What Does curl Do?

Definition

curl transfers data from or to a URL using many protocols (HTTPS, HTTP, SFTP, LDAP, etc.). It is built on libcurl, so flags you learn in scripts match what languages embed for HTTP clients. Unlike a browser, curl does not execute JavaScript or render pages—it speaks the wire protocol directly.

Everyday HTTP patterns

Goal	Example shape	Notes
Headers only	`curl -I URL`	HEAD request—great for cache/TTL checks
Follow redirects	`curl -L URL`	301/302 chains to final body
POST JSON	`curl -d '{"a":1}' -H 'Content-Type: application/json' URL`	Quote carefully in shells
Custom Host/SNI	`curl --resolve example.com:443:127.0.0.1 https://example.com`	Local vhost testing
Timing trace	`curl -w '%{time_connect} %{time_total}\n' -o /dev/null -s URL`	DNS/TCP/TLS phases in variables

TLS and trust

curl validates certificates against the system trust store. -k / --insecure disables verification—acceptable only for quick lab tests, never for production secrets. Use --cacert for private CAs.

curl vs wget

curl excels at verbs, headers, and APIs; wget historically shines at recursive static mirroring. Many tasks can be done with either—pick based on flags you remember and script clarity.

Frequently Asked Questions

Q.Is curl only for downloading files?

No. curl sends and receives arbitrary HTTP bodies—JSON APIs, OAuth token exchanges, uploads with multipart forms—downloads are just one use case.

Q.What does curl -I do?

It sends a HEAD request and prints response headers only—useful to inspect status codes, cache headers, and redirects without downloading the body.

Q.Why do I see certificate verify failed?

The server chain is incomplete, uses a private CA, or SNI/hostname mismatches. Fix trust material or hostname; avoid -k except in controlled tests.

Q.How do I send a Bearer token?

Add a header: curl -H "Authorization: Bearer TOKEN" https://api.example.com/resource—never commit tokens to shell history; use env vars or secret stores.

Q.Does curl follow redirects by default?

No—add -L. Without it, 301/302 responses print the redirect body or nothing depending on server.

Q.How do I limit time spent waiting?

Use --connect-timeout and --max-time to cap connect and total duration—critical in scripts hitting flaky endpoints.

Q.Can curl use HTTP/2 or HTTP/3?

Modern curl builds support HTTP/2 with --http2 where negotiated; HTTP/3 requires appropriate curl/OpenSSL or QUIC stacks—check curl -V on your machine.

Q.Is curl the same as libcurl in my app?

They share the same engine—options map closely between CLI flags and the library API, which is why curl examples translate well into language bindings.