Variable inside double quotes doesn't work as argument [duplicate] - bash

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Why does shell ignore quoting characters in arguments passed to it through variables? [duplicate]
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I'm trying to pass arguments in a variable to curl and it fails. I turned on set -x to get more information, and I'm really confused about all the single quotes I'm seeing here.
curlopts="-ksS -H 'Content-Type:application/x-www-form-urlencoded'"
$ curl "$curlopts" https://localhost
+ curl '-ksS -H '\''Content-Type:application/x-www-form-urlencoded'\''' https://localhost
curl: option -ksS -H 'Content-Type:application/x-www-form-urlencoded': is unknown
curl: try 'curl --help' or 'curl --manual' for more information
It works without quotes like curl $curlopts https://localhost but I thought it's generally bad practice to use a variable without quotes.

Answer 1

This:
curlopts="-ksS -H 'Content-Type:application/x-www-form-urlencoded'"
sets curlopts to -ksS -H 'Content-Type:application/x-www-form-urlencoded'.
After the above, this:
curl "$curlopts" https://localhost
is equivalent to this:
curl "-ksS -H 'Content-Type:application/x-www-form-urlencoded'" https://localhost
meaning that it calls curl with two arguments: -ksS -H 'Content-Type:application/x-www-form-urlencoded', and https://localhost.
But of course, you want to call curl with four arguments: -ksS, -H, Content-Type:application/x-www-form-urlencoded, and https://localhost.
To achieve that, I recommend using an array:
curlopts=(-ksS -H Content-Type:application/x-www-form-urlencoded)
curl "${curlopts[#]}" https://localhost
where the "${arrayname[#]}" syntax expands, magically, into a separate double-quoted word for each element of the array.
(Note: I've removed the single-quotes because they aren't needed here — none of the characters in Content-Type:application/x-www-form-urlencoded require quoting — but if they make you feel happier, you can safely re-add them.)