How to capture response metrics with cURL write-out

Capturing response metrics with curl using write-out exposes HTTP status codes, timing breakdowns, and transfer sizes for each HTTP request, which simplifies analysis of slow endpoints and validation of API behavior under different conditions. Compact metric summaries make it easier to compare responses over time or across environments without storing full response bodies.

The curl –write-out option expands placeholders such as {http_code}, {time_total}, and {size_download} after the transfer completes, printing a formatted summary to standard output. Combining –write-out with –output for the response body and –silent for progress suppression keeps metrics separate from content while still surfacing errors with –show-error.

On Ubuntu and other Linux distributions, write-out variables behave consistently across HTTP and HTTPS, but certain placeholders remain empty for protocols that do not expose specific timing or size values. Logging formats must account for shell quoting, avoid including sensitive data such as tokens in URLs or headers, and rotate metric logs so repeated requests do not exhaust disk space.

1. Define a write-out format file that records key response metrics in a stable, parseable layout.

   $ cat > curl-format.txt <<'EOF'
   http_code=%{http_code}
   remote_ip=%{remote_ip}
   remote_port=%{remote_port}
   scheme=%{scheme}
   time_namelookup=%{time_namelookup}
   time_connect=%{time_connect}
   time_appconnect=%{time_appconnect}
   time_pretransfer=%{time_pretransfer}
   time_starttransfer=%{time_starttransfer}
   time_total=%{time_total}
   size_download=%{size_download}
   speed_download=%{speed_download}
   EOF

   Grouping write-out variables in a dedicated file keeps metrics consistent across commands and avoids complex shell escaping of {variable} placeholders.

2. Send an HTTPS request using curl with the write-out format file and capture the response body separately.

   $ curl --silent --show-error --output response.html --write-out @curl-format.txt https://example.org/
   http_code=200
   remote_ip=93.184.216.34
   remote_port=443
   scheme=https
   time_namelookup=0.004321
   time_connect=0.015842
   time_appconnect=0.078901
   time_pretransfer=0.079456
   time_starttransfer=0.123456
   time_total=0.234567
   size_download=1256
   speed_download=5357.000000

   The –silent option suppresses the progress bar, –show-error keeps error messages visible, and –output response.html stores the body separately from the metrics.

3. Append write-out metrics to a log file so every request adds a new set of timing and size values.

   $ curl --silent --show-error --output /dev/null --write-out @curl-format.txt https://example.org/ >> curl-metrics.log

   Metric logs recorded during frequent or automated requests can grow rapidly and may reveal sensitive paths or query strings, so unbounded logging on shared systems risks information leakage and disk exhaustion.

4. Capture metrics for multiple requests in a single run using a simple loop that repeats the curl call.

   $ for i in $(seq 1 3); do curl --silent --show-error --output /dev/null --write-out @curl-format.txt https://example.org/ >> curl-metrics.log; echo >> curl-metrics.log; done

   Appending an empty line after each run separates metric blocks, which simplifies parsing and visual inspection in tools such as awk or grep.

5. Inspect the metrics log and confirm that each request adds a new block of values with the expected status codes and timings.

   $ cat curl-metrics.log
   http_code=200
   remote_ip=93.184.216.34
   remote_port=443
   scheme=https
   time_namelookup=0.004321
   time_connect=0.015842
   time_appconnect=0.078901
   time_pretransfer=0.079456
   time_starttransfer=0.123456
   time_total=0.234567
   size_download=1256
   speed_download=5357.000000
   
   http_code=200
   remote_ip=93.184.216.34
   remote_port=443
   scheme=https
   time_namelookup=0.003987
   time_connect=0.014201
   time_appconnect=0.077532
   time_pretransfer=0.078044
   time_starttransfer=0.120001
   time_total=0.230012
   size_download=1256
   speed_download=5461.000000

   Stable values for http_code and reasonable ranges for time_total and speed_download indicate successful requests and usable timing data for trend analysis.