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curl is a command-line tool for transferring data with URLs. It supports HTTP, HTTPS, FTP, and many other protocols. Essential for API testing, web scraping, and general HTTP operations.

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Basic Usage

• curl <url> - GET request, output to stdout

• curl -o file.html <url> - Save to file

• curl -O <url> - Save with remote filename

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HTTP Methods

• curl -X GET <url> - GET request (default)

• curl -X POST <url> - POST request

• curl -X PUT <url> - PUT request

• curl -X DELETE <url> - DELETE request

• curl -X PATCH <url> - PATCH request

• curl -I <url> - HEAD request (headers only)

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Headers

• -H "Header: Value" - Add custom header

• -H "Content-Type: application/json" - Set content type

• -H "Authorization: Bearer token" - Auth header

• -A "User-Agent" - Set User-Agent

• -e "Referer" - Set Referer header

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Data/Body

• -d "data" - Send POST data

• -d @file.json - POST data from file

• --data-urlencode "param=value" - URL encode data

• -F "file=@path/file.txt" - Form upload

• -F "field=value" - Form field

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Output Options

• -o filename - Output to file

• -O - Save with remote filename

• -s - Silent mode (no progress)

• -S - Show errors in silent mode

• -v - Verbose output

• -i - Include response headers

• -w "%{http_code}" - Write out format

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Authentication

• -u user:password - Basic auth

• --digest - Digest auth

• --ntlm - NTLM auth

• --negotiate - Negotiate auth

• -E cert.pem - Client certificate

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Cookies

• -b "name=value" - Send cookie

• -b cookies.txt - Read cookies from file

• -c cookies.txt - Save cookies to file

• -c - - Print cookies to stdout

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SSL/TLS

• -k - Ignore SSL errors

• --cacert file - CA certificate

• --cert file - Client certificate

• --key file - Private key

• --tlsv1.2 - Force TLS 1.2

• --tlsv1.3 - Force TLS 1.3

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Proxy

• -x proxy:port - HTTP proxy

• -x socks5://proxy:port - SOCKS5 proxy

• --proxy-user user:pass - Proxy auth

• --noproxy "localhost" - Bypass proxy

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Redirects & Timeouts

• -L - Follow redirects

• --max-redirs N - Max redirects

• --connect-timeout N - Connection timeout

• -m N - Max time for operation

• --retry N - Retry count

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Common Examples

Simple GET

curl https://api.example.com/data

Basic GET request.

POST JSON

curl -X POST -H "Content-Type: application/json" -d '{"key":"value"}' https://api.example.com/data

POST JSON data.

Download File

curl -O https://example.com/file.zip

Download and save with original name.

With Auth

curl -u admin:password https://api.example.com/admin

Basic authentication.

Headers Only

curl -I https://example.com

Get response headers only.

Verbose Debug

curl -v https://example.com

See full request/response details.

Follow Redirects

curl -L https://example.com/redirect

Follow HTTP redirects.

Upload File

curl -F "file=@/path/to/file.txt" https://example.com/upload

Upload file via form.

Status Code Only

curl -s -o /dev/null -w "%{http_code}" https://example.com

Get only HTTP status code.

Through Proxy

curl -x http://proxy:8080 https://example.com

Route through HTTP proxy.

Save Cookies

curl -c cookies.txt -b cookies.txt https://example.com/login

Manage session cookies.

API Token

curl -H "Authorization: Bearer eyJhbGc..." https://api.example.com/user

Request with bearer token.

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Write-Out Variables

Use with -w option:

• %{http_code} - HTTP status code
• %{time_total} - Total time
• %{time_connect} - Time to connect
• %{size_download} - Downloaded bytes
• %{speed_download} - Download speed
• %{url_effective} - Final URL
• %{redirect_url} - Redirect URL

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Tips

• Use -s for scripts (silent mode)

• Use -v to debug connection issues

• Use -L to handle redirects automatically

• Use -k only for testing (insecure)

• Combine -sS for silent but show errors

• Use @filename to read data from files

• Essential for API testing and debugging

• Supports many protocols beyond HTTP

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