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From: Rene Kita <mail@rkta.de>
Newsgroups: comp.unix.shell
Subject: Re: Basic ps Tips
Date: Mon, 5 Aug 2024 06:08:30 -0000 (UTC)
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Joerg Mertens <joerg-mertens@t-online.de> wrote:
> Rene Kita <mail@rkta.de> wrote:
>> Joerg Mertens <joerg-mertens@t-online.de> wrote:
[...]
>>> The error message also is printed, when you run ps with a valid
>>> flag plus some string, like `ps -a xyz´, so it seems to be independent
>>> of the `-C´-option.  Maybe it has to do with parsing of the old-style
>>> flags vs. the dashed ones.
>> 
>> You got me curious and I got my debugger out for my morning coffee.
>> 
>> The error message comes from a function kvm_openfiles, which is called
>> after the flag parsing is done. From a quick glance, this function takes
>> what looks like three file names. If the second one is NULL, it will
>> default to /dev/mem. Trying to open /dev/mem it will error out - which
>> could be due to me being on a VPS.
>> 
>> While the man page mentions kvm(3) in the SEE ALSO section, there is no
>> mentioning nor explanation what additional arguments can be passed to
>> ps.
> 
> As I understand it, `ps´ can not only be used to examine the processes
> of your currently running system, but also those of another system
> represented by an image file.  Normally you would use the `N´ option 
> plus a filename to tell ps to do this, but the original way seems to
> have been to just add the filename to the commandline as an additional
> argument.
> 
> Now when you call `ps xy´, the argument xy is interpreted as a string
> of old style options which can be written without dashes.  Some
> people still write `ps aux´ for example to look at their processes.

Guilty. :-)

> But when you write `ps a xy´ (or `ps -a xy´), xy is interpreted as
> a "kernel image to be examined" (see the first argument to
> kvm_openfiles(3)), which leads to the `permission denied´ error.
> 
> To get back to the `-C´ option - it is there for compatibility
> reasons with old scripts.  You can find the relevant commit message
> under http://cvsweb.openbsd.org/src/bin/ps/ps.c when you search for the
> string `-C´.

That's what I thought. I was just surprised that it's not documented.
But the man page seems to be silent about all old style things. Looks
like it's by design.