How to set AND expand variables in a heredoc section

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?

It is behaving as expected. The code inside the heredoc runs on a remote host and it doesn't see the `count=0` initialization. — codeforester, Mar 29 '17 at 18:51
Is there some way to pass the variable (and a couple of others) into the heredoc execution? — user2824889, Mar 29 '17 at 19:01
There is no "heredoc execution". The heredoc defines a string. The string is passed to ssh, where it is evaluated by the shell. — William Pursell, Mar 29 '17 at 19:09

score 4 · Answer 1 · answered Mar 29 '17 at 18:56

4

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.

answered Mar 29 '17 at 18:56

anubhava

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Also note that value of `$count` is passed from current shell to remote shell – anubhava Mar 29 '17 at 19:21

score 1 · Answer 2 · edited May 23 '17 at 12:02

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"

clt60 · Answer 3 · 2017-03-29T19:34:58.127

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

How to set AND expand variables in a heredoc section

3 Answers3