Ssh / popen / stdin weirdness

Ruby
1

Sorry I can’t figure out how to find the cause of this myself. I’m sure
there’s some article somewhere I’m not finding.

on RHEL4 u3, ruby 1.8, create:

===
app.rb:

my_pipe = IO.popen("ssh foreign-host ‘cat filename’)
file_contents = my_pipe.read
my_pipe.close

inp = $stdin.read
puts "returning " + inp

====

Run this app from this host (call it ServerB):
ruby app.rb <<EOF
hi there
EOF

You get “returning hi there”, as expected.

Now, from ServerA (public keys have been pushed), run:

ssh ServerB ‘ruby app.rb’ <<EOF
hi there
EOF

it fails, in that inp receives nothing from $stdin.

now, go back to app.rb, remove the first three lines so there’s no call
to IO.pipe.

run it again, it works.

why does $stdin get “moved” when the app is called remotely and opens a
pipe?

2

====

run it again, it works.

why does $stdin get “moved” when the app is called remotely
and opens a pipe?

SSH doesn’t fork a TTY when a command is given. Try calling ssh with
the -t flag.
-Doug

3

“Mike C.” [email protected] writes:

Sorry I can’t figure out how to find the cause of this myself. I’m
sure there’s some article somewhere I’m not finding.

What’s happening is that the standard input is getting read by the ssh
process itself, if the standard input is ready when the ssh process is
running.

Try this on ServerB: (i.e. where you ran it locally and it succeeded)

echo hi there | ruby abb.rb

I’m betting that you won’t get “hi there” being returned, because “hi
there” will end up as standard input to the ssh process.

To fix this, change this line:
my_pipe = IO.popen("ssh foreign-host ‘cat filename’)
into:
my_pipe = IO.popen(“ssh foreign-host ‘cat filename’ < /dev/null”)

Now ssh won’t pull on your standard input stream. If you prefer to
put the bit of input redirection at the front of the line, you can
also do something like this:
my_pipe = IO.popen(“echo|ssh foreign-host ‘cat filename’”)
or even this, though I think it’s less portable:
my_pipe = IO.popen(“</dev/null ssh foreign-host ‘cat filename’”)

Now, why this makes a difference running it locally versus remotely
I’m not entirely sure, especially since I’d think that a here document
would be sent all at once even to a local process, but when I was
running it locally just by typing lines and then control-D, the line
returned was the first line typed after the ssh process had completed.

4

my_pipe = IO.popen("ssh foreign-host ‘cat filename’)

Not sure if this is causing a problem, but you are missing a closeing
quote in this line.