curl 是一種指令行工具，作用是發出網絡請求，然後擷取資料，顯示在"标準輸出"（stdout）上面。它支援多種協定，下面列舉其常用功能。
一、檢視網頁源碼
直接在 curl 指令後加上網址，就可以看到網頁源碼。以網址 www.sina.com為例（選擇該網址，主要因為它的網頁代碼較短）。
$ curl www.sina.com
<html>
<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>
<hr><center>nginx</center>
</body>
</html>
如果要把這個網頁儲存下來，可以使用 -o 參數：
$ curl -o [檔案名] www.sina.com
二、自動跳轉
有的網址是自動跳轉的。使用 -L 參數，curl 就會跳轉到新的網址。
$ curl -L www.sina.com
鍵入上面的指令，結果自動跳轉為 www.sina.com.cn。
三、顯示頭資訊
-i 參數可以顯示 http response 的頭資訊，連同網頁代碼一起。-I 參數則隻顯示 http response 的頭資訊。
$ curl -i www.sina.com
HTTP/1.1 301 Moved Permanently
Server: nginx
Date: Tue, 23 Aug 2016 08:30:16 GMT
Content-Type: text/html
Location: http://www.sina.com.cn/
Expires: Tue, 23 Aug 2016 08:32:16 GMT
Cache-Control: max-age=120
Age: 102
Content-Length: 178
X-Cache: HIT from xd33-83.sina.com.cn
<html>
<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>
<hr><center>nginx</center>
</body>
</html>
四、顯示通信過程
-v 參數可以顯示一次 http 通信的整個過程，包括端口連接配接和 http request 頭資訊。
$ curl -v www.sina.com
* Rebuilt URL to: www.sina.com/
* Hostname was NOT found in DNS cache
* Trying 202.108.33.60...
* Connected to www.sina.com (202.108.33.60) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.35.0
> Host: www.sina.com
> Accept: */*
>
< HTTP/1.1 301 Moved Permanently
* Server nginx is not blacklisted
< Server: nginx
< Date: Tue, 23 Aug 2016 08:48:14 GMT
< Content-Type: text/html
< Location: http://www.sina.com.cn/
< Expires: Tue, 23 Aug 2016 08:50:14 GMT
< Cache-Control: max-age=120
< Age: 40
< Content-Length: 178
< X-Cache: HIT from xd33-81.sina.com.cn
<
<html>
<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>
<hr><center>nginx</center>
</body>
</html>
* Connection #0 to host www.sina.com left intact
如果覺得上面的資訊還不夠，那麼下面的指令可以檢視更詳細的通信過程。
$ curl --trace output.txt www.sina.com
或者
$ curl --trace-ascii output.txt www.sina.com
運作後，打開 output.txt 檔案檢視。
五、發送表單資訊
發送表單資訊有 GET 和 POST 兩種方法。
GET 方法相對簡單，隻要把資料附在網址後面就行。
$ curl example.com/form.cgi?data=xxx
POST 方法必須把資料和網址分開，curl 就要用到 --data 或者 -d 參數。
$ curl -X POST --data "data=xxx" example.com/form.cgi
如果你的資料沒有經過表單編碼，還可以讓 curl 為你編碼，參數是 --data-urlencode。
$ curl -X POST--data-urlencode "date=April 1" example.com/form.cgi
六、HTTP動詞
curl 預設的 HTTP 動詞是 GET，使用 -X 參數可以支援其他動詞。
$ curl -X POST www.example.com
$ curl -X DELETE www.example.com
七、User Agent字段
這個字段是用來表示用戶端的裝置資訊。伺服器有時會根據這個字段，針對不同裝置，傳回不同格式的網頁，比如手機版和桌面版。
浏覽器的 User Agent 是：
Mozilla/5.0 (Windows NT 10.0; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.84 Safari/537.36
curl 可以這樣模拟：
$ curl --user-agent "[User Agent]" [URL]
八、cookie
使用 --cookie 參數，可以讓 curl 發送 cookie。
$ curl --cookie "name=xxx" www.example.com
至于具體的 cookie 的值，可以從 http response 頭資訊的 Set-Cookie 字段中得到。
九、增加頭資訊
有時需要在 http request 之中，自行增加一個頭資訊。–header 參數就可以起到這個作用。
$ curl --header "Content-Type:application/json" http://example.com
十、HTTP認證
有些網域需要 HTTP 認證，這時 curl 需要用到 --user 或者 -u 參數。
$ curl --user name:password example.com
附錄 curl 指令完整的參數
$ curl --help
Usage: curl [options...] <url>
Options: (H) means HTTP/HTTPS only, (F) means FTP only
--anyauth Pick "any" authentication method (H)
-a, --append Append to target file when uploading (F/SFTP)
--basic Use HTTP Basic Authentication (H)
--cacert FILE CA certificate to verify peer against (SSL)
--capath DIR CA directory to verify peer against (SSL)
-E, --cert CERT[:PASSWD] Client certificate file and password (SSL)
--cert-type TYPE Certificate file type (DER/PEM/ENG) (SSL)
--ciphers LIST SSL ciphers to use (SSL)
--compressed Request compressed response (using deflate or gzip)
-K, --config FILE Specify which config file to read
--connect-timeout SECONDS Maximum time allowed for connection
-C, --continue-at OFFSET Resumed transfer offset
-b, --cookie STRING/FILE String or file to read cookies from (H)
-c, --cookie-jar FILE Write cookies to this file after operation (H)
--create-dirs Create necessary local directory hierarchy
--crlf Convert LF to CRLF in upload
