我正在尝试在我的计算机上安装 FreeCAD（运行 18.10），并且在安装时遇到了一些麻烦。我想使用最新的稳定版本ppa:freecad-maintainers/freecad-stable，因为主存储库中的版本已经过时并且在启动时有几个错误。问题是，即使将 PPA 添加到我的源代码后，apt仍然会从官方存储库安装该版本。
这是我运行的输出sudo apt update
Hit:1 http://download.virtualbox.org/virtualbox/debian cosmic InRelease
Hit:2 http://us.archive.ubuntu.com/ubuntu cosmic InRelease
Get:3 http://us.archive.ubuntu.com/ubuntu cosmic-updates InRelease [83.2 kB]
Get:4 http://security.ubuntu.com/ubuntu cosmic-security InRelease [83.2 kB]
Hit:5 http://ppa.launchpad.net/freecad-maintainers/freecad-stable/ubuntu cosmic InRelease
Get:6 http://us.archive.ubuntu.com/ubuntu cosmic-backports InRelease [74.6 kB]
Hit:7 http://ppa.launchpad.net/js-reynaud/kicad-5/ubuntu cosmic InRelease
Fetched 241 kB in 1s (343 kB/s)
Reading package lists... Done
Building dependency tree
Reading state information... Done
All packages are up to date.
所以你会认为来自 FreeCAD 维护者稳定 PPA 的最新包应该可用，对吧？然而，输出apt policy freecad是
freecad:
Installed: (none)
Candidate: 0.16.6712+dfsg1-3build1
Version table:
0.16.6712+dfsg1-3build1 500
500 http://us.archive.ubuntu.com/ubuntu cosmic/universe amd64 Packages
如果有用，这是我的/etc/apt/sources.list
# deb cdrom:[Ubuntu 18.10 _Cosmic Cuttlefish_ - Release amd64 (20181017.3)]/ cosmic main restricted
# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu/ cosmic main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic main restricted
## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu/ cosmic-updates main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic-updates main restricted
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ cosmic universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic universe
deb http://us.archive.ubuntu.com/ubuntu/ cosmic-updates universe
# deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic-updates universe
## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team, and may not be under a free licence. Please satisfy yourself as to
## your rights to use the software. Also, please note that software in
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://us.archive.ubuntu.com/ubuntu/ cosmic multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic multiverse
deb http://us.archive.ubuntu.com/ubuntu/ cosmic-updates multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic-updates multiverse
## N.B. software from this repository may not have been tested as
## extensively as that contained in the main release, although it includes
## newer versions of some applications which may provide useful features.
## Also, please note that software in backports WILL NOT receive any review
## or updates from the Ubuntu security team.
deb http://us.archive.ubuntu.com/ubuntu/ cosmic-backports main restricted universe multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu/ cosmic-backports main restricted universe multiverse
## Uncomment the following two lines to add software from Canonical's
## 'partner' repository.
## This software is not part of Ubuntu, but is offered by Canonical and the
## respective vendors as a service to Ubuntu users.
# deb http://archive.canonical.com/ubuntu cosmic partner
# deb-src http://archive.canonical.com/ubuntu cosmic partner
deb http://security.ubuntu.com/ubuntu cosmic-security main restricted
# deb-src http://security.ubuntu.com/ubuntu cosmic-security main restricted
deb http://security.ubuntu.com/ubuntu cosmic-security universe
# deb-src http://security.ubuntu.com/ubuntu cosmic-security universe
deb http://security.ubuntu.com/ubuntu cosmic-security multiverse
# deb-src http://security.ubuntu.com/ubuntu cosmic-security multiverse
# This system was installed using small removable media
# (e.g. netinst, live or single CD). The matching "deb cdrom"
# entries were disabled at the end of the installation process.
# For information about how to configure apt package sources,
# see the sources.list(5) manual.
deb [arch=amd64] http://download.virtualbox.org/virtualbox/debian cosmic contrib
# deb-src [arch=amd64] http://download.virtualbox.org/virtualbox/debian cosmic contrib
这是输出ls -al /etc/apt/sources.list.d
total 24
drwxr-xr-x 2 root root 4096 Dec 5 10:33 .
drwxr-xr-x 6 root root 4096 Dec 16 11:03 ..
-rw-r--r-- 1 root root 0 Dec 5 11:02 elmer-csc-ubuntu-ubuntu-elmer-csc-ppa-cosmic.list
-rw-r--r-- 1 root root 0 Dec 5 11:02 elmer-csc-ubuntu-ubuntu-elmer-csc-ppa-cosmic.list.save
-rw-r--r-- 1 root root 172 Dec 5 11:02 freecad-maintainers-ubuntu-freecad-stable-cosmic.list
-rw-r--r-- 1 root root 172 Dec 5 11:02 freecad-maintainers-ubuntu-freecad-stable-cosmic.list.save
-rw-r--r-- 1 root root 140 Dec 5 11:02 js-reynaud-ubuntu-kicad-5-cosmic.list
-rw-r--r-- 1 root root 140 Dec 5 11:02 js-reynaud-ubuntu-kicad-5-cosmic.list.save
如果我忘记提供任何可能有用的东西，请告诉我。提前感谢您的帮助！