Shell Script to Log CAN Messages with FIFO

Question

Edit 1-29-25: My goal is to have access to the last N number of actions that happened. To do this, I imagine that I need to continuously log CANbus messages in the Linux environment in a buffer. Then, when commanded by the user, the system would copy the last N number of messages into a different file. I imagine that the buffer would start getting quite big after some time so I would need to delete the oldest lines.

I am trying to create a Linux shell script to log CANbus messages on a CAN network and when the file gets over a certain number of lines or a filesize, the first line will get deleted and the CAN messages will continue logging, AKA a First-In First-Out system.

I am running the following commands while SSH'ed into a Linux PLC via PuTTY.

I am using the command candump -L can0 > test.log to log CANbus messages. While this command is running, the test.log file populates with the CANbus messages on the CAN network.

Then, I log in with a different PuTTY window and use ls -l test.log every few seconds to check that the test.log file is indeed increasing in size, indicating that the file is actively logging CAN traces.

Then, while the CAN traces are still actively logging, I use the command sed -i '1d' test.log to delete the first line of the test.log log file. After this, I run ls -l test.log every few seconds again to check the file size, but now the file size stays the same, indicating that the CAN trace has stopped logging even though the candump command from above is still running.

TLDR: If a file gets edited while the candump command is actively logging CAN traces into the file, the logging seems to stop.

Is there a way to keep the CANbus logging active while deleting lines from the log file? Or is there a more efficient way to create this script/application to log CANbus messages with a FIFO system?

Well, sed -i doesn't actually modify a file, it created a new one and replaces the old one with it. candump still writes in the old file, but that file isn't accessible by you (ie. any other process) anymore. — Fravadona
– Fravadona, Commented Jan 27 at 23:20
doing this via shell script will be messy. a python/ruby script would probably be best - have them capture the output via a pipe, buffer N lines into shared file/memory that others can tail ... — ticktalk
– ticktalk, Commented Jan 28 at 7:46
Fravadona is right about sed actions, so don't spend time on that. But t's not clear what you're using your FIFO system for. What is the use case? Do you just want to have access to the last 100 actions that happened? Or should some 2ndary system be monitoring these 100 actions and cause something else to happen? Could you install a small database (sqlLite or numerous others) to keep this data for you? Please update your question rather than reply in comments. It would be nice to delete me comment here when all is clarified in the body of your question. Good luck! — shellter
– shellter, Commented Jan 29 at 5:14

KamilCuk · Accepted Answer · 2025-01-29 18:39:39Z

2

There is the system logger.

Run in the background or as a systemd service:

candump -L can0 | logger -t candump

To get last 10 messages, do:

journalctl -n 10 SYSLOG_IDENTIFIER=candump

No need to do rotation, filtering, caring about disc space, it's all implemented.

answered Jan 29 at 18:39

KamilCuk

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