signal handling via ssh

signal handling via ssh

[Sebastian Stark]

Dear list,

I have a question that is maybe off-topic, so please forgive me if it turns out that this is not at all zsh or even shell-related.

If I do the following:

  ssh -t $server "less /var/log/syslog; exec \$SHELL -l"

end then press 'q' I end up on $server in a login shell. That's what I want.

However, If I do this:

  ssh -t $server "less +F /var/log/syslog; exec \$SHELL -l"

and press '^C' I'm logged off from $server. The '^C' does not seem to go through to only the less process.

How can I achieve something like the above but such that ^C does not end the ssh connection?

Thanks for any help.


Sebastian

Re: signal handling via ssh

[Bart Schaefer]

On Apr 1, 11:27am, Sebastian Stark wrote:
}
}   ssh -t $server "less +F /var/log/syslog; exec \$SHELL -l"
} 
} and press '^C' I'm logged off from $server. The '^C' does not seem to
} go through to only the less process.
}
} How can I achieve something like the above but such that ^C does not
} end the ssh connection?

The ^C is a tty-driver interrupt, which means it's sent to the "process
group leader" which controls the pseudo-tty allocated by ssh -t.  In
this example the group leader is the entire shell process that has been
invoked to run those two commands in sequence.

So what you need to do is stop the group leader from reacting to the
signal while allowing "less" to do so.  You can do the first with the
trap command, but that ignores the signal in all sub-processes as well,
so you have to restore the signal handling in a new process created
explicitly for that purpose.  It all comes out like this:

ssh -t $server "trap '' INT; \
                (trap - INT && exec less +F /var/log/syslog); \
	        exec \$SHELL -l"

-- 

Re: signal handling via ssh

[Sebastian Stark]

Am 01.04.2011 um 16:14 schrieb Bart Schaefer:

> On Apr 1, 11:27am, Sebastian Stark wrote:
> }
> }   ssh -t $server "less +F /var/log/syslog; exec \$SHELL -l"
> } 
> } and press '^C' I'm logged off from $server. The '^C' does not seem to
> } go through to only the less process.
> }
> } How can I achieve something like the above but such that ^C does not
> } end the ssh connection?
> 
> The ^C is a tty-driver interrupt, which means it's sent to the "process
> group leader" which controls the pseudo-tty allocated by ssh -t.  In
> this example the group leader is the entire shell process that has been
> invoked to run those two commands in sequence.
> 
> So what you need to do is stop the group leader from reacting to the
> signal while allowing "less" to do so.  You can do the first with the
> trap command, but that ignores the signal in all sub-processes as well,
> so you have to restore the signal handling in a new process created
> explicitly for that purpose.  It all comes out like this:
> 
> ssh -t $server "trap '' INT; \
>                (trap - INT && exec less +F /var/log/syslog); \
> 	        exec \$SHELL -l"

Works for me, thank you very much for both, code and explanation!


Sebastian