curl

curl

curl is a command-line tool and library for transferring data with URLs, supporting HTTP, HTTPS, FTP, SCP, SFTP, and dozens of other protocols on Linux, macOS, and Windows.

What curl Does and When to Use It

curl sends requests to URLs and displays the response. System administrators use curl to test HTTP endpoints, debug API responses, download files, inspect SSL certificates, and verify connectivity to remote services. curl supports over 25 protocols, including HTTP/1.1, HTTP/2, HTTP/3, FTP, SFTP, SCP, LDAP, SMTP, and IMAP.

curl differs from wget in scope and design. wget specializes in recursive downloads and mirroring websites. curl focuses on single-request transfers with fine-grained control over headers, authentication, cookies, and TLS settings. curl outputs to stdout by default; wget saves to a file by default.

curl ships pre-installed on macOS and most Linux distributions. For official documentation, see curl.se/docs.

How to Install curl

=== "Ubuntu / Debian"

sudo apt install curl

=== "RHEL / CentOS / Fedora"

sudo dnf install curl

=== "macOS"

curl ships with macOS. Update via Homebrew:

brew install curl

Verify the installed version:

curl --version

Core Concepts of curl

curl Request Methods

curl defaults to GET for HTTP requests. Use -X (or --request) to specify a different HTTP method. Common methods: GET (retrieve data), POST (submit data), PUT (replace data), PATCH (partial update), DELETE (remove data). For POST requests, use -d to send form data or --json to send JSON.

curl Headers and Output

curl sends and receives HTTP headers alongside the body. Use -H to add custom request headers. Use -I (or --head) to fetch only response headers. Use -v (verbose) to display the full request/response exchange including TLS handshake details.

curl Authentication

curl supports multiple authentication schemes: basic auth ( -u user:pass), bearer tokens ( -H "Authorization: Bearer TOKEN"), client certificates ( --cert), and OAuth. Use --netrc to read credentials from ~/.netrc instead of passing them on the command line.

curl and TLS/SSL

curl verifies SSL certificates against the system's CA bundle by default. Use --cacert to specify a custom CA certificate. Use -k (or --insecure) to skip certificate verification — only for debugging, never in production. Use --cert and --key for mutual TLS (client certificate authentication).

Common Tasks with curl

How to Make a GET Request with curl

curl sends a GET request and prints the response body to stdout:

curl https://api.example.com/users

How to Send a POST Request with JSON Data Using curl

curl sends a POST request with a JSON payload using the --json flag (curl 7.82+) or -d with a Content-Type header:

curl --json '{"name":"Alice","email":"alice@example.com"}' https://api.example.com/users

Equivalent command for older curl versions:

curl -X POST -H "Content-Type: application/json" -d '{"name":"Alice"}' https://api.example.com/users

How to View HTTP Response Headers with curl

curl displays only the response headers with the -I flag. This sends a HEAD request:

curl -I https://example.com

Display both headers and body with -i:

curl -i https://example.com

How to Debug an HTTP Connection with curl Verbose Mode

curl's -v flag shows the full connection lifecycle — DNS resolution, TCP handshake, TLS negotiation, request headers sent, and response headers received:

curl -v https://example.com

How to Download a File with curl

curl saves the response body to a file with -o (custom filename) or -O (original filename from URL):

curl -O https://example.com/archive.tar.gz
curl -o myfile.tar.gz https://example.com/archive.tar.gz

Resume an interrupted download with -C -:

curl -C - -O https://example.com/largefile.iso

How to Check an SSL Certificate with curl

curl displays SSL certificate details during a verbose HTTPS request. Combine -v with --head to inspect the certificate without downloading the full response:

curl -vI https://example.com 2>&1 | grep -A 6 "Server certificate"

curl Troubleshooting

Related Tools and Guides

wget is an alternative to curl for downloading files, with recursive download support. See the comparison: curl vs wget: when to use which.

OpenSSL provides lower-level SSL/TLS inspection and certificate management. See the OpenSSL article.