bash - Need to expand a variable in a HEREDOC that is in quotes

Is it possible to use a HEREDOC and get variable expansion to happen when the variable is in quotes?

Example: $package is an environment variable that is set before the script runs

#!/bin/bash

cat > out.json <<'EOF'
{
    "apps": [
    {
        "cwd":"/usr/local/$package"
    }
}
EOF

Expected output if $package="www"

{
    "apps": [
    {
        "cwd":"/usr/local/www"
    }
}

Answer

TL;DR

Use < instead of <<'EOF'.

---

This is what POSIX says about here-documents [emphasis mine]:

[n]<    here-document
delimiter

[…]

If any part of word is quoted, the delimiter shall be formed by performing quote removal on word, and the here-document lines shall not be expanded. Otherwise, the delimiter shall be the word itself.

If no part of word is quoted, all lines of the here-document shall be expanded for parameter expansion, command substitution, and arithmetic expansion. […]

In you case word is 'EOF' (quoted!), so $package inside the here-document is not expanded. Drop these quotes and you will get the desired result.

cat > out.json <{
    "apps": [
    {
        "cwd":"/usr/local/$package"
    }
}
EOF

Note this has nothing to do with quotes that surround the variable inside the here-document.