How to use the command 'curl' (with examples)

Curl is a widely used command-line tool that allows you to transfer data to or from a server. It supports various protocols, including HTTP, FTP, and POP3. Curl is a versatile tool that can be used for a variety of purposes, such as downloading files from a URL, sending form-encoded data, and authenticating with a server.

Use case 1: Download the contents of a URL to a file

Code:

curl http://example.com --output path/to/file

Motivation:

Downloading the contents of a URL to a file can be useful for various tasks, such as saving HTML code for analysis or storing data for future reference. By using curl, you can easily fetch the contents of a URL and save it to a specified file location.

Explanation:

• curl: The command itself
• http://example.com: The URL of the file you want to download
• --output: Specifies the output file
• path/to/file: The path and filename where the downloaded file will be saved

Example output:

The contents of the URL http://example.com will be downloaded and saved to the file specified in the --output parameter.

Use case 2: Download a file, saving the output under the filename indicated by the URL

Code:

curl --remote-name http://example.com/filename

Motivation:

Sometimes, you may want to download a file and preserve the original filename specified by the URL. By using the --remote-name option, curl will save the file with the same name as indicated by the URL.

Explanation:

• curl: The command itself
• --remote-name: Saves the output under the filename indicated by the URL
• http://example.com/filename: The URL of the file you want to download

Example output:

The file located at http://example.com/filename will be downloaded and saved with the same name.

Use case 3: Download a file, following location redirects, and automatically continuing a previous file transfer, returning an error on server error

Code:

curl --fail --remote-name --location --continue-at - http://example.com/filename

Motivation:

This use case is helpful when you want to make sure that the file is downloaded correctly and any errors are reported. By using the combination of --fail, --location, and --continue-at -, curl will follow any location redirects, resume a previous file transfer, and return an error if the server encounters any issues.

Explanation:

• curl: The command itself
• --fail: Returns an error on server error (non-successful HTTP codes)
• --remote-name: Saves the output under the filename indicated by the URL
• --location: Follows any location redirects
• --continue-at -: Automatically continues (resumes) a previous file transfer. The hyphen after continue-at specifies to continue from where it left off
• http://example.com/filename: The URL of the file you want to download

Example output:

If the file transfer encounters any server error, it will return an error. Otherwise, the file will be downloaded, following location redirects and continuing a previous transfer if applicable.

Use case 4: Send form-encoded data (POST request of type application/x-www-form-urlencoded)

Code:

curl --data 'name=bob' http://example.com/form

Motivation:

Sending form-encoded data is useful for submitting data to a server, typically through a form submission. By using the --data option, you can specify the data to be sent in the HTTP request body.

Explanation:

• curl: The command itself
• --data 'name=bob': Specifies the form-encoded data to be sent
• http://example.com/form: The URL where the data should be sent

Example output:

The form-encoded data “name=bob” will be sent to the URL http://example.com/form .

Use case 5: Send a request with an extra header, using a custom HTTP method

Code:

curl --header 'X-My-Header: 123' --request PUT http://example.com

Motivation:

Sometimes, you may need to send a request with additional headers or use a custom HTTP method. By using the --header option, you can include custom headers, and the --request option allows you to specify the HTTP method to be used.

Explanation:

• curl: The command itself
• --header 'X-My-Header: 123': Specifies the extra header to include in the request
• --request PUT: Specifies the HTTP method to be used (in this case, PUT)
• http://example.com: The URL where the request should be sent

Example output:

A request with the specified extra header and HTTP method will be sent to the URL http://example.com .

Use case 6: Send data in JSON format, specifying the appropriate content-type header

Code:

curl --data '{"name":"bob"}' --header 'Content-Type: application/json' http://example.com/users/1234

Motivation:

Sending data in JSON format is common when interacting with APIs that expect JSON payloads. By using the --header option to specify the content-type and the --data option to provide the JSON data, you can easily send JSON payloads to the server.

Explanation:

• curl: The command itself
• --data '{"name":"bob"}': Specifies the JSON data to be sent
• --header 'Content-Type: application/json': Sets the appropriate content-type header
• http://example.com/users/1234: The URL where the request should be sent

Example output:

The JSON data ‘{“name”:“bob”}’ will be sent to the URL http://example.com/users/1234 with the appropriate content-type header.

Use case 7: Pass a username and password for server authentication

Code:

curl --user myusername:mypassword http://example.com

Motivation:

Sometimes, you may need to authenticate with a server using a username and password. By using the --user option, you can provide the credentials required for authentication.

Explanation:

• curl: The command itself
• --user myusername:mypassword: Specifies the username and password for authentication
• http://example.com: The URL where the request should be sent

Example output:

A request will be sent to the URL http://example.com with the provided username and password for authentication.

Use case 8: Pass client certificate and key for a resource, skipping certificate validation

Code:

curl --cert client.pem --key key.pem --insecure https://example.com

Motivation:

In some cases, you may need to pass a client certificate and key for authentication with a server. By using the --cert and --key options, you can provide the required credentials. The --insecure option allows you to skip certificate validation if necessary.

Explanation:

• curl: The command itself
• --cert client.pem: Specifies the path to the client certificate file
• --key key.pem: Specifies the path to the client key file
• --insecure: Skips certificate validation
• https://example.com: The URL where the request should be sent

Example output:

A request will be sent to the URL https://example.com , providing the client certificate and key. Certificate validation will be skipped if the --insecure option is used.

Conclusion:

Curl is a powerful command-line tool that allows you to transfer data to or from a server, supporting various protocols. By familiarizing yourself with the different use cases of curl, you can effectively use it for tasks like downloading files, sending data, and authenticating with servers.