I HATE YOU, OPENSSH!

Sometimes, but just sometimes, I really, really hate OpenSSH. Consider the following code:

#!/bin/bash

while read hostname; do
	ssh -o PasswordAuthentication=no -i secret.key "${hostname}" \
				      "echo \$(hostname): \$(date -u)"
done < listofhosts.txt

The file listofhosts.txt contains - surprise - a list of hostnames, one per line. The expected output of this small script is something like this:

flora: Tue Jan 13 09:39:18 UTC 2015
pallas: Tue Jan 13 09:39:19 UTC 2015
athene: Tue Jan 13 09:39:21 UTC 2015
lutetia: Tue Jan 13 09:39:22 UTC 2015

Simple enough as far as scripts go, right? WRONG!
Now, what you DO get is this:

flora: Tue Jan 13 09:40:07 UTC 2015

Why? Because OpenSSHs ssh command drains STDIN. Why? No idea! Because it can, I guess. To get the above expected output, you need to do this:

#!/bin/bash

exec 3<listofhosts.txt

while read hostname <&3; do
	ssh -o PasswordAuthentication=no -i secret.key "${hostname}" \
				      "echo \$(hostname): \$(date -u)"
done

Hope that helps you somewhere down the line.

EOF

Category: blog

Tags: openssh rant shellscripting