PM2 scale command fails with -1 argument due to option parsing

What's going wrong?

When using pm2 scale <app_name> -1 to reduce the number of instances for a PM2-managed application, an error occurs because the -1 is being treated as an option, not as a valid argument to decrease the number of instances.

Root Cause

The issue arises because commander.js, which PM2 uses for argument parsing, treats -1 as an option due to the leading hyphen (-), resulting in the error. As seen in commander.js documentation, the hyphen (-) is reserved for option flags, causing unintended behavior when attempting to pass a negative number to scale down instances.

Suggested Improvement

To resolve this, it would be ideal to:

• Support the -1 argument as a valid input for reducing the number of instances.
• Consider using a different symbol (such as ~ or another syntax) to indicate a relative scale change, e.g., pm2 scale <app_name> ~1 to decrease by one instance.

How could we reproduce this issue?

1. Start an application using PM2:
   pm2 start --name <app_name> app.js

2. Attempt to scale the application down by 1 instance using:
   pm2 scale <app_name> -1

Supporting information

--- PM2 report ----------------------------------------------------------------
Date                 : Wed Dec 25 2024 10:31:22 GMT+0800 (台北標準時間)
===============================================================================
--- Daemon -------------------------------------------------
pm2d version         : 5.4.3
node version         : 22.11.0
node path            : not found
argv                 : C:\Program Files\nodejs\node.exe,C:\Program Files (x86)\nvm\v22.11.0\node_modules\pm2\lib\Daemon.js
argv0                : node
user                 : undefined
uid                  : N/A
gid                  : N/A
uptime               : 33min
===============================================================================
--- CLI ----------------------------------------------------
local pm2            : 5.4.3
node version         : 22.11.0
node path            : not found
argv                 : C:\Program Files\nodejs\node.exe,C:\Program Files\nodejs\node_modules\pm2\bin\pm2,report
argv0                : C:\Program Files\nodejs\node.exe
user                 : user
===============================================================================
--- System info --------------------------------------------
arch                 : x64
platform             : win32
type                 : Windows_NT
cpus                 : 12th Gen Intel(R) Core(TM) i7-1260P
cpus nb              : 16
freemem              : 9051037696
totalmem             : 25358680064
home                 : C:\Users\user

References
commander.js documentation (v2.15.1)
PM2 code that handles scaling
https://github.com/Unitech/pm2/blob/e3a327fab7a6b385ca0d960c878347dce320ae61/lib/API.js#L1828-L1835

Start with lib/API.js at the scaling entry point referenced in the issue, then trace how the CLI parser receives the scale argument and compare that behavior with commander.js. Done means pm2 scale <app_name> -1 is accepted and reduces the instance count by one, with the existing scale behavior preserved.