Variable expansion inside single quotes

Question

I am quite new to shell scripting.

I have the following script:

out="FAILURE"
curl -X POST -d 'json={"json":"message"}' http://localhost:8888/json.tail.test

I want to replace "message" with the $out's value. I tried different ways but could not get that done. Can someone please suggest me?

Answer 1

Do this:

out="FAILURE"
curl -X POST -d 'json={"json":"'$out'"}' http://localhost:8888/json.tail.test

Basically, enclose everything except $out inside single quotes. Single quotes protect double quotes but suppress the expansion of variables like $out.