Mastering curl: Essential Command‑Line Options for HTTP Requests

Introduction

curl is a widely used command‑line tool for making HTTP requests; its name means “client URL”. With dozens of options it can replace graphical tools such as Postman when used skillfully.

-A (User‑Agent)

Specifies the User-Agent header. The default string is curl/[version]. Example:

$ curl -A 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/76.0.3809.100 Safari/537.36' https://google.com

To omit the header: $ curl -A '' https://google.com Alternatively, use -H 'User-Agent: …' to set it directly.

-b (Send Cookie)

Sends a Cookie header. Example: $ curl -b 'foo=bar' https://google.com Multiple cookies can be sent with a semicolon‑separated list or by reading a cookie file:

$ curl -b 'foo1=bar;foo2=bar2' https://google.com

$ curl -b cookies.txt https://www.google.com

-c (Save Cookie)

Writes cookies received from the server to a file:

$ curl -c cookies.txt https://www.google.com

-d (POST Data)

Sends data in the request body and automatically sets Content-Type: application/x-www-form-urlencoded. It also switches the request method to POST, so -X POST can be omitted.

$ curl -d 'login=emma&password=123' -X POST https://google.com/login

Multiple -d flags are concatenated, and a file can be used with @filename:

$ curl -d '@data.txt' https://google.com/login

--data-urlencode

Like -d but URL‑encodes the data automatically:

$ curl --data-urlencode 'comment=hello world' https://google.com/login

-e (Referer)

Sets the Referer header:

$ curl -e 'https://google.com?q=example' https://www.example.com

The same effect can be achieved with -H 'Referer: …'.

-F (Form Upload)

Uploads a file using multipart/form-data:

$ curl -F '[email protected]' https://google.com/profile

Optional MIME type and filename can be specified:

$ curl -F '[email protected];type=image/png' https://google.com/profile

$ curl -F '[email protected];filename=me.png' https://google.com/profile

-G (GET with Data)

Appends data as a query string to the URL, turning the request into GET:

$ curl -G -d 'q=kitties' -d 'count=20' https://google.com/search

Combine with --data-urlencode for URL‑encoded parameters.

-H (Custom Header)

Adds arbitrary HTTP headers:

$ curl -H 'Accept-Language: en-US' https://google.com

$ curl -H 'Accept-Language: en-US' -H 'Secret-Message: xyzzy' https://google.com

JSON payloads can be sent by setting Content-Type: application/json and using -d:

$ curl -d '{"login": "emma", "pass": "123"}' -H 'Content-Type: application/json' https://google.com/login

-i (Show Response Headers)

Prints the response headers before the body.

$ curl -i https://www.example.com

-I / --head (HEAD Request)

Sends a HEAD request and displays only the response headers:

$ curl -I https://www.example.com

$ curl --head https://www.example.com

-k (Insecure SSL)

Disables SSL certificate verification:

$ curl -k https://www.example.com

-L (Follow Redirects)

Follows HTTP redirects automatically:

$ curl -L -d 'tweet=hi' https://api.twitter.com/tweet

--limit-rate

Limits transfer speed, useful for simulating slow networks:

$ curl --limit-rate 200k https://google.com

-o (Save to File)

Saves the response body to a specified file (similar to wget):

$ curl -o example.html https://www.example.com

-O (Save with Remote Name)

Saves the response using the remote filename:

$ curl -O https://www.example.com/foo/bar.html

-s (Silent)

Suppresses progress and error messages. Combine with -o /dev/null for completely silent execution:

$ curl -s https://www.example.com

$ curl -s -o /dev/null https://google.com

-S (Show Errors)

Shows only error messages; often used together with -s:

$ curl -S -s -o /dev/null https://google.com

-u (User Authentication)

Provides basic authentication credentials: $ curl -u 'bob:12345' https://google.com/login Credentials can also be embedded in the URL:

$ curl https://bob:[email protected]/login

-v (Verbose)

Outputs the full request/response exchange for debugging:

$ curl -v https://www.example.com

--trace -

prints raw binary data as well:

$ curl --trace - https://www.example.com

-x (Proxy)

Specifies a proxy server. If no scheme is given, HTTP is assumed:

$ curl -x socks5://james:[email protected]:8080 https://www.example.com

$ curl -x james:[email protected]:8080 https://www.example.com

-X (Custom Method)

Overrides the HTTP method used for the request:

$ curl -X POST https://www.example.com