persistence

Generate a startup script:
```
pm2 startup
```
This command generates the command to make PM2 start on boot. Run the output command to enable it.

8. Zero-Downtime Reload

Reload all applications with zero downtime:
```
pm2 reload all
```
This command is especially useful for reloading apps without downtime (great for production).

9. Updating PM2

Update PM2 to the latest version:
```
pm2 update
```
Reload PM2’s daemon process:
```
pm2 updatePM2
```

10. Miscellaneous Commands

Get system information (CPU, RAM usage):
```
pm2 info <app_name or id>
```
Clear logs for all applications:
```
pm2 flush
```
Kill the PM2 daemon (stops all applications and the PM2 process):
```
pm2 kill
```

Basic JSON configuration file (ecosystem.config.js) for managing two applications using PM2. This file includes environment variables, watch mode, and other configurations. One application is a Node.js (Express) app, and the other is a Python app.

`ecosystem.config.js` File

module.exports = {
  apps: [
    {
      name: "expressApp",                   // Name of the first app
      script: "app.js",                     // Entry file for the Node.js app
      watch: true,                          // Watch for file changes
      env: {                                // Environment variables for development
        NODE_ENV: "development",
        PORT: 3000,
        DB_URI: "mongodb://localhost:27017/devdb"
      },
      env_production: {                     // Environment variables for production
        NODE_ENV: "production",
        PORT: 8080,
        DB_URI: "mongodb://localhost:27017/proddb"
      },
      ignore_watch: ["node_modules"],       // Ignore changes in the node_modules folder
      instances: 2,                         // Run 2 instances (for load balancing)
      exec_mode: "cluster"                  // Use cluster mode for zero-downtime reloads
    },
    {
      name: "pythonApp",                    // Name of the second app
      script: "python",                     // Command to run Python
      args: "app.py",                       // Script file for the Python app
      interpreter: "python3",               // Specify Python 3 interpreter
      watch: ["./"],                        // Watch for changes in the root directory
      env: {                                // Environment variables for development
        ENV_MODE: "development",
        API_KEY: "dev_api_key"
      },
      env_production: {                     // Environment variables for production
        ENV_MODE: "production",
        API_KEY: "prod_api_key"
      }
    }
  ]
};

Explanation of the Configurations

name: Name of the application, which you can use with PM2 commands.
script: Specifies the script or command to start the application (app.js for the Express app, app.py for the Python app).
watch: Enables PM2's file-watching feature to restart the app if files change.
ignore_watch: Prevents PM2 from restarting on changes in specified folders (like node_modules).
instances: Sets the number of instances (useful for load balancing).
exec_mode: When set to "cluster", PM2 will run the app in cluster mode, allowing zero-downtime restarts.
env and env_production: Defines environment variables for development and production modes.

mizanmahi/pm2.md