[sf-lug] ps and grep

[Tom Haddon]

On Thu, 2008-03-06 at 18:00 -0800, jim stockford wrote:
> yes. for any command that uses piping, the bash shell
> loads the right-most command first, then works its way
> back to the left-most. you can verify this by looking at
> the process numbers:
> $ ps aux | grep " " | grep "  "
> 
> the shell loads grep with an argument to look for any
> line with two space characters, then it loads grep with
> an argument to look for any line with one space
> character, then it loads the ps command with its aux
> argument.

Cool, makes perfect sense, thanks!

Tom

> 
> 
> 
> On Mar 6, 2008, at 5:48 PM, Tom Haddon wrote:
> 
> >
> > On Thu, 2008-03-06 at 17:47 -0800, jim stockford wrote:
> >>
> >> at the point you type the  ps  command and hit
> >> enter, you've kicked off a new process, something
> >> like
> >> $ ps aux | grep doit
> >>
> >> the pipe character is also a command delimiter:
> >> you've kicked off two processes, the first one loaded
> >> is the  grep  process and the second one loaded is
> >> the  ps  process. after all, nothing runs unless it's a
> >> process, and for  ps  to get the output of  grep  then
> >> grep has to be loaded first so's to be in place, right?
> >
> > Do you mean that for grep to process ps's output, grep has to be loaded
> > first? If so, this all makes sense, and I think I have it.
> >
> > Thanks! Tom
> >
> >>
> >> but it's only after those two processes are loaded
> >> that the  ps  command starts its work, which means
> >> it's dumping all processes including the  grep
> >> process and the  ps  process itself. you've asked
> >> grep to filter out everything but any line that has
> >> doit on it, so if the doit process is running, you get
> >> that line, and you also get the line for which doit is
> >> an argument: the grep process.
> >>
> >> make sense?
> >>
> >>
> >>
> >> On Mar 6, 2008, at 5:17 PM, Tom Haddon wrote:
> >>
> >>> Bit of an elementary question, this, but can someone remind me why:
> >>>
> >>> ps fuwxx | grep <something>
> >>>
> >>> returns "grep <something>" in the list if finds? Intuition would
> >>> suggest
> >>> that the ps is happening first, and so the grep command wouldn't show
> >>> in
> >>> the list. One of those things that was explained to me once, but 
> >>> seems
> >>> to have slipped through my sieve-like memory...
> >>>
> >>> Cheers, Tom
> >>>
> >>>
> >>>
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> >>>
> >>
> >
> >
>