ssh "!command" escape sequence - does it work?

[Karl Auer]

On Fri, 2020-09-11 at 13:11 +0100, Chris Green wrote:
> Yes, as it's supposed to be.  As you say though the rest of the
> !command escape seems to be, at the very least, not quite what one
> expects.  I've searched around quite a bit but I've not found
> anything very helpful.

I turned on debug with "-vvv", and:

otherhost:~$ [enter ~C] 
ssh> !echo freddy
debug3: Executing /bin/bash -c "echo freddy"
[enter]
otherhost:~$

I.e., nothing was actually echoed, but:

otherhost: ~$ /bin/bash -c "echo freddy"
freddy

myhost: ~$ /bin/bash -c "echo freddy"
freddy

This led me to think about what stderr and stdout might be when
executing a program deep in the bowels of ssh.

Now watch this :-)

myhost:~$ ssh otherhost
otherhost:~$ 
ssh> !echo boo > /tmp/boo.txt

otherhost:~$ exit
logout
Connection to otherhost closed.
myhost:~$ ls -la /tmp/boo.txt
-rw-r--r-- 1 kauer kauer 4 Sep 11 22:47 /tmp/boo.txt
myhost:~$ cat /tmp/boo.txt
boo

Ta daaa!

I.e., it was working all along. I bet that, like me, you were trying to
do something simple like local console output :-)

More investigation needed, but it makes sense not to allow you to
scribble on the remote's stdout, though it seems happy to scribble on
stderr.

The code that actually does this is a function called ssh_local_cmd in
sshconnect.c. It does a fork/exec sequence. You can get errors written
to syslog with the ssh "-y" option.

Regards, K.

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au)
http://www.biplane.com.au/kauer

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