What is curl?

curl is an open-source command-line tool for transferring data with URLs, and libcurl is the library behind it, embedded in a very wide range of operating systems, devices and applications. It is released under an MIT-style license. For API work it is the lowest common denominator: every API documentation site shows curl examples, every platform has it, and a curl command is the most portable way to hand someone a reproducible request. It speaks HTTP/0.9 through HTTP/3 and over two dozen other protocols, and supports authentication schemes from Basic and Digest to Bearer tokens and AWS SigV4.

Why use it in the API Lifecycle?

Reproducible requests: A single command captures method, URL, headers and body, which makes it the standard format for bug reports and documentation examples.
Full HTTP control: Custom methods (-X), arbitrary headers (-H), HTTP version selection (--http1.1, --http2, --http3), redirect following (-L) and ranges (-r).
JSON shortcut: --json (since 7.82.0) posts a body with Content-Type: application/json and Accept: application/json set for you.
Wire-level visibility: -v, --trace-ascii and -D show exactly what was sent and received, including TLS negotiation and response headers.
Scriptable checks: -w '%{response_code}' and other write-out variables expose status, timings and sizes for shell scripts and health checks.
Connection control: --resolve and --connect-to send a request to a specific IP or host while keeping the original Host and TLS name, which is how you test a server behind a load balancer or CDN.

Best Practices

Use --fail-with-body (since 7.76.0) in scripts and CI. Without it curl exits 0 on a 500 Internal Server Error; with plain --fail it exits 22 but discards the body, which is usually the Problem Details document that explains the failure.
Debug with -v or --trace-ascii -, not just the body. Most API bugs are in headers: a missing Content-Type, a Vary that defeats caching, or a redirect that drops Authorization.
Use --retry for transient failures in scripts. It retries timeouts and 408, 429, 500, 502, 503 and 504, and honors Retry-After. Avoid --retry-all-errors on non-idempotent requests; curl’s own documentation warns it can duplicate data. See Retries and Backoff.
Keep tokens out of shell history and process lists. Read headers from a file with -H @headers.txt, or use --variable and --expand-header (since 8.3.0) to pull a value from an environment variable, instead of typing -H "Authorization: Bearer ..." inline.
Test conditional requests with --etag-save and --etag-compare to confirm the server returns 304 Not Modified for an unchanged ETag. See Conditional Requests.
Add --compressed when checking response size or caching. Without it curl does not send Accept-Encoding, so you see the uncompressed path that real clients rarely take.
Time requests with -w variables such as %{time_connect}, %{time_starttransfer} and %{time_total} to separate network and TLS cost from server processing time.
For repeatable suites rather than one-off checks, move to Hurl, which runs on libcurl and adds captures and assertions in a plain-text file.