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I want to be able to send signals (SIGINT is the most important) through ssh.

This command:

ssh server "sleep 1000;echo f" > foo

will start sleep on server and after 1000 seconds it will put 'f\n' in the file foo on my local machine. If I press CTRL-C (i.e. send SIGINT to ssh) it will kill ssh, but it will not kill sleep on the remote server. I want it to kill sleep on the remote server.

So I tried:

ssh server -t "sleep 1000;echo f" > foo

But if stdin is not a terminal I get this error:

Pseudo-terminal will not be allocated because stdin is not a terminal.

and then SIGINT is still not forwarded.

So I tried:

ssh server -t -t "sleep 1000;echo f" > output

But then the output in foo is not 'f\n' but instead 'f\r\n' which is disastrous in my situation (as my output is binary data).

In the above I use "sleep 1000;echo f", but in reality that is supplied by the user, thus it can contain anything. However, if we can make it work for "sleep 1000;echo f" we can most likely make it work for all realistic situations.

I really do not care about getting a pseudo-terminal at the other end, but I have been unable to find any other way of getting ssh to forward my SIGINT.

Is there another way?


The user could give commands that read binary data from stdin, such as:

seq 1000 | gzip | ssh server "zcat|bzip2; sleep 1000" | bzcat > foo

The user could give commands that are cpu intensive, such as:

ssh server "timeout 1000 burnP6"


The version that seems to work for me is:

your_preprocessing |
  uuencode a | ssh -tt -oLogLevel=quiet server "stty isig -echoctl -echo ; uudecode -o - |
your_command |
  uuencode a" | uudecode -o - |

Thanks to digital_infinity for pointing me in the right direction.

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I think your actual usage of ssh must be more complicated than what you show as examples, because you can get the behavior you want with a simple rearrangement: sleep 1000 && ssh server "echo f" > foo (It has to be &&, not ;, so that killing sleep prevents the ssh command from running.) If I'm right, please make your examples more representative of your actual usage, so a better answer can be given. –  Warren Young Jun 4 '12 at 21:31
Correct: sleep and echo are actually user supplied scripts and not literally sleep and echo. So we do not know what they do and should assume the worst. –  Ole Tange Jun 4 '12 at 22:05
Sooo...you're going to come up with a better example command, right? One where a simple rearrangement doesn't fix the problem? You ask a good question, and I'd like to see it answered, but you're less likely to get an answer if "so don't do that, then" is a reasonable reply. –  Warren Young Jun 4 '12 at 22:56
Oh by the way... Don't do that ;) –  Tim Jun 4 '12 at 23:08
As elabrated in the question: """In the above I use "sleep 1000;echo f", but in reality that is supplied by the user, thus it can contain anything""" –  Ole Tange Jun 4 '12 at 23:16

2 Answers 2

up vote 3 down vote accepted

Short answer:

ssh -t fs "stty isig intr ^N -echoctl ; trap '/bin/true' SIGINT; sleep 1000; echo f" > foo

and stop the program by CTRL+N.

Long explanation:

  1. You must use stty option intr to change your server or local interrupt character to not collide with each other. In the command above I've changed the server interrupt character to CTRL+N. You can change your local interrupt character and leave the server's one without any changes.
  2. If you don't want the interrupt character to be in your output (and any other control character) use stty -echoctl.
  3. You must assure that control characters are switched on on the server bash invoked by sshd . If you don't you can end up with processes still hanging around after you logout. stty isig
  4. You actually catch SIGINT signal by trap '/bin/true' SIGINT with empty statement. Without the trap you will not have any stdout after SIGINT signal on your end.
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It seems not to deal nicely with binary input: seq 1000 | gzip | ssh -t -t server "stty isig intr ^N -echoctl ; trap '/bin/true' SIGINT; sleep 1; zcat|bzip2" | bzcat > foo; –  Ole Tange Jun 6 '12 at 11:28
You cannot interrupt by console if you set standard input for ssh for something other then console. In this case you must interrupt by the stdin stream. –  digital_infinity Jun 6 '12 at 11:58
I found that this seq 1000 | gzip | ssh -tt dell-test "zcat|bzip2" | bzcat > foo does not work too. To have this command work we need to remove -tt . So the pseudo terminal allocation probably takes some input from stdin –  digital_infinity Jun 6 '12 at 12:23
I tried the uuencode. In this version it does not work: cat foo.gz | perl -ne 'print pack("u",$_)' | ssh -t -t server 'perl -ne "print unpack(\"u\",\$_)"| zcat | gzip | perl -ne "print pack(\"u\",\$_)"' | perl -ne 'print unpack("u",$_)' > foo2.gz –  Ole Tange Jun 6 '12 at 12:56
I have this: (sleep 1; seq 1000 ) | gzip | uuencode bin | ssh -tt dell-test "stty isig intr ^N -echoctl -echo ; trap '/bin/true' SIGINT; sleep 1; uudecode -o /dev/stdout | zcat |bzip2 | uuencode bin2 " | uudecode -o /dev/stdout | bzcat >foo . Although the interrupt character does not work - we need some transmission method for interrupt character while stdin is busy. –  digital_infinity Jun 6 '12 at 13:27

I think you could find PID of the process you're running on server and send a signal using another ssh command (like this: ssh server "kill -2 PID").

I use this method for sending reconfiguration signals to applications running on a different machine (my applications catch SIGUSR1 and read a config file). In my case finding PID is easy, because I have unique process names and I can find the PID by sending a ps request via ssh.

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I do not see a bullet proof way of finding the PID of the program being run on remote. Remember it is given by the user. It could be: ssh server 'exec $(echo fyrrc 1000| /usr/games/rot13)' –  Ole Tange Jun 6 '12 at 11:40

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