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How to set and expand variables in the heredoc section

I have a heredoc that needs to call existing variables from the main script and set its own variables to be used later. Something like that:

count=0

ssh $other_host <<ENDSSH
  if [[ "${count}" == "0" ]]; then
    output="string1"
  else
    output="string2"
  fi
  echo output
ENDSSH

It doesn't work because 'output' is getting nothing.

I tried to use 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 the value "string2" because the $ counter was not expanded.

How do I use a heredoc that expands variables from the parent script and sets its own variables?

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3 answers


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

to have it removed on the remote host not locally.

+3


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it's best not to use stdin (for example using here-docs) to pass commands ssh

.

If you use a command line argument to pass shell commands, you can better separate what is being extended 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"
+1


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You can escape variables as @anubhava said, or if you are getting too many variables to escape, 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
0


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