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Re: A background ssh can take over the tty from bash?


From: Clark Wang
Subject: Re: A background ssh can take over the tty from bash?
Date: Sun, 11 Jun 2017 11:44:36 +0800

On Sat, Jun 10, 2017 at 11:00 PM, John McKown <address@hidden>
wrote:

> On Sat, Jun 10, 2017 at 9:19 AM, Clark Wang <address@hidden> wrote:
>
>> (I'm using bash 4.4.12 on Debian 8)
>>
>> Please follow the following steps to reproduce.
>>
>> From tty #1:
>>
>> $ tty
>> /dev/pts/11
>> $ ssh -o ControlMaster=yes -o ControlPath=/tmp/socket.tmp -N -f 127.0.0.1
>> address@hidden's password:
>> $ ps -C ssh uw
>> USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
>> root       716  0.0  0.2  44428   692 ?        Ss   22:04   0:00 ssh -o
>> ControlMaster=yes -o ControlPath=/tmp/socket.tmp -N -f 127.0
>> $
>> $
>> ​​
>> ssh -o ControlMaster=no -o ControlPath=/tmp/socket.tmp 127.0.0.1 sleep
>> 9999 &
>> [1] 725
>> $    <-- Then I cannot input anything except CTRL-C
>>
>> Go to tty #2:
>>
>> $ ps t pts/11 jw
>>  PPID   PID  PGID   SID TTY      TPGID STAT   UID   TIME COMMAND
>> 32714   725   725 32714 pts/11   32714 S        0   0:00 ssh -o
>> ControlMaster=no -o ControlPath=/tmp/socket.tmp 127.0.0.1 sleep 9999
>>   579 32714 32714 32714 pts/11   32714 Ss+      0   0:00 bash
>> $
>>
>> If I kill the "ssh -o ControlMaster=no -o ControlPath=/tmp/socket.tmp
>> 127.0.0.1 sleep 9999" then tty #1 (pts/11) would be able to accept my
>> input
>> again. Seems like the background "ssh" at tty #1 is consuming all input. I
>> cannot understand this since it should be stopped by SIGTTIN if it tries
>> to
>> read from the tty.
>>
>> Idea?
>>
>> -clark
>>
>
> ​IMO, not a bug in BASH, so this is the wrong forum. But, since I'm
> already blathering on, I'd say the problem is that you have invoked "ssh"
> incorrectly.​ In particular, ssh _does_ read from stdin normally. But, from
> the man page:
>
>  -f      Requests ssh to go to background just before command execution.
> This is useful if ssh is going to ask for passwords
>              or passphrases, but the user wants it in the background.
> This implies -n.  The recommended way to start X11 programs
>              at a remote site is with something like ssh -f host xterm.
>
>              If the ExitOnForwardFailure configuration option is set to
> “yes”, then a client started with -f will wait for all
>              remote port forwards to be successfully established before
> placing itself in the background.
>
>
> So instead of:
>
>
> ssh -o ControlMaster=no -o ControlPath=/tmp/socket.tmp 127.0.0.1 sleep 9999
> &
>
> Try:
>
>
> ssh -f -o ControlMaster=no -o ControlPath=/tmp/socket.tmp 127.0.0.1 sleep
> 9999
>

Thanks but I want it to run as a background job in bash.


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