curl -w Option Guide

In everyday web development and operations work, the curl command is an incredibly powerful tool. It can simulate requests from various browsers, perform debugging and testing. Not only can curl initiate HTTP requests, but it can also provide very detailed response information, such as HTTP status codes, request time, and more. By using the -w option, curl can output detailed statistics about the request, helping developers monitor and optimize web application performance.

This article will detail how to use the curl -w option to get HTTP request status codes, request time, response content type, and other performance metrics.

Get HTTP Status Code

To get the HTTP status code of a request, curl provides a very simple method. Use the -w option with the %{http_code} variable to output the HTTP status code of the response.

Example:

curl -s -w '%{http_code}' https://www.avayuan.com -o /dev/null

• -s: Silent mode, curl will not output the progress bar or other extra information.
• -w '%{http_code}': Specifies the output for the HTTP status code.
• -o /dev/null: Discards the response body, only the status code is shown.

Output:

200

A status code of 200 means the request was successful.

Get Total Request Time

If you need to get the total time of an HTTP request, you can use the %{time_total} variable. This value represents the total time from making the request to receiving the response, in seconds.

Example:

curl -s -w '%{time_total}' https://www.avayuan.com -o /dev/null

Output:

0.480300

This indicates that the total time from making the request to receiving the response was 0.4803 seconds.

Get Detailed Time Consumption

In addition to total time, curl provides multiple time-related metrics to help us more precisely analyze each phase of the request’s performance. Common time consumption metrics include:

• time_namelookup: Time spent on domain name lookup.
• time_connect: Time spent establishing the TCP connection (i.e., the time for the three-way handshake).
• time_redirect: Time spent handling redirects.
• time_pretransfer: Time from the start of the request until the response starts transferring.
• time_starttransfer: Time from the start of the request until the first byte is ready to transfer.

Example:

curl -s -w 'Lookup Time: %{time_namelookup}s\nConnect Time: %{time_connect}s\nRedirect Time: %{time_redirect}s\nPretransfer Time: %{time_pretransfer}s\nStart Transfer Time: %{time_starttransfer}s\n' http://www.avayuan.com -o /dev/null

Output:

Lookup Time: 0.008517s
Connect Time: 0.023489s
Redirect Time: 0.000000s
Pretransfer Time: 0.023700s
Start Transfer Time: 0.048536s

With these metrics, you can more precisely analyze the performance of the request and identify bottlenecks, whether they are related to DNS lookup, connection time, redirection, or transfer time.

Get Response Content Type

If you need to check the content type of the response (e.g., whether the returned data is an HTML page, JSON, or something else), you can use %{content_type} to get the Content-Type field from the response headers.

Example:

curl -s -w '%{content_type}' http://www.avayuan.com -o /dev/null

Output:

text/html; charset=UTF-8

This indicates that the returned content is in HTML format and the character encoding is UTF-8.

Output Multiple Metrics

The curl -w option allows you to output multiple performance metrics at once. You can include multiple variables in the format string and separate them with spaces or line breaks.

curl -s -w '%{time_total}s %{http_code}' http://www.avayuan.com -o /dev/null

Output:

0.520615s 200

This command will output both the total request time (in seconds) and the HTTP status code.

Output on Different Lines

If you prefer each metric to be output on different lines, you can use \n to break the lines.

curl -s -w '%{time_total}s\n%{http_code}' http://www.avayuan.com -o /dev/null

Output:

0.460350s
200

Here, we separated the total time and status code onto two lines.

Summary

By using the curl -w option, developers can easily obtain various performance metrics of HTTP requests, including status codes, total request time, time consumption for each stage, and response content type. Mastering these techniques can help you better debug and optimize web applications, improving website response speed and stability.

Commonly Used curl -w Options

Field	Description
%{http_code}	The HTTP status code, which indicates the result of an HTTP request. Different status codes have different meanings; for example, 200 means the request was successful, while 404 means the resource was not found.
%{time_total}	Total time (in seconds) taken from the moment the HTTP request is made until the full response is received. This value provides a comprehensive reflection of the overall request process duration.
%{time_namelookup}	DNS lookup time (in seconds), the time taken to resolve the domain name to an IP address. If the DNS resolution is slow, it will impact subsequent steps like connection and data transfer.
%{time_connect}	TCP connection time (in seconds), the time it takes to establish a TCP connection, including the time for the three-way handshake.
%{time_redirect}	Redirect time (if applicable), the time spent handling redirects when the request encounters one or more 3xx HTTP status codes.
%{time_pretransfer}	Time (in seconds) from the start of the request until the response begins to transfer. This includes DNS resolution, TCP connection, and initial transfer preparation.
%{time_starttransfer}	Time (in seconds) from the start of the request until the first byte of the response starts to transfer. This metric shows when the server begins to respond.
%{content_type}	The Content-Type of the response (e.g., text/html), indicating the format of the returned data.

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