[lug] Bash Scripting Ping

[D. Stimits]

> 
>     How's Jed's suggestion different from what you need?
> 
A single ping is a single line of text, followed by termination of the process providing the text. I am interested in parsing sections of a continuous, never ending string of text whenever a newline is found. Each newline providing a "token" to process, but the text stream never ending.

How do I trigger something to occur on a subset of a continuous string of text delimited by newlines without actually ending that continuous source of a text stream? That is the brain teaser, to arbitrarily pick and process subsets of a continuous stream of characters in bash. It does not need to be "ping", that is just the case that is puzzling me.

Perhaps an updated pseudo code:
stream = `ping` # ping never ends, but embeds newlines.
while (stream.subset_token_via_newline); do
do stuff with newline delimited token, not interrupting the stream;
done

Versus:
while (true); do
token = something_generating_a_single_token;
do something with token;
done

>     Best regards,
>     kh
> 
> 
>     On 9 November 2020 02:27:21 GMT+08:00, "D. Stimits" <stimits at comcast.net> wrote:
> 
>         > > 
> > 
> >             > > > On 11/08/2020 11:21 AM Jed S. Baer <blug at jbaer.cotse.net> wrote:
> > > 
> > > 
> > >             On Sun, 8 Nov 2020 10:53:09 -0700 (MST)
> > >             D. Stimits wrote:
> > > 
> > > 
> > >                 > > > > I am curious about something in bash scripting which does not seem to be
> > > >                 particularly easy. I wanted to script "ping -O -D address" such that each
> > > >                 line gets processed by some logic if the ping fails. Redirecting ping to
> > > >                 a variable does not work because it only "returns" when the ping exits
> > > >                 (each line is not an exit). Even if I were to fork and exec, the forked
> > > >                 process would itself have the same problem.
> > > > 
> > > >                 Is there some simple/clever way to process each line of a ping in bash
> > > >                 without killing off the ping itself? My goal was to send it through some
> > > >                 database and statistics type processing as success/failure lines occur.
> > > > 
> > > >             > > > I'm not sure of quite what you want. Is it necessary to process this in
> > >             real time? And you say "...if ping fails", but then "... as success/failure
> > >             lines occur."
> > > 
> > >         > > 
> > 
> >         I'd like to know if there is a way to pipe the output of ping such that each line is equivalent to a loop iteration. Pseudo code:
> >            while (ping text line); do
> >               line | edit line; # e.g., substituting microseconds for date.
> >               print edit line if conditions are met; # e.g., a few lines of fail prior to print.
> >            done
> > 
> >         I'm not actually concerned that this is ping so much as I am trying to figure out how a newline from a continuous command can be used to trigger a loop iteration. It has become something of a "brain teaser" for me. It is easy if the ping itself ends with a single ping. It is also easy if you run ping all day and only then process the results upon exit of ping. I am now just really curious how the newline delimited continuous output of ping can be used to trigger something else despite the continuous output itself not stopping.
> > 
> >         > 
> > 
> >             > > > The super simple thing is a bash read loop. But you won't know the exit
> > >             status until the ping finishes.
> > > 
> > >             $ ping -c 3 -D -O n.n.n.n | while read LINE;
> > > 
> > >                 > > > > do
> > > >                 echo $LINE
> > > >                 done
> > > > 
> > > >             > > > Bash read will do token separation using IFS, so you could do
> > >             | while read FIELD1 FIELD2 etc
> > > 
> > >             Offhand, if the exit status of ping must be known before processing, then
> > >             redirect ping output to a file, check status, then you can still do a bash
> > >             read loop to process the file. If needed, the bash mktemp command will
> > >             generate a unique tmp filename for your ping output.
> > > 
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> > >             - Linus Torvalds
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