--crlfile FILE Get a CRL list in PEM format from the given file
-d, --data DATA HTTP POST data (H)
--data-ascii DATA HTTP POST ASCII data (H)
--data-binary DATA HTTP POST binary data (H)
--data-urlencode DATA HTTP POST data url encoded (H)
--delegation STRING GSS-API delegation permission
--digest Use HTTP Digest Authentication (H)
--disable-eprt Inhibit using EPRT or LPRT (F)
--disable-epsv Inhibit using EPSV (F)
--dns-servers DNS server addrs to use: 1.1.1.1;2.2.2.2
--dns-interface Interface to use for DNS requests
--dns-ipv4-addr IPv4 address to use for DNS requests, dot notation
--dns-ipv6-addr IPv6 address to use for DNS requests, dot notation
-D, --dump-header FILE Write the headers to this file
--egd-file FILE EGD socket path for random data (SSL)
--engine ENGINE Crypto engine (SSL). "--engine list" for list
-f, --fail Fail silently (no output at all) on HTTP errors (H)
-F, --form CONTENT Specify HTTP multipart POST data (H)
--form-string STRING Specify HTTP multipart POST data (H)
--ftp-account DATA Account data string (F)
--ftp-alternative-to-user COMMAND String to replace "USER [name]" (F)
--ftp-create-dirs Create the remote dirs if not present (F)
--ftp-method [MULTICWD/NOCWD/SINGLECWD] Control CWD usage (F)
--ftp-pasv Use PASV/EPSV instead of PORT (F)
-P, --ftp-port ADR Use PORT with given address instead of PASV (F)
--ftp-skip-pasv-ip Skip the IP address for PASV (F)
--ftp-pret Send PRET before PASV (for drftpd) (F)
--ftp-ssl-ccc Send CCC after authenticating (F)
--ftp-ssl-ccc-mode ACTIVE/PASSIVE Set CCC mode (F)
--ftp-ssl-control Require SSL/TLS for ftp login, clear for transfer (F)
-G, --get Send the -d data with a HTTP GET (H)
-g, --globoff Disable URL sequences and ranges using {} and []
-H, --header LINE Custom header to pass to server (H)
-I, --head Show document info only
-h, --help This help text
--hostpubmd5 MD5 Hex encoded MD5 string of the host public key. (SSH)
-0, --http1.0 Use HTTP 1.0 (H)
--http1.1 Use HTTP 1.1 (H)
--http2.0 Use HTTP 2.0 (H)
--ignore-content-length Ignore the HTTP Content-Length header
-i, --include Include protocol headers in the output (H/F)
-k, --insecure Allow connections to SSL sites without certs (H)
--interface INTERFACE Specify network interface/address to use
-4, --ipv4 Resolve name to IPv4 address
-6, --ipv6 Resolve name to IPv6 address
-j, --junk-session-cookies Ignore session cookies read from file (H)
--keepalive-time SECONDS Interval between keepalive probes
--key KEY Private key file name (SSL/SSH)
--key-type TYPE Private key file type (DER/PEM/ENG) (SSL)
--krb LEVEL Enable Kerberos with specified security level (F)
--libcurl FILE Dump libcurl equivalent code of this command line
--limit-rate RATE Limit transfer speed to this rate
-l, --list-only List only mode (F/POP3)
--local-port RANGE Force use of these local port numbers
-L, --location Follow redirects (H)
--location-trusted like --location and send auth to other hosts (H)
-M, --manual Display the full manual
--mail-from FROM Mail from this address (SMTP)
--mail-rcpt TO Mail to this/these addresses (SMTP)
--mail-auth AUTH Originator address of the original email (SMTP)
--max-filesize BYTES Maximum file size to download (H/F)
--max-redirs NUM Maximum number of redirects allowed (H)
-m, --max-time SECONDS Maximum time allowed for the transfer
--metalink Process given URLs as metalink XML file
--negotiate Use HTTP Negotiate Authentication (H)
-n, --netrc Must read .netrc for user name and password
--netrc-optional Use either .netrc or URL; overrides -n
--netrc-file FILE Set up the netrc filename to use
-N, --no-buffer Disable buffering of the output stream
--no-keepalive Disable keepalive use on the connection
--no-sessionid Disable SSL session-ID reusing (SSL)
--noproxy List of hosts which do not use proxy
--ntlm Use HTTP NTLM authentication (H)
--oauth2-bearer TOKEN OAuth 2 Bearer Token (IMAP, POP3, SMTP)
-o, --output FILE Write output to <file> instead of stdout
