cURL (Client URL) Command & Library

cURL (Client URL) is an open-source command-line tool and cross-platform C library (libcurl) created by Daniel Stenberg in 1997. It is designed to transfer data across diverse network protocols—including HTTP, HTTPS, FTP, FTPS, SFTP, SCP, and WebSocket. Because of its ubiquity across Linux, macOS, and Windows operating systems, cURL is the undisputed universal standard for testing REST APIs, downloading web assets, troubleshooting network routing, and sharing reproducible API reproduction snippets.

Convert complex cURL commands to native Fetch, Axios, Python Requests, Go, or Rust code using our cURL Command Converter.

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Technical Specifications at a Glance

Specification	Details
Creator & Year	Daniel Stenberg (1997)
Underlying Library	libcurl (Bindings for 50+ languages)
Supported Protocols	HTTP, HTTPS, FTP, FTPS, IMAP, POP3, SMTP, SFTP, SCP, TELNET, DICT, WS
HTTP Standards	HTTP/1.0, HTTP/1.1, HTTP/2, HTTP/3 (QUIC)
Default HTTP Method	GET (Switches to POST when -d or --data is provided)
Configuration File	~/.curlrc

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Essential cURL Flags & Syntax Cheat Sheet

Flag	Long Option	Description	Example Usage
-X	--request	Specifies custom HTTP method (GET, POST, PUT, DELETE).	curl -X DELETE https://api.example.com/item/1
-H	--header	Passes extra HTTP request headers.	curl -H "Authorization: Bearer $TOKEN" ...
-d	--data	Sends HTTP POST data (triggers application/x-www-form-urlencoded).	curl -d "user=alice" https://api.example.com/login
--json	--json	Sends JSON payload and automatically sets Content-Type: application/json.	curl --json '{"name":"Alice"}' https://api.example.com
-i	--include	Prints response HTTP headers alongside response body.	curl -i https://example.com
-I	--head	Performs a HEAD request (headers only, no body).	curl -I https://example.com
-L	--location	Follows HTTP 3xx redirects automatically.	curl -L https://short.url
-k	--insecure	Bypasses SSL/TLS certificate verification (debugging only).	curl -k https://localhost:8443
-s	--silent	Disables progress meters and error messages.	curl -s https://api.ipify.org

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Practical cURL Recipes for Developers

1. Sending a JSON POST Request with Authentication

curl -X POST https://api.example.com/v1/projects \
  -H "Authorization: Bearer eyJhbGciOiJIUzI1Ni..." \
  -H "Content-Type: application/json" \
  -d '{
    "name": "DevFlow Tools",
    "isPublic": true,
    "tags": ["developer", "productivity"]
  }'

2. Inspecting Response Time Latency

curl -w "DNS: %{time_namelookup}s\nConnect: %{time_connect}s\nTLS: %{time_appconnect}s\nTotal: %{time_total}s\n" \
  -o /dev/null -s https://api.example.com/health

3. Downloading Large Files with Progress and Resume

# -C - resumes broken downloads, -O preserves remote file name
curl -C - -O https://releases.example.com/large-database-dump.sql.gz

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Code Examples: Translating cURL to Native Code

Modern JavaScript (Fetch API)

// Translated from: curl -X POST https://api.devflow.tools/webhook -H "Content-Type: application/json" -d '{"event":"ping"}'
async function sendWebhookPing() {
  const response = await fetch('https://api.devflow.tools/webhook', {
    method: 'POST',
    headers: {
      'Content-Type': 'application/json',
    },
    body: JSON.stringify({ event: 'ping' }),
  });

  if (!response.ok) {
    throw new Error(`HTTP error! status: ${response.status}`);
  }

  return response.json();
}

Python (using requests)

import requests

url = "https://api.devflow.tools/webhook"
headers = {"Content-Type": "application/json"}
payload = {"event": "ping"}

response = requests.post(url, json=payload, headers=headers, timeout=10)
print(f"Status Code: {response.status_code}")
print(response.json())

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Common Pitfalls & Security Notes

1. Quoting JSON Payloads in Shell: In Unix shells (bash/zsh), payload strings enclosed in double quotes will expand variables like $token unexpectedly, while Windows CMD requires escaping inner quotes as \". Always use single quotes around JSON payloads in bash (-d '{"key":"value"}').
2. Never Ignore Certificates in Production: Using -k or --insecure exposes connections to Man-in-the-Middle (MitM) eavesdropping. If debugging self-signed internal certificates, pass --cacert /path/to/rootCA.crt instead.
3. Redacting Sensitive Tokens from Bash History: Running curl -H "Authorization: Bearer <secret>" logs credentials into your ~/.bash_history file. Use environment variables or --config flag to pass headers discreetly.

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Frequently Asked Questions

What is the difference between cURL and Wget?

While both are command-line file retrieval utilities, curl supports bidirectional communication across over 25 protocols with fine-grained HTTP header and payload control. In contrast, wget is optimized for recursive directory mirroring, batch file downloading, and resuming transfers over unreliable links.

How do I debug SSL handshake or TLS certificate issues with cURL?

Add the verbose flag -v (or --trace-ascii debug.log) to display the complete TLS negotiation, cipher suite agreements, ALPN protocol choices, and remote certificate subject validation details.

How can I turn a cURL snippet into code for my project?

Rather than manually transcribing query parameters, multipart form data, and authentication headers, paste your cURL string into our cURL to Fetch & Axios Converter to generate ready-to-use code across 10+ languages.