curl

The response body goes to standard output. -s (silent) hides the progress meter, which curl draws on standard error whenever its output goes to a pipe or a file, as it does here; at an interactive terminal plain curl URL prints just the body.

-o NAME chooses the file name; -O reuses the last part of the URL (report.csv). When saving to a file curl shows its progress meter, as the second command does: total size, percent received and average speed.

/old-page answers 301 with a Location header and an empty body, and plain curl stops there. With -L it requests the new location and prints the real page.

-I sends a HEAD request, so the server returns the status line and headers but no body: a quick way to check a status, content type, size or caching headers.

-i prints the response headers, a blank line, then the body. Unlike -I, this is a normal GET.

Without -f, a 404 is still a successful transfer: curl prints the error body and exits 0. With -f it prints nothing from the body, reports the status (shown because of -S) and exits 22, which set -e or && can act on.

--fail-with-body (curl 7.76 and later) exits 22 like -f but still prints the response, which often explains what went wrong.

-o /dev/null discards the body and -w prints a template after the transfer. Variables such as %{http_code}, %{size_download} and %{time_total} turn curl into a health check or a timing probe.

-d makes the request a POST with Content-Type: application/x-www-form-urlencoded, joining several -d values with &. The test server echoes back what it received.

--json (curl 7.82 and later) posts the data and sets both Content-Type and Accept to application/json. On older curl, write -H 'Content-Type: application/json' -d '...'. --json @file.json reads the body from a file.

-X sets the method name and each -H adds a header (or replaces one curl would send, like Accept). The echo shows both arrived.

-G turns the -d data into a query string on a GET instead of a POST body, and --data-urlencode encodes the value for you: in the echoed path, spaces became +, > became %3e and % became %25.

Without credentials the server answers 401 with a JSON error. -u user:password sends a Basic Authorization header. Leave out :password and curl prompts for it, which keeps it out of your shell history.

-v traces the exchange on standard error: > lines are what curl sent, < lines what came back. The grep keeps just those, dropping the connection details; a full -v also shows DNS results, connection attempts and TLS details.

Nothing listens on port 9999. curl makes three attempts in all, one second apart, printing an error for each, then gives up with exit status 7 ("failed to connect"). Plain --retry only retries transient errors such as timeouts and 5xx answers; --retry-connrefused adds refused connections.