--pass PASS Pass phrase for the private key (SSL/SSH)
--post301 Do not switch to GET after following a 301 redirect (H)
--post302 Do not switch to GET after following a 302 redirect (H)
--post303 Do not switch to GET after following a 303 redirect (H)
-#, --progress-bar Display transfer progress as a progress bar
--proto PROTOCOLS Enable/disable specified protocols
--proto-redir PROTOCOLS Enable/disable specified protocols on redirect
-x, --proxy [PROTOCOL://]HOST[:PORT] Use proxy on given port
--proxy-anyauth Pick "any" proxy authentication method (H)
--proxy-basic Use Basic authentication on the proxy (H)
--proxy-digest Use Digest authentication on the proxy (H)
--proxy-negotiate Use Negotiate authentication on the proxy (H)
--proxy-ntlm Use NTLM authentication on the proxy (H)
-U, --proxy-user USER[:PASSWORD] Proxy user and password
--proxy1.0 HOST[:PORT] Use HTTP/1.0 proxy on given port
-p, --proxytunnel Operate through a HTTP proxy tunnel (using CONNECT)
--pubkey KEY Public key file name (SSH)
-Q, --quote CMD Send command(s) to server before transfer (F/SFTP)
--random-file FILE File for reading random data from (SSL)
-r, --range RANGE Retrieve only the bytes within a range
--raw Do HTTP "raw", without any transfer decoding (H)
-e, --referer Referer URL (H)
-J, --remote-header-name Use the header-provided filename (H)
-O, --remote-name Write output to a file named as the remote file
--remote-name-all Use the remote file name for all URLs
-R, --remote-time Set the remote file's time on the local output
-X, --request COMMAND Specify request command to use
--resolve HOST:PORT:ADDRESS Force resolve of HOST:PORT to ADDRESS
--retry NUM Retry request NUM times if transient problems occur
--retry-delay SECONDS When retrying, wait this many seconds between each
--retry-max-time SECONDS Retry only within this period
--sasl-ir Enable initial response in SASL authentication
-S, --show-error Show error. With -s, make curl show errors when they occur
-s, --silent Silent mode. Don't output anything
--socks4 HOST[:PORT] SOCKS4 proxy on given host + port
--socks4a HOST[:PORT] SOCKS4a proxy on given host + port
--socks5 HOST[:PORT] SOCKS5 proxy on given host + port
--socks5-hostname HOST[:PORT] SOCKS5 proxy, pass host name to proxy
--socks5-gssapi-service NAME SOCKS5 proxy service name for gssapi
--socks5-gssapi-nec Compatibility with NEC SOCKS5 server
-Y, --speed-limit RATE Stop transfers below speed-limit for 'speed-time' secs
-y, --speed-time SECONDS Time for trig speed-limit abort. Defaults to 30
--ssl Try SSL/TLS (FTP, IMAP, POP3, SMTP)
--ssl-reqd Require SSL/TLS (FTP, IMAP, POP3, SMTP)
-2, --sslv2 Use SSLv2 (SSL)
-3, --sslv3 Use SSLv3 (SSL)
--ssl-allow-beast Allow security flaw to improve interop (SSL)
--stderr FILE Where to redirect stderr. - means stdout
--tcp-nodelay Use the TCP_NODELAY option
-t, --telnet-option OPT=VAL Set telnet option
--tftp-blksize VALUE Set TFTP BLKSIZE option (must be >512)
-z, --time-cond TIME Transfer based on a time condition
-1, --tlsv1 Use TLSv1 (SSL)
--trace FILE Write a debug trace to the given file
--trace-ascii FILE Like --trace but without the hex output
--trace-time Add time stamps to trace/verbose output
--tr-encoding Request compressed transfer encoding (H)
-T, --upload-file FILE Transfer FILE to destination
--url URL URL to work with
-B, --use-ascii Use ASCII/text transfer
-u, --user USER[:PASSWORD][;OPTIONS] Server user, password and login options
--tlsuser USER TLS username
--tlspassword STRING TLS password
--tlsauthtype STRING TLS authentication type (default SRP)
-A, --user-agent STRING User-Agent to send to server (H)
-v, --verbose Make the operation more talkative
-V, --version Show version number and quit
-w, --write-out FORMAT What to output after completion
--xattr Store metadata in extended file attributes
-q If used as the first parameter disables .curlrc
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