使用curl断点续传下载文件

办公网络网速不是很好，使用Chrome下载一些软件时不时会中断，恶心的是Chrome居然不支持断点续传下载(为什么chrome的下载不支持断点续传呢?)，迅雷自然是不能装的，那怎么办？还好我有大名鼎鼎的curl，看官网的介绍，是不是有点6到没朋友~~

Supports...

DICT, FILE, FTP, FTPS, GOPHER, GOPHERS, HTTP, HTTPS, IMAP, IMAPS, LDAP, LDAPS, MQTT, POP3, POP3S, RTMP, RTMPS, RTSP, SCP, SFTP, SMB, SMBS, SMTP, SMTPS, TELNET and TFTP. curl supports SSL certificates, HTTP POST, HTTP PUT, FTP uploading, HTTP form based upload, proxies, HTTP/2, HTTP/3, cookies, user+password authentication (Basic, Plain, Digest, CRAM-MD5, SCRAM-SHA, NTLM, Negotiate and Kerberos), file transfer resume, proxy tunneling and more.

What's curl used for?

curl is used in command lines or scripts to transfer data. curl is also used in cars, television sets, routers, printers, audio equipment, mobile phones, tablets, settop boxes, media players and is the Internet transfer engine for thousands of software applications in over ten billion installations.

curl is used daily by virtually every Internet-using human on the globe.

而且 Windows 10 已经内置支持curl(Tar and Curl Come to Windows)， 通过 PowerShell/CMD 可以直接使用，一口气上六楼，很方便！

C:\>curl
curl: try 'curl --help' for more information

C:\>curl -V -v
curl 7.55.1 (Windows) libcurl/7.55.1 WinSSL
Release-Date: 2017-11-14, security patched: 2019-11-05
Protocols: dict file ftp ftps http https imap imaps pop3 pop3s smtp smtps telnet tftp
Features: AsynchDNS IPv6 Largefile SSPI Kerberos SPNEGO NTLM SSL

言归正传，下面我们使用酷酷的`curl`下载一些文件(注：以下演示均在高速网络下进行)

1. 最简单的使用curl --remote-name(-O) URL 或者 curl --output(-o) fileName URL， 以下载最新64位windows版本Curl为例

   C:\>mkdir Software
   
   C:\>cd Software
   
   C:\Software>curl -O https://curl.se/windows/dl-7.77.0_2/curl-7.77.0_2-win64-mingw.zip
     % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                    Dload  Upload   Total   Spent    Left  Speed
    100 5154k  100 5154k    0     0  5154k      0  0:00:01 --:--:--  0:00:01 6467k
   

1. 使用-continue-at(-C)断点续传下载，这次用PowerShell下载一个大文件(SSMS)，关于该选项的使用，参见以下用户手册注解

   -C, --continue-at

   Use "-C -" to tell curl to automatically find out where/how to resume the transfer. It then uses the given output/input files to figure that out.

   • 在Powershell中使用需要带上后缀名,即curl.exe，不然不能正确识别

     PS C:\Software> curl -C - -O https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
     Invoke-WebRequest : 无法处理参数，因为参数名称“C”具有二义性。可能的匹配项包括:  -Credential -CertificateThumbprint -Certificate -ContentType。
     所在位置 行:1 字符: 6
     + curl -C - -O https://download.microsoft.com/download/4/6/8/4681f3b2-f ...
     +      ~~
         + CategoryInfo          : InvalidArgument: (:) [Invoke-WebRequest]，ParameterBindingException
         + FullyQualifiedErrorId : AmbiguousParameter,Microsoft.PowerShell.Commands.InvokeWebRequestCommand
     
     PS C:\Software> curl --continue-at - -O https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
     Invoke-WebRequest : 找不到接受实际参数“-”的位置形式参数。
     所在位置 行:1 字符: 1
     + curl --continue-at - -O https://download.microsoft.com/download/4/6/8 ...
     + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         + CategoryInfo          : InvalidArgument: (:) [Invoke-WebRequest]，ParameterBindingException
         + FullyQualifiedErrorId : PositionalParameterNotFound,Microsoft.PowerShell.Commands.InvokeWebRequestCommand
     

   • 通过关闭无线/有线网络，模拟网络不稳定的情况

     PS C:\Software> curl.exe -C - -O https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
       % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                      Dload  Upload   Total   Spent    Left  Speed
      16  565M   16 94.7M    0     0  4852k      0  0:01:59  0:00:20  0:01:39     0
     curl: (56) Send failure: Connection was reset
     
