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To request a JSON representation with cURL, send an HTTP request and ask for JSON with an Accept header:
curl -sS -H 'Accept: application/json' 'https://api.example.com/resource'The endpoint still decides what it returns. Its documentation defines the URL, authentication, query parameters, content-negotiation behavior, and response schema. For a JSON request body, use --json on curl 7.82.0 or newer, or the explicit Content-Type and --data-binary form on older versions.
Get JSON from an API with cURL
A GET request normally retrieves an API representation. The following command suppresses the progress meter while preserving diagnostics for failures:
curl -sS -H 'Accept: application/json' 'https://api.example.com/resource'-sSor--silent --show-errorhides normal transfer progress but still prints errors.-Hor--headeradds theAccept: application/jsonrequest header.- The quoted URL prevents the shell from interpreting characters such as
&and?.
An Accept header is a request, not a guarantee. Some APIs always return JSON; others select a representation only when the header is supported; an endpoint may return HTML for an error or redirect. Check the API documentation and the response headers before assuming the body is JSON.
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Add authentication and query parameters
Use the authentication method documented by the API. A bearer-token example is:
curl -sS
-H 'Accept: application/json'
-H "Authorization: Bearer $API_TOKEN"
--get 'https://api.example.com/resource'
--data-urlencode 'status=active'
--data-urlencode 'limit=20'--get keeps the parameters in the query string. --data-urlencode safely encodes spaces, ampersands, and other special characters. Do not put secrets directly in shell history when an environment variable or a credential mechanism supported by the API is available.
Format or select fields with jq
cURL writes the response bytes to standard output. jq is optional, but it makes JSON readable and can select values:
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curl -sS -H 'Accept: application/json' 'https://api.example.com/resource' | jq .For an array under data, print each item’s name:
curl -sS 'https://api.example.com/resource' | jq -r '.data[].name'jq . pretty-prints without changing the data model. The -r option emits raw strings without JSON quotes, which is useful in shell pipelines. If the server returns invalid JSON, jq reports a parse error; inspect the unfiltered response and HTTP headers before changing the filter.
POST JSON with modern cURL
curl 7.82.0 introduced --json. It is a shortcut for sending the body with --data-binary, setting Content-Type: application/json, and asking for a JSON response with Accept: application/json:
curl --json '{"name":"Ada","active":true}'
'https://api.example.com/endpoint'The option can be used more than once. It accepts inline data, a file, or standard input:
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curl --json @payload.json 'https://api.example.com/endpoint'
curl --json @- 'https://api.example.com/endpoint' < payload.jsonUse a file for a sizeable or reusable payload. Keep shell quoting in mind: single quotes preserve JSON double quotes on POSIX shells, while PowerShell uses different quoting rules.
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POST with authentication and an explicit method
curl -sS --json @payload.json
-H "Authorization: Bearer $API_TOKEN"
'https://api.example.com/endpoint'Most APIs infer POST when a request body is supplied. If the API requires another method, follow its documentation; do not assume that adding a body alone changes the intended operation.
POST JSON on older cURL versions
Before 7.82.0, write the headers and body options explicitly:
curl -sS -X POST
-H 'Content-Type: application/json'
-H 'Accept: application/json'
--data-binary @payload.json
'https://api.example.com/endpoint'For a small inline body:
curl -sS -X POST
-H 'Content-Type: application/json'
-H 'Accept: application/json'
--data-binary '{"name":"Ada","active":true}'
'https://api.example.com/endpoint'--data-binary sends the bytes as provided, including line endings. Unlike --data, it does not strip carriage returns or newlines. That distinction matters for signatures and exact payloads.
Choose an input method
| Approach | Minimum/version note | Best use | Trade-off |
|---|---|---|---|
--json '…' |
curl 7.82.0 or newer | Short JSON requests with conventional JSON headers | Less explicit control; it does not validate syntax |
--json @file or --json @- |
curl 7.82.0 or newer | Reusable files and piped input | Requires managing the input file or pipeline |
--data-binary @file plus headers |
Works with older versions | Precise byte handling and maximum header control | More verbose |
Pipe to jq |
Separate jq installation | Pretty output or field extraction | Transforms display output; it is not part of the HTTP request |
Check your version with curl --version. The --json shortcut was added in curl 7.82.0, documented by the curl project in 2022.
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cURL does not parse or validate the value passed to --json. The curl documentation explicitly warns that there is no verification that the supplied data is actual JSON or syntactically correct. A malformed body is transmitted and the server may return a parsing error.
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When jq is available, validate a file without making a request:
jq empty payload.jsonFor generated data, produce JSON with a serializer rather than concatenating strings. This avoids unescaped quotes, newlines, and booleans represented as text.
