(Tcl) curl with Variable Substitution in a JSON Request Body

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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.

Note: This example requires Chilkat v11.5.0 or greater.

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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