[PATCH] commands: dmesg: clear logbuffer fully if not requested otherwise

[Sascha Hauer]

On Fri, Oct 17, 2025 at 01:04:36PM +0200, Jonas Rebmann wrote:
> Previously, `dmesg -c` would clear the logbuffer except for the last 10
> lines. This comes with a fair amount of surprise given linux `dmesg -c`
> clears the whole logbuffer.
> 
> Clear the complete logbuffer given `dmesg -c` but with an optional
> argument `dmesg -c [<num>]` to keep <num> lines.
> 
> While at it, move the deleteion of log lines to the log_print function
> to prevent a corner case of lost log message if they arrive between the
> log_print() and log_clean() in dmesg.
> 
> If loglevels are selected using -l or -p in addition to cleaning being
> selected using -c, only clean those messages shown by the loglevel
> selection.

I find this behaviour rather surprising. The only useful thing I could
think of we can do with this is "Discard unimportant messages", but only
by printing them. It would be more useful if we had a -C (uppercase)
option to discard messages without printing them. But even with that,
there are many things we can do with this behaviour that are not useful
and only a few that are actually useful. We could delete for example all
"info" messages and keep the more and less important messages, but why
would we want to do this?

I think you should rather use the filter options -l and -p to limit the
output to the messages you are interested in and finally use -c to clear
the buffer.

Sascha

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