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Table of Contents
2. Testing Different HTTP Methods
GET Request (Retrieve Data)
POST Request (Create Data)
PUT Request (Update Data)
DELETE Request (Remove Data)
3. Handling Authentication
API Key in Header
Bearer Token (OAuth/JWT)
4. Viewing and Debugging Responses
5. Saving and Processing Responses
Save response to a file:
Pretty-print JSON with jq:
6. Handling HTTPS and SSL (When Needed)
Final Tips
Home System Tutorial LINUX How to Use `cURL` for API Testing on Linux

How to Use `cURL` for API Testing on Linux

Jul 29, 2025 am 01:39 AM
curl api test

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.

How to Use `cURL` for API Testing on Linux

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.

How to Use `cURL` for API Testing on Linux

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:

How to Use `cURL` for API Testing on Linux
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 (like Content-Type or Authorization)
  • -d to send data in the request body
  • -i to include response headers
  • -v for verbose output (helpful for debugging)

Example – Simple GET request:

How to Use `cURL` for API Testing on Linux
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 to jq.

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 with grep, jq, or jsonlint 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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