On Wed, Feb 1, 2012 at 7:13 PM, Stephen Harris <lists@xxxxxxxxxx> wrote:
> On Wed, Feb 01, 2012 at 07:03:33PM -0800, Peter Blajev wrote:
> > On Wed, Feb 1, 2012 at 2:53 PM, Stephen Harris <lists@xxxxxxxxxx> wrote:
> >
> > > On Wed, Feb 01, 2012 at 01:07:31PM -0800, Peter Blajev wrote:
> > > > echo " server2
> > > > server2" | \
> > > > while read confLine; do
> > > > echo "--> $confLine"
> > > > ssh peter@$confLine ls
> > > > echo "--> END $confLine"
> > > > done
>
> > > > Any idea what would cause the ssh command to break the while loop?
> > >
> > > "ssh" is reading from stdin and passing the data over to the remote
> > > machine. You can test this with
> > > ssh peter@$confLine 'read x ; echo we got $x'
> > >
> > > To stop it doing this, use the "-n" flag
> > > ssh -n peter@$confLine ls
> >
> > Unfortunately I can't always use the (-n) option. If I wan't to send data
> > through the pipe then the (-n) won't work. For example (on top of my
> head):
> > mysqldump dB | ssh peter@remoteServer "mysql dB"
>
> In this situation, ssh will read from the pipe and not from the "echo"
> statement and so it won't break your while loop.
>
> The thing you need to understand is how redirection works.
> echo foo | while read
> do
> ...
> done
>
> Everything from the "while" to the "done" will have stdin configured to
> read from the pipe (the output of "echo"). Here's an example:
> % echo "a
> b
> c
> d" | while read a
> do
> echo We have $a
> read b
> echo And $b
> done
> We have a
> And b
> We have c
> And d
>
> You can see that the "read" statement inside the loop is reading from
> the pipe and so draining input.
>
> An "ssh" on its own, in this situation, would drain _all_ the input.
> The "-n" flag tells ssh to not do this. But, equally, when you do
> sqldump | ssh
> then you've told ssh to take input from the output of sqldump, and so it's
> no longer reading the from outer loop.
>
> Instead of "ssh -n" you could do "ssh < /dev/null" or "echo | ssh" or
> other options, and get the same sort of effect; you're setting up the
> stdin to ssh to be from somewhere _other_ than your main echo.
Wow. Great lesson. Thank you Stephen.
It makes more sense now.
I'll save this email for sure.
--
Peter
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