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From: Rene Kita <mail@rkta.de>
Newsgroups: comp.unix.shell
Subject: Re: Basic ps Tips
Date: Sun, 4 Aug 2024 06:56:36 -0000 (UTC)
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Sender: Rene Kita <kt@ob.rkta.de>
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Joerg Mertens <joerg-mertens@t-online.de> wrote:
> Rene Kita <mail@rkta.de> wrote:
>> Ed Morton <mortonspam@gmail.com> wrote:
>>> On 8/3/2024 2:08 AM, Rene Kita wrote:
>>>> Jerry Peters <jerry@example.invalid> wrote:
>>>>> Lawrence D'Oliveiro <ldo@nz.invalid> wrote:
>>>>>> On Sat, 27 Jul 2024 00:40:49 -0000 (UTC), I wrote:
>>>> [...]
>>>>>>      ps -p$(pgrep -d, bash) -wwo pid,ppid,lstart,tty,etime,cmd
>>>>>>
>>>>>> says not to truncate the output, which is handy for long command lines.
>>>>>   
>>>>> Or just use ps -C <command>:
>>>> [...]
>>>>> Does noone know about -C? I keep seeing things like 'ps -ef | grep
>>>>> <something> in scripts to see if <something is running, rather than
>>>>> using 'ps -C'.
>>>> 
>>>> I did not know about it.
>>>> 
>>>> The man page on OpenBSD does not mention -C, but calling 'ps -C' does
>>>> not give an error. But:
>>>> #v+
>>>> $ ps -C ksh
>>>> ps: /dev/mem: Permission denied'
>>>> #v-
>>>> 
>>>> Dunno what to make out of it, but apparently one reason to use grep
>>>> instead of -C is portability.
>>> 
>>> Its described in the man page for FreeBSD ps, 
>>> https://man.freebsd.org/cgi/man.cgi?ps(1), as:
>>> 
>>>     -C      Change  the  way         the  CPU  percentage is calculated by 
>>> using a
>>>            "raw" CPU calculation that ignores "resident" time  (this  nor-
>>>            mally has no effect).
>>> 
>>> so maybe you're running FreeBSD instead of the OpenBSD version.
>> 
>> I'm pretty sure I know which version of BSD I'm running and I would be
>> very surprised if OpenBSD would ship the FreeBSD version of ps...
>> 
>> But let's have a look at the source:
>> #v+
>>         while ((ch = getopt(argc, argv,
>>             "AaCcefgHhjkLlM:mN:O:o:p:rSTt:U:uvW:wx")) != -1)
>>                 switch (ch) {
>>                 case 'A':
>>                         all = 1;
>>                         xflg = 1;
>>                         break;
>>                 case 'a':
>>                         all = 1;
>>                         break;
>>                 case 'C':
>>                         break;                  /* no-op */
>> #v-
>> 
>> '-C' does nothing. I did not look further to see where that error is
>> coming from.
> 
> 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.

> Regards