Bash：用字符串格式化环境变量

Question

How does one format a string leveraging an environment variable within at the command line? For example, I want to curl and pass some variable, ie:

curl -X POST --data-urlencode 'payload={"text": "I want to print an environment variable here, eg a path: $PATH"}' https://someapi.com/

Answer 1

The easy answer is to change the kind of quotes you're using:

curl -X POST --data-urlencode \
  'payload={"text": "I want to print an environment variable here, eg a path: '"$PATH"'"}' \
   https://someapi.com/

Notably, this is still using single-quotes on the outside (so you don't need to change your payload), but then it ends the single quotes, starts double quotes, and embeds your substitution in those double quotes (before ending them and switching back to single quotes, within which literal -- rather than syntactic -- double quotes can be embedded).

A variant on this approach, which avoids the need for syntactic quotes mixed into the document content, is to used an unquoted heredoc, as advised by @chepner in the comments :

curl -X POST --data-urlencode @- https://someapi.com/ <<EOF
payload={"text": "I want to print an environment variable here, eg a path: $PATH"}
EOF

---

The better answer is to use a tool that knows how to format JSON; jq is a widely popular choice. Consider the following example:

text="I want to print an environment variable here, eg a path: \"$PATH\""
curl -X POST --data-urlencode @- https://someapi.com/ <<EOF
payload=$(printf '%s\n' "$text" | jq -R '{text: .}')
EOF

This way you're guaranteed valid output, even if your environment variable contains backslashes, literal quotes, nonprintable characters, or whatever else may come.