cURL is a powerful command-line tool for API testing on Linux, pre-installed on most distributions, enabling direct interaction with web services. 2. The basic syntax is curl [options] [URL], with common options including -X for HTTP methods, -H for headers, -d for request body data, -i to include response headers, and -v for verbose debugging output. 3. Different HTTP methods can be tested: GET for retrieving data, POST for creating data by sending JSON with -d and setting Content-Type header, PUT for updating resources, and DELETE for removing them. 4. Authentication can be handled via API keys using -H "x-api-key: your-key" or Bearer tokens with -H "Authorization: Bearer token", where tokens should be stored in environment variables for security. 5. Response inspection is enhanced with -i to view headers, -v for full request/response details including SSL handshake, and -I to fetch only headers (for GET requests). 6. Responses can be saved to files using redirection (e.g., > output.json) or processed with tools like jq for formatted JSON output, requiring -s to suppress progress info. 7. For HTTPS with self-signed certificates, use -k to skip SSL verification in testing, or preferably --cacert to specify a trusted certificate. 8. Best practices include quoting JSON in -d to prevent shell issues, using environment variables for secrets, combining cURL with grep or jq for automated checks, and writing shell scripts to validate API behavior such as checking health endpoints, ensuring robust and repeatable API testing workflows.
Using cURL
for API testing on Linux is a straightforward and powerful way to interact with web services directly from the command line. Since cURL
is pre-installed on most Linux distributions, it’s a go-to tool for developers and testers to verify API behavior, debug requests, and automate basic checks.

Here’s how to effectively use cURL
for API testing:
1. Understanding Basic cURL Syntax
The general syntax for making an API request with cURL
is:

curl [options] [URL]
For API testing, you’ll commonly use options like:
-X
to specify the HTTP method (GET, POST, PUT, DELETE, etc.)-H
to add headers (likeContent-Type
orAuthorization
)-d
to send data in the request body-i
to include response headers-v
for verbose output (helpful for debugging)
Example – Simple GET request:

curl -X GET https://jsonplaceholder.typicode.com/posts/1
This fetches a sample post from a free REST API.
2. Testing Different HTTP Methods
Most APIs support CRUD operations via standard HTTP methods. Here’s how to test each one.
GET Request (Retrieve Data)
curl -X GET \ -H "Accept: application/json" \ https://api.example.com/users
Tip:
-H "Accept: application/json"
tells the server you expect JSON in return.
POST Request (Create Data)
curl -X POST \ -H "Content-Type: application/json" \ -d '{"name": "John", "email": "john@example.com"}' \ https://api.example.com/users
-H "Content-Type: application/json"
ensures the server knows you're sending JSON.-d
contains the request body. Use single quotes around the JSON to avoid shell interpretation issues.
PUT Request (Update Data)
curl -X PUT \ -H "Content-Type: application/json" \ -d '{"name": "Jane", "email": "jane@example.com"}' \ https://api.example.com/users/1
Updates the user with ID 1.
DELETE Request (Remove Data)
curl -X DELETE https://api.example.com/users/1
No data needed—just sends a delete signal.
3. Handling Authentication
Many APIs require authentication. Common methods include API keys and Bearer tokens.
API Key in Header
curl -X GET \ -H "x-api-key: your-api-key-here" \ https://api.example.com/data
Bearer Token (OAuth/JWT)
curl -X GET \ -H "Authorization: Bearer your-jwt-token" \ https://api.example.com/protected
Keep tokens secure—avoid hardcoding them in scripts. Use environment variables instead:
export TOKEN="your-token" curl -H "Authorization: Bearer $TOKEN" https://api.example.com/secure
4. Viewing and Debugging Responses
Use these options to inspect what’s really happening.
Show response headers:
curl -i -X GET https://api.example.com/users
The
-i
flag includes HTTP headers in output—useful to check status codes, content type, rate limits, etc.Verbose mode (for debugging):
curl -v -X GET https://api.example.com/users
Shows full request and response handshake, including SSL negotiation.
Only show response headers (no body):
curl -I -X GET https://api.example.com
Note: This only works for GET requests and shows the
HEAD
response.
5. Saving and Processing Responses
You can redirect output to a file or pipe it to tools like jq
for JSON parsing.
Save response to a file:
curl -X GET https://api.example.com/users > users.json
Pretty-print JSON with jq
:
curl -s https://api.example.com/users | jq
-s
silences progress output so only JSON is passed tojq
.
6. Handling HTTPS and SSL (When Needed)
By default, cURL
verifies SSL certificates. If you're testing on a local server with a self-signed cert, you might get an error.
To skip SSL verification (only for testing!):
curl -k -X GET https://self-signed.example.com
Better alternative: Use --cacert
to specify a custom certificate:
curl --cacert /path/to/cert.pem https://api.example.com
Final Tips
- Always quote JSON in
-d
to prevent shell errors. - Use environment variables for secrets.
- Combine
cURL
withgrep
,jq
, orjsonlint
for automated checks. - Write shell scripts to automate test sequences:
#!/bin/bash RESPONSE=$(curl -s -X GET https://api.example.com/health) echo "$RESPONSE" | grep -q "status\":\"ok" && echo "API is up" || echo "API is down"
Basically,
cURL
gives you full control over HTTP requests and is perfect for quick API validation, debugging, or integration testing in Linux environments. With a few well-crafted commands, you can simulate almost any client behavior.The above is the detailed content of How to Use `cURL` for API Testing on Linux. For more information, please follow other related articles on the PHP Chinese website!

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