Using Environment Variables in cURL Command - Unix
Single quotes inhibit variable substitution, so use double quotes. The inner double quotes must then be escaped.
... -d "{\"username\":\"$USERNAME\",\"password\":\"$PASSWORD\"}"
For less quoting, read from standard input instead.
curl -k -X POST -H 'Content-Type: application/json' -d @- <<EOF{ "username": "$USERNAME", "password": "$PASSWORD"}EOF-d @foo reads from a file named foo. If you use - as the file name, it reads from standard input. Here, standard input is supplied from a here document, which is treated as a double-quoted string without actually enclosing it in double quotes.
curl -k -X POST -H 'Content-Type: application/json' -d '{"username":"'$USERNAME'","password":"'$PASSWORD'"}'Here the variable are placed outside of "'" quotes and will be expanded by shell (just like in echo $USERNAME). For example assuming that USRNAME=xxx and PASSWORD=yyy the argv[7] string passed to curl is {"username":"xxx","password":"yyy"}
And yes, this will not work when $USERNAME or $PASSWORD contain space characters.