有時候為了測試網絡情況，需要返回每個階段的耗時時間，比如DNS解析耗時，建立連接所消耗的時間，從建立連接到准備傳輸所使用的時間，從建立連接到傳輸開始所使用的時間，整個過程耗時，下載的數據量，下載速度，上傳數據量，上傳速度等等。
使用 cURL 獲取站點的各類響應時間 – dns解析時間,響應時間,傳輸時間
這是一個本人博客站點執行 curl 命令的情況。輸出通常是 HTML 代碼，通過 -o 參數發送到 /dev/null。-s 參數去掉所有狀態信息。-w 參數讓 curl 輸出的計時器的狀態信息。 
一次http請求中的各個時間段-dns解析,等待服務器響應,獲取內容等
下邊對-w參數做個詳細的解釋，由我(DigDeeply)翻譯。有不對的地方請大家指出。(英文原文：http://curl.haxx.se/docs/manpage.html)
以下是可用的變量名：
-w, --write-out
以下變量會按CURL認為合適的格式輸出，輸出變量需要按照%{variable_name}的格式，如果需要輸出%，double一下即可，即%%，同時，\n是換行，\r是回車，\t是TAB。
url_effective The URL that was fetched last. This is most meaningful if you've told curl to follow location: headers.
filename_effective The ultimate filename that curl writes out to. This is only meaningful if curl is told to write to a file with the --remote-name or --output option. It's most useful in combination with the --remote-header-name option. (Added in 7.25.1)
http_code http狀態碼，如200成功,301轉向,404未找到,500服務器錯誤等。(The numerical response code that was found in the last retrieved HTTP(S) or FTP(s) transfer. In 7.18.2 the alias response_code was added to show the same info.)
http_connect The numerical code that was found in the last response (from a proxy) to a curl CONNECT request. (Added in 7.12.4)
time_total 總時間，按秒計。精確到小數點后三位。 （The total time, in seconds, that the full operation lasted. The time will be displayed with millisecond resolution.）
time_namelookup DNS解析時間,從請求開始到DNS解析完畢所用時間。(The time, in seconds, it took from the start until the name resolving was completed.)
time_connect 連接時間,從開始到建立TCP連接完成所用時間,包括前邊DNS解析時間，如果需要單純的得到連接時間，用這個time_connect時間減去前邊time_namelookup時間。以下同理，不再贅述。(The time, in seconds, it took from the start until the TCP connect to the remote host (or proxy) was completed.)
time_appconnect 連接建立完成時間，如SSL/SSH等建立連接或者完成三次握手時間。(The time, in seconds, it took from the start until the SSL/SSH/etc connect/handshake to the remote host was completed. (Added in 7.19.0))
time_pretransfer 從開始到准備傳輸的時間。(The time, in seconds, it took from the start until the file transfer was just about to begin. This includes all pre-transfer commands and negotiations that are specific to the particular protocol(s) involved.)
time_redirect 重定向時間，包括到最后一次傳輸前的幾次重定向的DNS解析，連接，預傳輸，傳輸時間。(The time, in seconds, it took for all redirection steps include name lookup, connect, pretransfer and transfer before the final transaction was started. time_redirect shows the complete execution time for multiple redirections. (Added in 7.12.3))
time_starttransfer 開始傳輸時間。在發出請求之后，Web 服務器返回數據的第一個字節所用的時間(The time, in seconds, it took from the start until the first byte was just about to be transferred. This includes time_pretransfer and also the time the server needed to calculate the result.)
size_download 下載大小。(The total amount of bytes that were downloaded.)
size_upload 上傳大小。(The total amount of bytes that were uploaded.)
size_header 下載的header的大小(The total amount of bytes of the downloaded headers.)
size_request 請求的大小。(The total amount of bytes that were sent in the HTTP request.)
speed_download 下載速度，單位-字節每秒。(The average download speed that curl measured for the complete download. Bytes per second.)
speed_upload 上傳速度,單位-字節每秒。(The average upload speed that curl measured for the complete upload. Bytes per second.)
content_type 就是content-Type，不用多說了，這是一個訪問我博客首頁返回的結果示例(text/html; charset=UTF-8)；(The Content-Type of the requested document, if there was any.)
num_connects Number of new connects made in the recent transfer. (Added in 7.12.3)
num_redirects Number of redirects that were followed in the request. (Added in 7.12.3)
redirect_url When a HTTP request was made without -L to follow redirects, this variable will show the actual URL a redirect would take you to. (Added in 7.18.2)
ftp_entry_path The initial path libcurl ended up in when logging on to the remote FTP server. (Added in 7.15.4)
ssl_verify_result ssl認證結果，返回0表示認證成功。( The result of the SSL peer certificate verification that was requested. 0 means the verification was successful. (Added in 7.19.0))
若多次使用-w參數，按最后一個的格式輸出。If this option is used several times, the last one will be used.