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HTTP Client with curl

curl is a very helpful and useful tool when you have to deal with HTTP calls.

Since it may not be available in your distro, make sure that you install it:

$ sudo apt-get install curl

Making GET requests

The simplest thing you can do with curl is to make a GET request to a URL

$ curl https://hackbulgaria.com/

<some html here>

If you want to see the response headers, you can call curl with -I:

$ curl -I https://hackbulgaria.com/
HTTP/1.1 200 OK
Server: nginx/1.8.0
Date: Wed, 10 Feb 2016 12:03:57 GMT
Content-Type: text/html; charset=utf-8
Connection: keep-alive
X-Frame-Options: SAMEORIGIN
Content-Language: bg
Vary: Accept-Language, Cookie

This is very helpful in order to debug things.

If you want to see request headers, you can do the following things:

  1. Start a netcat server at some port: $ nc -l 8000
  2. Execute: $ curl https://localhost:8000
  3. On the nc tab you will see the request headers
GET / HTTP/1.1
User-Agent: curl/7.37.1
Host: localhost:8000
Accept: */*

The other way is to execute:

$ curl -vs https://hackbulgaria.com > /dev/null

... some output ...
> GET / HTTP/1.1
> User-Agent: curl/7.37.1
> Host: hackbulgaria.com
> Accept: */*
> 
< HTTP/1.1 200 OK
* Server nginx/1.8.0 is not blacklisted
< Server: nginx/1.8.0
< Date: Wed, 10 Feb 2016 12:06:01 GMT
< Content-Type: text/html; charset=utf-8
< Transfer-Encoding: chunked
< Connection: keep-alive
< X-Frame-Options: SAMEORIGIN
< Content-Language: bg
< Vary: Accept-Language, Cookie
<

-vs means verbose and silent and we redirect the HTML output to /dev/null

Making POST requests

We are going to use our simple Flask HTTP server that is located in materials folder.

In order to run it, you have to follow these two simple steps:

  1. Install requirements: $ pip install -r requirements.txt in a proper virtualenv.
  2. Run: $ python3 simple_http_server.py

In order to make a POST request, we can use the -X flag from the curl command.

$ curl -X POST http://localhost:5000
{
    "get_data": {},
    "post_data": {},
    "method": "POST",
    "headers": {
        "Host": "localhost:5000",
        "Content-Type": "",
        "Content-Length": "",
        "User-Agent": "curl/7.37.1",
        "Accept": "*/*"
    }
}%

If we want to send some POST data, we can use the -d flag:

$ curl -X POST -d "name=rado&age=23" http://localhost:5000
{
    "get_data": {},
    "post_data": {
        "age": "23",
        "name": "rado"
    },
    "method": "POST",
    "headers": {
        "Host": "localhost:5000",
        "Content-Type": "application/x-www-form-urlencoded",
        "Content-Length": "16",
        "User-Agent": "curl/7.37.1",
        "Accept": "*/*"
    }
}%

Also, we can send a GET request with parameters, placed in the URL:

curl http://localhost:5000\?name\=rado\&age\=23
{
    "get_data": {
        "age": "23",
        "name": "rado"
    },
    "post_data": {},
    "method": "GET",
    "headers": {
        "Host": "localhost:5000",
        "Content-Type": "",
        "Content-Length": "",
        "User-Agent": "curl/7.37.1",
        "Accept": "*/*"
    }
}%

If we want to send some custom headers, we can use -H flag:

$ curl -H "Content-Type: application/json" http://localhost:5000\?name\=rado\&age\=23
{
    "get_data": {
        "age": "23",
        "name": "rado"
    },
    "post_data": {},
    "method": "GET",
    "headers": {
        "Host": "localhost:5000",
        "Content-Type": "application/json",
        "Content-Length": "",
        "User-Agent": "curl/7.37.1",
        "Accept": "*/*"
    }
}

You can see a list of valid HTTP headers here

Play with that simple server.