platformio/platformio-core

PIO device monitor adds \r before echoed \n automatically

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#5097 opened on Mar 3, 2025

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device managementhelp wanted

Description

Hello!

Thank you all for your hard work on this project!

I was curious about whether a certain behavior was intended with regards to the serial monitor on PlatformIO. I have a STM32 attached via a USB-UART adapter to my Linux (Ubuntu 24.04) machine. When I run:

pio device monitor --port /dev/ttyUSB0 --baud 115200 --eol LF --raw

I'm expecting that only \n gets sent to the remote device when I press Enter, which I can confirm when I step through our code for the UART that receives and echoes back the same characters. However, in the terminal, it appears to print a \r automatically when an \n is transmitted from the STM32. I am quite sure that the STM32 does not transmit anything other than what it receives, so there's probably some issue with how I'm invoking the device monitor, or maybe there's a bug.

I have also tried:

pio device monitor --port /dev/ttyUSB0 --baud 115200 --eol LF --filter direct pio device monitor --port /dev/ttyUSB0 --baud 115200 --eol LF --filter default pio device monitor --port /dev/ttyUSB0 --baud 115200 --eol LF --filter nocontrol pio device monitor --port /dev/ttyUSB0 --baud 115200 --eol LF --filter printable

And even just:

pio device monitor --port /dev/ttyUSB0 --baud 115200 --eol LF

But none of them prevent a \r being added. To clarify, the behavior I'm looking for when I hit Enter to send an LF is as follows:

Typing this long string...
                          ^     <--- pressed Enter here
                          |     <--- cursor appears here

But what happens is this:

Typing this long string...
                          ^     <--- pressed Enter here
| <--- cursor appears here

How can I filter out the \r from being added before \n using the device monitor options? Will I have to specify a custom filter as seems to be described here? Or is this a possible bug?

Contributor guide

Tech stack
python
Domain
cli
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
2
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
under 1 hour
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic PythonUnderstanding of serial terminal EOL handling
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
70
Research direction
The issue describes an unexpected automatic addition of \r before \n when using `pio device monitor` with ` eol LF`. To resolve, examine the device monitor code in the `platformio core` repository, particularly the filter and EOL handling logic in `device/monitor/`. Look at how filters like 'raw', 'default', 'nocontrol', etc., process received data. Check if there's a default conversion that adds \r for LF only mode. Review related files such as `device/monitor/filter.py` or similar. The goal is to identify where the extra \r is injected and why it persists despite the ` eol LF` flag.