We can assign a variable for curl using single quote ' and wrap some other variables in

• a single quote => ' $variable '
• a double-quote single-quote => "' $variable '"
• a single+double+single quote => '"' $variable '"'

Lets test each case, but first watch out for this catch that if we use a single quote ' for variable assignment, that variable is not evaluated.

watch out

Please notice the assignment is done by a single quote CURL_DATA='content'

cmd='ls'

CURL_DATA='{
    "cmd": "$cmd",    <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us

{
    "cmd": "$cmd",    <===== we need ls not $cmd
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

a single quote ' $variable '

• The value of a variable is evaluated
• neither single quote ' nor applies double one "

cmd='ls'

CURL_DATA='{
    "cmd": '$cmd',    <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us:

{
    "cmd": ls,    <===== neither 'ls' nor "ls", just ls
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

a double-quote single-quote "' $variable '"

• variable is evaluated
• will be surrounded by a double quote "

cmd='ls'

CURL_DATA='{
    "cmd": "'$cmd'",    <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us

{
    "cmd": "ls",    <===== we have double quote " variable "
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

a single+double+single quote => '"' $variable '"'

• variable is evaluated
• will be surrounded by a single quote '

cmd='ls'

CURL_DATA='{
    "cmd": '"'$cmd'"',
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA";

Will give us

{
    "cmd": 'ls',    <===== we have a single quote ' variable '
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

summary

# no quote at all
$cmd => $cmd

# a double quote (" $variable ")
"$cmd" => "$cmd"

# a single quote (' $variable ')
'$cmd' => ls

# a single quote + a double quote ("' $variable '")
"'$cmd'" => "ls"

# a single-double-single quote ('"' $variable '"')
'"'$cmd'"' => 'ls'

which one we should use?

Since JSON needs a double quote " for its key or value we can use :

• a double-quote single-quote "' $variable '"

curl

cmd='ls'

CURL_DATA='{
    "cmd": "'$cmd'",
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA" | jq '.'

curl --data "$CURL_DATA" -X POST localhost:3232/cmd | jq '.'

---

NOTE:
The equivalent of ' for a variable evaluation is '" which means instead of using '$cmd' we can use '"$cmd"' and it gives us ls neither with a single quote nor a double quote , but it gets more confusing if we needed to apply for curl since we need a double quoted result "ls" and would have to wrap it in another double quote => "'"

This code works well, but the above is more readable

cmd='ls'

CURL_DATA='{
    "cmd": "'"$cmd"'",     <===== our variable
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}';

echo "$CURL_DATA" | jq '.'

curl --data "$CURL_DATA" -X POST localhost:3232/cmd | jq '.'

Will give us:

{
    "cmd": "ls",    <===== result
    "args": [ "-la" , "/tmp" ],
    "options": {
        "cwd": "/tmp"
    },
   "type": "sync"
}

finally

We can use either of:

    "cmd": "'$cmd'",    <===== will be: "ls"

or

    "cmd": "'"$cmd"'",    <===== will be: "ls" 

and "$CURL_DATA" as a normal variable

curl --data "$CURL_DATA" -X POST localhost:3232/cmd