How to set AND expand variables in a heredoc section - bash

Question

I have a heredoc that needs to call existing variables from the main script, and set its own variables to use later. Something like this:
count=0
ssh $other_host <<ENDSSH
if [[ "${count}" == "0" ]]; then
output="string1"
else
output="string2"
fi
echo output
ENDSSH
That doesn't work because 'output' doesn't get set to anything.
I tried using the solution from this question:
count=0
ssh $other_host << \ENDSSH
if [[ "${count}" == "0" ]]; then
output="string1"
else
output="string2"
fi
echo output
ENDSSH
It didn't work either. $output got set to "string2" because $count wasn't expanded.
How can I use a heredoc that expands variables from the parent script, and sets its own variables?

Answer 1

You can use:
count=0
ssh -t -t "$other_host" << ENDSSH
if [[ "${count}" == "0" ]]; then
output="string1"
else
output="string2"
fi
echo "\$output"
exit
ENDSSH
We use \$output so that it is expanded on remote host not locally.

Answer 2

It is better not to use stdin (such as by using here-docs) to pass commands to ssh.
If you use a command-line argument to pass your shell commands instead, you can better separate what is expanded locally and what will be executed remotely:
# Use a *literal* here-doc to read the script into a *variable*.
# Note how the script references parameter $1 instead of
# local variable $count.
read -d '' -r script <<'EOF'
[[ $1 == '0' ]] && output='zero' || output='nonzero'
echo "$output"
EOF
# The variable whose value to pass as a parameter.
# With value 0, the script will echo 'zero', otherwise 'nonzero'.
count=0
# Use `set -- '$<local-var>'...;` to pass the local variables as
# positional parameters, followed by the script code.
ssh localhost "set -- '$count'; $script"

Answer 3

You can escape the variables as #anubhava said, or, if you get too much variables for the escaping, you can do it in two steps:
# prepare the part which should not be expanded
# note the quoted 'EOF'
read -r -d '' commands <<'EOF'
if [[ "$count" == "0" ]]; then
echo "$count - $HOME"
else
echo "$count - $PATH"
fi
EOF
localcount=1
#use the unquoted ENDSSH
ssh me#nox.local <<ENDSSH
count=$localcount # count=1
#here will be inserted the above prepared commands
$commands
ENDSSH
will print something like:
1 - /usr/bin:/bin:/usr/sbin:/sbin