Introduction
In the world of system management and software development, transferring data to or from remote servers is a daily task. One of the most versatile and light tools to achieve this in Linux environments is the curl command. Although its name comes from 'Client URL', its functionality goes far beyond simple file download, allowing to interact with API, test endpoints and automate workflows without the need for graphic interfaces.
What's curl?
Curl is a command line tool and a libcurl library that allows you to transfer data using various protocols such as HTTP, HTTPS, FTPS, SCP, SFTP, IMAP, POP3 and SMTP. Its design focuses on simplicity and power, offering a direct syntax that can be integrated into shell scripts, cron jobs or CI / CD pipelines. Thanks to its open source license, it is available in virtually all Linux distributions.
Installation
In most modern distributions, curl comes pre-installed. If not present, it can be added by the corresponding package manager. In Debian or Ubuntu you run sudo apt update & sudo apt install curl. In Red Hat, CentOS or Fedora, sudo dnf install curl or sudo yum install curl is used according to the version. In Arch Linux the command is sudo pacman -S curl. After installation, simply write curl -version to verify.
Basic syntax
The simplest way to use curl is to provide a URL as an argument. For example, curl https: / / example.com returns the resource content to the standard output. To save the result in a file you add the option -or followed by the file name, such as curl -or archivo.html https: / / example.com. It can also be used -O (capital) to keep the original name of the remote resource.
Examples of use: download files
Suppose we want to get the latest version of a compressed package from a repository. The command curl -L -or latest.tar.gz https: / / example.com / packages / latest.tar.gz follows redirections thanks to -L and saves the file with the name indicated. If we only need to see the server headers, curl -I https: / / example.com shows information such as the status code, the content type and the last modification date.
Data delivery: POST and PUT requests
To interact with REST API it is common to send data to the request body. With curl can be done using -X POST or -X PUT together with -d to specify the data. For example, curl -X POST -d'name = juan & age = 30 'https: / / api.example.com / users sends a coded form as application / x-www-form-urlencoded. If JSON is preferred, add -H'Content-Type: application / json' and pass the object with -d '{name: juan, age: 30}'.
Useful options
Some flags that are very practical in the day-to-day include -v or -verbose to see the full exchange of headers and bodies, -s or -silent to delete the progress meter and error messages, -S or -show-error to show errors even when used -s, -L or -follow-location to follow HTTP redirections, -r or -range to download only a range of bytes useful to resume transfers, and -C to continue a interrupted download.
Authentication
Curl supports several authentication methods. For basic HTTP credentials it is used -u user: password or either -u user and the password is requested interactively. For carrier tokens, common to modern APIs, a header with -H'Authorization: Bearer tu _ token _ here 'is added. In FTP connections you can specify -u for the user and password, and for SFTP it is usually combined with SSH keys by the -key and -cert option if required.
Depuration and verbose
When a request does not behave as expected, the -v option is indispensable. It shows the TLS trading information, the headers sent and received, and the body of the response. To save this record in a file you can combine with -or salid.txt or redirect the standard output to a file. In addition, curl offers -trace-ascii archivo.txt that turns all the exchange into readable format for a deeper analysis.
Conclusion
The curl command is a Swiss knife for anyone working with Linux networks. Its ability to handle multiple protocols, its wealth of options and its ease of use in scripts make it an indispensable tool. Dominating curl not only accelerates data downloading and sending tasks, but also opens the door to sophisticated web services automation and infrastructure monitoring.
