Issues when spawn'ing ssh on expect ( {$var} vs. "$var" ) - linux

Question

I have a expect script that so far it is working fine ... it spawn a ssh session over an existing ssh tunnel like this:
spawn ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o {ProxyCommand=nc -X 5 -x localhost:8888 %h %p} "$user_ip"
$user_iphas the username and destination IP, as expected by ssh, like username#IP
The problem is that sometimes, port 8888 is being used by another ssh tunnel, and every time this happens I have to tweak the code and change the tunnel port.
So, I am trying to send the port in a variable, to avoid touch the code all the time.
I am getting the tunnel port from command line, as:
set proxy_port [lindex $argv 2]
and then
spawn ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o {ProxyCommand=nc -X 5 -x localhost:"$proxy_port" %h %p} "$conn"
I see that proxy_portvariable is properly set, but when the script try to spawn the ssh, I get:
spawn ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -o {ProxyCommand=nc -X 5 -x localhost:8888 %h %p} USERID#10.0.0.1
command-line line 0: garbage at end of line; "-o".
send: spawn id exp5 not open
while executing
"send "$pwd\r""
(file "./amm-qev.new.exp" line 36)
In the error above, I see that the port was properly replaced, but it is complaining about ssh syntax.
What is wrong there that my weak eyes are not catching ?

Answer 1

Tcl's {...} syntax is like single-quoted string ('...') in shell where the $var would not be expanded. You should use double quotes.
spawn ssh -o UserKnownHostsFile=/dev/null \
-o StrictHostKeyChecking=no \
-o "ProxyCommand=nc -X 5 -x localhost:$proxy_port %h %p" \
"$conn"