Bash: optionally passing arguments to a command

I'm trying to add arguments to a command call depend on another variable. Please look at the shell scripting code:

curl \
  $([ -z "${title}" ] || echo --data-urlencode title=${title}) \
  http://example.com

In the example, if title is given not null, an argument will be added to curl.

This does not work correctly if title contains spaces. Also I couldn't surround $(...) with quotations, because if title is null, it will yield an unexpected empty argument to curl.

What should I do to make it work as expect.

I've solved the problem with the bash ${var:+...} syntax, (reference).

The script now changes to

curl \
    ${title:+ --data-urlencode "title=${title}"} \
    http://example.com

which works perfectly.

Also see:

• Bash - function with optional arguments and missing logic
• How to write a bash script that takes optional input arguments?