Python example for a JSON POST
import json
import requests
payload = {"name": "Ada", "active": True}
response = requests.post(
"https://api.example.com/endpoint",
json=payload,
headers={"Authorization": "Bearer " + API_TOKEN},
timeout=30,
)
response.raise_for_status()
print(response.json())The Python client serializes the object and sets the JSON content type. Keep the API’s authentication and schema requirements unchanged when adapting the example.
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const payload = { name: 'Ada', active: true };
const res = await fetch('https://api.example.com/endpoint', {
method: 'POST',
headers: {
'Content-Type': 'application/json',
'Accept': 'application/json',
'Authorization': `Bearer ${process.env.API_TOKEN}`
},
body: JSON.stringify(payload)
});
if (!res.ok) throw new Error(`HTTP ${res.status}`);
console.log(await res.json());Inspect status codes, headers, and error bodies
When a command fails, diagnose the HTTP exchange before changing the JSON:
curl -i -sS -H 'Accept: application/json' 'https://api.example.com/resource'
curl -D headers.txt -sS 'https://api.example.com/resource'
curl -v 'https://api.example.com/resource'-ior--includeprints response headers before the body.-D headers.txtor--dump-headersaves headers separately.-vor--verboseshows connection, request, and response diagnostics; avoid sharing its output when it contains credentials.
Look for the HTTP status, Content-Type, redirects, authentication challenges, and the server’s error JSON. A 401 or 403 points to credentials or permissions, a 404 to the URL or resource, a 415 to the request content type, and a 422 or 400 commonly indicates a schema or validation problem. These meanings are conventions; the API’s documentation and error body are authoritative.
Common problems and fixes
The output is HTML instead of JSON
Confirm the URL, authentication, and redirect behavior. The response may be a login page, proxy error, or an API error rendered as HTML. Use -i and inspect Content-Type. Add Accept: application/json only when the API documents it; the header cannot make a non-JSON endpoint become JSON.
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“Unknown option: –json”
Your curl predates 7.82.0. Use the explicit Content-Type, Accept, and --data-binary command, or upgrade curl through your operating system’s supported package channel.
HTTP 415 Unsupported Media Type
The server did not recognize the request body format. For a JSON POST, send Content-Type: application/json. With older curl, include it manually; with --json, curl supplies it.
HTTP 400, 422, or a JSON parse error
Validate the payload with jq empty, compare property names and types with the API schema, and ensure the endpoint expects the method you used. Remember that --json sends bytes but does not check their syntax.
jq says “parse error”
First save the unfiltered response, then inspect it with curl -i. A gateway error, HTML page, truncated transfer, or an API that returned plain text can all produce this message. Fix the HTTP response or endpoint before changing the jq expression.
Query parameters are missing or corrupted
Quote the URL and use --get --data-urlencode for query values. An unquoted ampersand can make the shell start a background job, and unescaped spaces can split one argument into several.
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Browser cookies, redirects, CSRF tokens, and interactive login state are not automatically present in cURL. Use the API’s documented token, API key, cookie, or OAuth flow. Never paste a secret into a public script or support log.
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Reliability, performance, and scripting practices
- Set a timeout appropriate to the API, such as
--max-time 30, so a hung connection does not block a job forever. - Use
--fail-with-bodywhere supported when scripts should treat HTTP errors as failures while retaining the response body for diagnosis. - Write machine-readable output to a file with
-o response.jsonand process it only after checking the command’s exit status. - Do not blindly retry POST requests. A timeout can occur after the server accepted the request; retry only when the API documents idempotency keys or safe retry behavior.
- For repeated calls, respect rate limits and backoff instructions from response headers or API documentation.
- Keep the response’s encoding and exact bytes when signatures, hashes, or downloads matter; use
--data-binaryfor request bodies and avoid unnecessary formatting steps.
What JSON means here
JSON is a text interchange format defined for interoperable data. RFC 8259, published by the RFC Editor and IETF in December 2017, describes its syntax and interoperability considerations. cURL transports the bytes; it does not enforce the full JSON specification or the API’s application-level schema. The server remains responsible for validating required fields, types, and business rules.
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Frequently Asked Questions
Does cURL automatically return JSON?
No. cURL returns whatever bytes the endpoint sends. Use the documented URL and request headers, then verify the response’s status and Content-Type.
Is jq required to use JSON with cURL?
No. jq only formats or queries the response after cURL receives it.
Can I send a JSON file from standard input?
Yes. On curl 7.82.0 or newer, use –json @- and redirect or pipe the JSON into cURL.
Why did my POST succeed but produce no visible output?
The server may return an empty body, a 204 status, or output that was redirected with -o. Inspect the status and headers with -i or -D.
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