The Basics of Curl

Curl stands for Client URL
The curl command is used to transfer data using the URL syntax
The curl command in general use returns the contents of the website the url points to.

[!NOTE] Notes

By default curl always does a GET request

It is not necessary to give the protocol to the curl command via curl https://google.com but in this case when it is not given curl defaults to the http protocol

The curl command will also not follow any redirect by default and would just return Redirecting...

[!TIP] Website to test bad SSL certs

badssl.com

Some useful flags and arguments

Basic (without options), returns the contents of the website curl <protocol>://<url>
To get the header info only curl -I <protocol>://<url>
To get both the content and the header info curl -i <protocol>://<url>
To output the content into a file (can also use stdout for this) curl -o <filename> <protocol>://<url> curl -O <protocol>://<url> The one with 'O' uses the file path given in the url to get and store the contents in the file with the same name and path.
To make curl follow the redirect curl -L <protocol>://<url>
To get a verbose output curl -v <protocol>://<url>
For websites with bad SSL certs the curl command will by default show an error, but we can still view the content by passing in the -k flag curl -k <protocol>://<url>
Globbing in curl curl https://dummy.restapiexample.com/api/v1/employee/[1-5] Curl can glob using basic regex and this above command will search for all five employees with id 1, 2, 3, 4 & 5
To give a POST request curl -d "name=Test&salary=10000&age=28" https://dummy.restapiexample.com/api/v1/create In this command the '-d' option means data and by default if you don't specify any method it does a POST request and the header is form content and data is url form data
To modify the header (-H) curl -d '{"name":"Test","salary":"10000","age":"29"}' -H "Content-type: application/json" https://dummy.restapiexample.com/api/v1/create In this case the data becomes a json data instead of url form data
To modify the method curl -X <http_method> <protocol>://<url>
Fake the host header curl --header "Host: example.com" http:/127.0.0.1/ This will also make the cookies for example.com work
Resolve curl --resolve example.com:443:127.0.0.1 https://example.com This under the hood populates the curl's DNS cache with the specified custom entry
Connect to another host by name (especially useful when working with load balancers) curl --connect-to example.com:443:host-47.example.com:443 https://example.com/

Although not very practical it is also possible to send an email using the curl command using the smtp protocol.

Although it might seem as a simple command curl hides tons of functionality inside it.

References and Further Reading

Bet you don't know this about cURL

The Basics of Curl

Some useful flags and arguments

References and Further Reading

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