openssh unix dev - Oct 2016 - Why stdin is required to be overwritten in SSH?

Hi SSH,

I have put the ssh command in `script.sh`, with the code:

~~~
#!/usr/bin/env bash
ssh -q server date
~~~

And I have one `main.sh` to call `script.sh` as below.

~~~
#!/usr/bin/env bash
# vim: set noexpandtab tabstop=2:

while read -r run
do
./script.sh
done < <(seq 10)
~~~

The `script.sh` can be called only once, say

~~~
$ ./main.sh
Tue Oct 18 12:26:05 CDT 2016
~~~

But 10 runs are expected.

If I modify the stdin when calling, like `main1.sh` as below.

~~~
#!/usr/bin/env bash
# vim: set noexpandtab tabstop=2:

while read -r run
do
./script.sh < /dev/null
done < <(seq 10)
~~~

It will return the right 10 calling.

~~~
$ ./main1.sh
Tue Oct 18 12:28:24 CDT 2016
Tue Oct 18 12:28:24 CDT 2016
Tue Oct 18 12:28:25 CDT 2016
Tue Oct 18 12:28:25 CDT 2016
Tue Oct 18 12:28:26 CDT 2016
Tue Oct 18 12:28:26 CDT 2016
Tue Oct 18 12:28:27 CDT 2016
Tue Oct 18 12:28:27 CDT 2016
Tue Oct 18 12:28:28 CDT 2016
Tue Oct 18 12:28:29 CDT 2016
~~~

Could you tell me why `stdin` needs to be overwritten in SSH? Thanks.

My machine info is as below.

~~~
$ bash --version
GNU bash, version 4.3.42(1)-release (x86_64-apple-darwin15.0.0)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

$ ssh -V
OpenSSH_6.9p1, LibreSSL 2.1.8
~~~

Best regards,
Jin

On Tue, Oct 18, 2016 at 10:34 AM, Jin Li <lijin.abc at gmail.com>
wrote:
> Could you tell me why `stdin` needs to be overwritten in SSH? Thanks.
Consider this (almost) equivalent simplified script:

while read -r run
do
  ssh localhost date
done < <(seq 10)

What's happening is that the remaining 9 lines are being read by ssh
and sent to the remote server where the "date" command ignores them
and they are discarded (but ssh doesn't know that the remote
"date" is
going to do that).  If you replace the "date" with "cat" you
can see
this:

$ cat t
while read -r run
do
  cat
done < <(seq 10)

$ bash t
2
3
4
5
6
7
8
9
10

This doesn't happen with a local "date" command because it never
reads
its stdin.

When you redirect ssh's stdin to /dev/null the ssh no longer consumes
the output from "seq".  ssh also has a "-n" option that does
this.

-- 
Darren Tucker (dtucker at zip.com.au)
GPG key 11EAA6FA / A86E 3E07 5B19 5880 E860  37F4 9357 ECEF 11EA A6FA (new)
    Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.

Hi Darren,

I got it. Many thanks for your excellent explanations.

Best regards,
Jin

On Tue, Oct 18, 2016 at 2:04 PM, Darren Tucker <dtucker at zip.com.au>
wrote:
> On Tue, Oct 18, 2016 at 10:34 AM, Jin Li <lijin.abc at gmail.com>
wrote:
>> Could you tell me why `stdin` needs to be overwritten in SSH? Thanks.
>
> Consider this (almost) equivalent simplified script:
>
> while read -r run
> do
>   ssh localhost date
> done < <(seq 10)
>
> What's happening is that the remaining 9 lines are being read by ssh
> and sent to the remote server where the "date" command ignores
them
> and they are discarded (but ssh doesn't know that the remote
"date" is
> going to do that).  If you replace the "date" with
"cat" you can see
> this:
>
> $ cat t
> while read -r run
> do
>   cat
> done < <(seq 10)
>
> $ bash t
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
> 10
>
> This doesn't happen with a local "date" command because it
never reads
> its stdin.
>
> When you redirect ssh's stdin to /dev/null the ssh no longer consumes
> the output from "seq".  ssh also has a "-n" option that
does this.
>
> --
> Darren Tucker (dtucker at zip.com.au)
> GPG key 11EAA6FA / A86E 3E07 5B19 5880 E860  37F4 9357 ECEF 11EA A6FA (new)
>     Good judgement comes with experience. Unfortunately, the experience
> usually comes from bad judgement.