TopolearnQuick latency breakdowns and status checks from the shell are invaluable when debugging slow or flaky services.
-w / --write-out prints a format string after the transfer, with %{variable} placeholders filled in. Combine it with -o /dev/null -s to print only what you ask for.
Common variables:
%{http_code}the response status,%{url_effective}the final URL after redirects,%{num_redirects}.%{size_download},%{speed_download},%{content_type},%{http_version},%{remote_ip}.- Timings in seconds, each measured from the start of the request:
%{time_namelookup}(DNS),%{time_connect}(TCP),%{time_appconnect}(TLS done),%{time_starttransfer}(first byte, often called TTFB),%{time_total}.
Because timings are cumulative, subtract neighbours to get each phase: TLS time is time_appconnect - time_connect; server think time is roughly time_starttransfer - time_appconnect.
Other handy forms: -w '%{json}' prints every variable as one JSON object (pipe it into jq), %{header_json} gives the response headers as JSON, %header{content-type} a single header, -w @format.txt reads the format from a file, and %{stderr} switches the rest of the output to stderr. \n in the format becomes a newline.
A status check one-liner: code=$(curl -s -o /dev/null -w '%{http_code}' "$URL"), then branch on $code.
Example
curl -s -o /dev/null -w '%{http_code}\n' https://example.com # 200
curl -sL -o /dev/null -w '%{url_effective} after %{num_redirects}\n' http://github.com
curl -s -o /dev/null -w 'dns=%{time_namelookup} tcp=%{time_connect} tls=%{time_appconnect} ttfb=%{time_starttransfer} total=%{time_total}\n' https://example.com
# dns=0.012 tcp=0.031 tls=0.068 ttfb=0.142 total=0.143
curl -s -o /dev/null -w '%{json}' https://example.com | jq '{http_code, time_total, http_version}'
code=$(curl -s -o /dev/null -w '%{http_code}' https://api.example.com/health)
[ "$code" = 200 ] || echo "unhealthy: $code"Remember
- -s -o /dev/null -w '...' prints only the metrics you want
- Timings are cumulative from the start; subtract to get phases
- -w '%{json}' dumps every variable as JSON
Watch out for
- Reading %{time_starttransfer} as server time alone; it includes DNS, TCP and TLS.