关于curl命令行下载页面为空的问题
平时经常用到wget/curl命令行下载页面，有时候会遇到curl命令行下载为空的情况，如：
zhuliting@zhuliting:~$ curl 'http://m.youku.com' -o youku.html
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
zhuliting@zhuliting:~$ file youku.html
youku.html: ERROR: cannot open `youku.html' (No such file or directory)
而wget下载，可以得到内容：
zhuliting@zhuliting:~$ wget 'http://m.youku.com' -O youku.html --2015-08-06 20:09:44-- http://m.youku.com/ 正在解析主机 m.youku.com (m.youku.com)... 211.151.146.60 正在连接 m.youku.com (m.youku.com)|211.151.146.60|:80... 已连接。 已发出 HTTP 请求，正在等待回应... 302 Moved Temporarily 位置：http://m.youku.com/wap/ [跟随至新的 URL] --2015-08-06 20:09:44-- http://m.youku.com/wap/ 再次使用存在的到 m.youku.com:80 的连接。 已发出 HTTP 请求，正在等待回应... 200 OK 长度： 16603 (16K) [text/html] 正在保存至: “youku.html” 100%[======================================================================================================>] 16,603 86.4KB/s 用时 0.2s 2015-08-06 20:09:44 (86.4 KB/s) - 已保存 “youku.html” [16603/16603]) zhuliting@zhuliting:~$ file youku.html youku.html: HTML document, UTF-8 Unicode text
如果这种情况，很可能是因为在下载过程中，发生了重定向。wget命令在下载时，默认能够自动下载重定向后的链接，而curl默认是不会的。通过对curl加参数-v进行调试，也可以发现，下载时发生了302重定向：
zhuliting@zhuliting:~$ curl 'http://m.youku.com' -o youku.html -v
* Rebuilt URL to: http://m.youku.com/
* Hostname was NOT found in DNS cache
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0* Trying 211.151.146.60...
* Connected to m.youku.com (211.151.146.60) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.35.0
> Host: m.youku.com
> Accept: */*
>
< HTTP/1.1 302 Moved Temporarily
* Server nginx/0.8.54 is not blacklisted
< Server: nginx/0.8.54
< Date: Thu, 06 Aug 2015 12:14:52 GMT
< Content-Type: text/html;charset=utf-8
< Connection: keep-alive
< Set-Cookie: JSESSIONID=ED87E6E12D2605176672773D570B8377; Path=/; HttpOnly
< Location: http://www.youku.com
< Content-Length: 0
<
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
* Connection #0 to host m.youku.com left intact
通过man curl发现，可以通过加选项-L进行重定向，来解决下载为空的问题：
zhuliting@zhuliting:~$ curl 'http://m.youku.com' -o youku.html -v -L
* Rebuilt URL to: http://m.youku.com/
* Hostname was NOT found in DNS cache
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0* Trying 211.151.146.60...
* Connected to m.youku.com (211.151.146.60) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.35.0
> Host: m.youku.com
> Accept: */*
>
< HTTP/1.1 302 Moved Temporarily
* Server nginx/0.8.54 is not blacklisted
< Server: nginx/0.8.54
< Date: Thu, 06 Aug 2015 12:17:43 GMT
< Content-Type: text/html;charset=utf-8
< Connection: keep-alive
< Set-Cookie: JSESSIONID=5ED7472830728CC0C65B60F9EDCCE087; Path=/; HttpOnly
< Location: http://www.youku.com
< Content-Length: 0
<
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0
* Connection #0 to host m.youku.com left intact
* Issue another request to this URL: 'http://www.youku.com'
* Rebuilt URL to: http://www.youku.com/
* Hostname was NOT found in DNS cache
* Trying 43.250.12.42...
* Connected to www.youku.com (43.250.12.42) port 80 (#1)
> GET / HTTP/1.1
> User-Agent: curl/7.35.0
> Host: www.youku.com
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: text/html
< Accept-Ranges: bytes
< ETag: "2574430301"
< Last-Modified: Thu, 06 Aug 2015 12:16:02 GMT
< Content-Length: 619907
< Connection: close
< Date: Thu, 06 Aug 2015 12:17:43 GMT
* Server b28www4 is not blacklisted
< Server: b28www4
<
{ [data not shown]
100 605k 100 605k 0 0 394k 0 0:00:01 0:00:01 --:--:-- 458k
* Closing connection 1