Tcl Requires Chilkat v11.5.0+

curl with Variable Substitution in a JSON Request Body

See more CURL Examples

This example shows how to use variables inside a JSON request body using the {{variable_name}} syntax. When the HTTP request’s Content-Type indicates JSON, Chilkat automatically applies proper JSON escaping to each substituted value, ensuring the resulting JSON remains valid. Tcl

load ./chilkat.dll

set success 0

#  Variable names are enclosed between {{ and }}
#  Important: The variable {{var_name}} should be placed inside the quotes.

#  This is correct:
#  curl -X POST https://api.example.com/messages \
#    -H "Content-Type: application/json" \
#    -d '{"text":"{{message}}"}'

#  This is incorrect:
#    -d '{"text":{{message}}}'

set sbCurl [new_CkStringBuilder]

CkStringBuilder_AppendLn $sbCurl "curl -X POST https://api.example.com/messages \\"
CkStringBuilder_AppendLn $sbCurl "  -H \"Content-Type: application/json\" \\"
CkStringBuilder_AppendLn $sbCurl "  -d '{\"text\":\"{{message}}\"}'"

set curl [new_CkHttpCurl]

#  In this example, the value we'll provide for the "message" variable
#  will contain chars that require JSON escaping.
CkHttpCurl_SetVar $curl "message" "He said \"Hello, world!\""

#  To demonstrate how the variables are replaced, this example does not execute the curl command. 
#  Instead, it generates the raw HTTP request that would be sent if the curl command were run.
set sbRawRequest [new_CkStringBuilder]

set success [CkHttpCurl_ToRawRequest $curl [CkStringBuilder_getAsString $sbCurl] $sbRawRequest]
if {$success == 0} then {
    puts [CkHttpCurl_lastErrorText $curl]
    delete_CkStringBuilder $sbCurl
    delete_CkHttpCurl $curl
    delete_CkStringBuilder $sbRawRequest
    exit
}

puts [CkStringBuilder_getAsString $sbRawRequest]

#  The output is shown below.
#  Notice that the quote chars around "Hello World!" are properly JSON escaped.

#  POST /messages HTTP/1.1
#  Host: api.example.com
#  Content-Type: application/json
#  Content-Length: 36
#  
#  {"text":"He said \"Hello, world!\""}

delete_CkStringBuilder $sbCurl
delete_CkHttpCurl $curl
delete_CkStringBuilder $sbRawRequest