apt的一些信息

/etc/apt/sources.list

如何还原为默认的源

网上有这样的贴子，我试了，竟然不行。
原因是这样的，比如，装好ubuntu之后，自带的源，从来也没有一次通过sudo apt update，
那么，当你换了国内源之后，即使将国内源删除，将背份的文件还源，也无济于事。
它还是会记得源来的源。
我试了各种方法，没有用处。

改为一个可用的原始源

20.04,如下，我用vim将其中的cn改为了us
:%s/cn/us/g

# See http://help.ubuntu.com/community/UpgradeNotes for how to upgrade to
# newer versions of the distribution.
deb http://us.archive.ubuntu.com/ubuntu jammy main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu jammy main restricted## Major bug fix updates produced after the final release of the
## distribution.
deb http://us.archive.ubuntu.com/ubuntu jammy-updates main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu jammy-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 jammy universe
# deb-src http://us.archive.ubuntu.com/ubuntu jammy universe
deb http://us.archive.ubuntu.com/ubuntu jammy-updates universe
# deb-src http://us.archive.ubuntu.com/ubuntu jammy-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 jammy multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu jammy multiverse
deb http://us.archive.ubuntu.com/ubuntu jammy-updates multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu jammy-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 jammy-backports main restricted universe multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu jammy-backports main restricted universe multiversedeb http://us.archive.ubuntu.com/ubuntu jammy-security main restricted
# deb-src http://us.archive.ubuntu.com/ubuntu jammy-security main restricted
deb http://us.archive.ubuntu.com/ubuntu jammy-security universe
# deb-src http://us.archive.ubuntu.com/ubuntu jammy-security universe
deb http://us.archive.ubuntu.com/ubuntu jammy-security multiverse
# deb-src http://us.archive.ubuntu.com/ubuntu jammy-security multiverse