最近打算用Python3写一个签名验证工具，安装pyOpenSSL时需要用到本机的libssl-dev库，进一步在Ubuntu上尝试安装库时出错，如下：
- ygu@guyongqiangx:~$ sudo apt-get install libssl-dev
- Reading package lists... Done
- Building dependency tree
- Reading state information... Done
- Some packages could not be installed. This may mean that you have
- requested an impossible situation or if you are using the unstable
- distribution that some required packages have not yet been created
- or been moved out of Incoming.
- The following information may help to resolve the situation:
- The following packages have unmet dependencies:
- libssl-dev : Depends: libssl1.0.0 (= 1.0.1f-1ubuntu2) but 1.0.1f-1ubuntu2.19 is to be installed
- Recommends: libssl-doc but it is not going to be installed
- E: Unable to correct problems, you have held broken packages.
度娘搜索提示说建议使用aptitude进行安装
- ygu@guyongqiangx:~$ sudo apt-get install aptitude
- Reading package lists... Done
- Building dependency tree
- Reading state information... Done
- aptitude is already the newest version.
- 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
发现我原来已经安装过aptitude了
使用aptitude再次安装libssl-dev，并选择降级的版本。
- ygu@guyongqiangx:~$ sudo aptitude install libssl-dev
- The following NEW packages will be installed:
- libssl-dev{b} libssl-doc{a}
- 0 packages upgraded, 2 newly installed, 0 to remove and 0 not upgraded.
- Need to get 2,031 kB of archives. After unpacking 7,801 kB will be used.
- The following packages have unmet dependencies:
- libssl-dev : Depends: libssl1.0.0 (= 1.0.1f-1ubuntu2) but 1.0.1f-1ubuntu2.19 is installed.
- The following actions will resolve these dependencies:
- ###
- Keep the following packages at their current version:
- 1) libssl-dev [Not Installed]
- ###
- # 这里提示时，一定要选n，选Y跟apt-get install操作一样
- Accept this solution? [Y/n/q/?] n
- The following actions will resolve these dependencies:
- ###
- Downgrade the following packages:
- 1) libssl1.0.0 [1.0.1f-1ubuntu2.19 (now) -> 1.0.1f-1ubuntu2 (trusty)]
- ###
- # 接受这里的降级处理，成功安装
- ###
- Accept this solution? [Y/n/q/?] y
- The following packages will be DOWNGRADED:
- libssl1.0.0
- The following NEW packages will be installed:
- libssl-dev libssl-doc{a}
- 0 packages upgraded, 2 newly installed, 1 downgraded, 0 to remove and 0 not upgraded.
- Need to get 2,857 kB of archives. After unpacking 7,784 kB will be used.
- Do you want to continue? [Y/n/?] y
- Get: 1 http://mirrors.aliyun.com/ubuntu/ trusty/main libssl1.0.0 amd64 1.0.1f-1ubuntu2 [825 kB]
- Get: 2 http://mirrors.aliyun.com/ubuntu/ trusty/main libssl-doc all 1.0.1f-1ubuntu2 [965 kB]
- Get: 3 http://mirrors.aliyun.com/ubuntu/ trusty/main libssl-dev amd64 1.0.1f-1ubuntu2 [1,066 kB]
- Fetched 2,857 kB in 50s (57.1 kB/s)
- Preconfiguring packages ...
- dpkg: warning: downgrading libssl1.0.0:amd64 from 1.0.1f-1ubuntu2.19 to 1.0.1f-1ubuntu2
- (Reading database ... 234481 files and directories currently installed.)
- Preparing to unpack .../libssl1.0.0_1.0.1f-1ubuntu2_amd64.deb ...
- Unpacking libssl1.0.0:amd64 (1.0.1f-1ubuntu2) over (1.0.1f-1ubuntu2.19) ...
- Selecting previously unselected package libssl-dev:amd64.
- Preparing to unpack .../libssl-dev_1.0.1f-1ubuntu2_amd64.deb ...
- Unpacking libssl-dev:amd64 (1.0.1f-1ubuntu2) ...
- Selecting previously unselected package libssl-doc.
- Preparing to unpack .../libssl-doc_1.0.1f-1ubuntu2_all.deb ...
- Unpacking libssl-doc (1.0.1f-1ubuntu2) ...
- Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
- Setting up libssl1.0.0:amd64 (1.0.1f-1ubuntu2) ...
- Setting up libssl-dev:amd64 (1.0.1f-1ubuntu2) ...
- Setting up libssl-doc (1.0.1f-1ubuntu2) ...
- Processing triggers for libc-bin (2.19-0ubuntu6.9) ...
安装成功。
简单总结：
apt-get安装时会默认选择预设的依赖版本。
aptitude安装时会提示是否选择默认设定的依赖版本，如果选n，会对库进行依赖降级，从而安装期望的版本。
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