     PS C:\Software> curl.exe -C - -O https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
     ** Resuming transfer from byte position 172521856
       % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                      Dload  Upload   Total   Spent    Left  Speed
       9  470M    9 46.9M    0     0  1502k      0  0:05:20  0:00:32  0:04:48 3720k
     curl: (56) Send failure: Connection was reset
     

   • 可以使用--retry选项让下载失败后自动进行(此时网络没连接，在观察到重试现象后，打开网络连接)

     PS C:\Software> curl.exe -C - -O --retry 10 https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
     ** Resuming transfer from byte position 221771295
       % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                      Dload  Upload   Total   Spent    Left  Speed
       0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0Warning: Transient problem: timeout Will retry in 1 seconds. 10 retries left.
       0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0Warning: Transient problem: timeout Will retry in 2 seconds. 9 retries left.
       0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0Warning: Transient problem: timeout Will retry in 4 seconds. 8 retries left.
       0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0Warning: Transient problem: timeout Will retry in 8 seconds. 7 retries left.
       0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0Warning: Transient problem: timeout Will retry in 16 seconds. 6 retries left.
     100  423M  100  423M    0     0  4667k      0  0:01:33  0:01:33 --:--:-- 7028k
     PS C:\Software>
     

   • 下载完成后使用断点续传及重试选项，不会覆盖已下载的文件；反之，则不成立

     PS C:\Software> curl.exe -C - -O https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
     ** Resuming transfer from byte position 666228072
       % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                      Dload  Upload   Total   Spent    Left  Speed
       0   237    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
     curl: (33) HTTP server doesn't seem to support byte ranges. Cannot resume.
     
     PS C:\Software> curl.exe -C - -O --retry 3 https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
     ** Resuming transfer from byte position 666228072
       % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                      Dload  Upload   Total   Spent    Left  Speed
       0   237    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
     Warning: Transient problem: FTP error Will retry in 1 seconds. 3 retries left.
       0   237    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
     Warning: Transient problem: FTP error Will retry in 2 seconds. 2 retries left.
       0   237    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
     Warning: Transient problem: FTP error Will retry in 4 seconds. 1 retries left.
       0   237    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
     curl: (33) HTTP server doesn't seem to support byte ranges. Cannot resume.
     
     PS C:\Software> curl.exe -O https://download.microsoft.com/download/4/6/8/4681f3b2-f327-4d3d-8617-264b20685be0/SSMS-Setup-ENU.exe
       % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                      Dload  Upload   Total   Spent    Left  Speed
     100  635M  100  635M    0     0  16.2M      0  0:00:39  0:00:39 --:--:-- 24.2M
     

对于有些资源，网速不好的时候，甚至跳转到具体下载链接都很困难，以下载最新的 windows 64 位版本的 PostgreSQL 13.X为例，官网找到下载资源列表，页面跳转后一直不开始下载， 点击下载链接也没反应。。。

别捉急，curl一样可以帮到你。鼠标右键复制下载链接，使用--head(-I) 或者 --include(-i)选项(也可使用--verbose(-v)选项，这样，你可以看到所谓的TCP三次握手是怎样发生的)，具体下载链接在Location首部字段里，然后使用上面提到的断点续传，DONE！

C:\>curl --head https://sbp.enterprisedb.com/getfile.jsp?fileid=1257713
HTTP/1.1 302 Found
Content-Type: text/html; charset=utf-8
Content-Length: 0
Connection: keep-alive
Server: nginx/1.16.1
Date: Sat, 10 Jul 2021 08:50:09 GMT
Location: https://get.enterprisedb.com/postgresql/postgresql-13.3-2-windows-x64.exe
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
X-Cache: Miss from cloudfront
Via: 1.1 a43db2746d5ea9543e11897b6654f9b6.cloudfront.net (CloudFront)
X-Amz-Cf-Pop: HKG62-C1
X-Amz-Cf-Id: KuxMKejZWH-BMtFbzdKMCXsJE1ctLNMoSIcSYHLIkWQmbFnbexDsNg==

写在最后，准备这篇随笔的时候，翻阅了不少资料，发现有两个资源是很不错的, 一个是官网的教程, 另一个是官网提到的Everything curl，Everything curl另有中文翻译版, 找到下载章节，一个感觉就是：我有必要写这篇博客吗？写个寂寞。。。最后，附上原文地址, 原创不易，且行且珍惜~~