Accidentally removed /bin. How do I restore it?

If your current system still has a running shell and internet access, this can be done using tools existing elsewhere on the system. I’m assuming you only deleted /bin. /bin of course has the most convenient utility that you could use in such a situation (busybox), but without that, we’ll have to get a little creative.

Since you already have a running shell, and since sudo is in /usr/bin, let’s get ourselves a running root shell before we do further damage. But /bin/bash and most other shells are gone! Luckily, Linux still has an in-memory copy of the shell you’re using. So:

sudo /proc/$$/exe

Strictly speaking we don’t need a root shell for much of what follows. But anyway.

Now, dpkg still works, at least for finding which packages have files in /bin:

dpkg -S /bin

We can use awk to process it and get the package names, and xargs and apt-get to download the packages (all in /usr/bin). If you have a temporary directory that you can use, cd there, because your current directory is going to get a bit messy:

dpkg -S /bin | awk -F '[, :]' '{NF--}1' | xargs apt-get download

Now, the biggest problem we have is that /bin/tar is missing, and without it, dpkg can’t extract the archives. We can get two-thirds of the way there, because:

1. .deb files are actually ar archives (again in /usr/bin):

   ar x tar_*.deb
   

2. Consisting of two .tar.* archives, data and control:

   $ echo *.tar.*
   control.tar.gz data.tar.xz
   

3. While the gzip utilities are in /bin, unxz is in /usr/bin:

   unxz data.tar.xz
   

Now we have a data.tar file without tar to extract tar from it.

Python to the rescue! This is where sudo is really needed:

$ sudo python -c 'import tarfile; tarfile.open("data.tar").extractall("/")'
$ echo /bin/*
/bin/tar

Now we can use dpkg to extract the remaining deb files to get a reasonably complete /bin:

for i in *.deb; do dpkg-deb -x "$i" /; done

However, we should still do a proper installation of the deb files, so that symlinks etc. that would be created by packages are re-created:

sudo apt install --reinstall ./*.deb

Or:

sudo dpkg -i *.deb
sudo apt-get install -f

Notes:

1. We can’t use Python 2 to directly extract the data.tar.xz file, since Python 2 only supports gzip and bzip2 compression. Python 3, however, does support it, so you can use Python 3 directly without unxz:

   sudo python3 -c 'import tarfile; tarfile.open("data.tar.xz").extractall("/")'
   

2. After getting back /bin/tar, you still need to extract some of the deb files before you can use apt-get: the shells, coreutils, etc. Easier to just extract all of them and re-install later.

You could temporarily put files from a live CD or another system into your /bin to make your system usable, then replace them with files from your Ubuntu installation by running apt-get install --reinstall for packages which have stuff in /bin.

Some additions to this excellent answer, after I encountered this issue (along with deleting /boot, /etc, /lib and /lib64):

• chroot requires /lib and /lib64 to be present; otherwise you will get the following error:
  failed to run command ‘/bin/bash’: No such file or directory
  I copied these from the LiveCD OS and didn’t have any problems restoring. YMMV depending on the packages you have installed on the system
• I cannot edit the answer referenced above, but there is a typo:
  cp /etc/resolv.conf /mnt/etc/resolv.cof
  should be
  cp /etc/resolv.conf /mnt/etc/resolv.conf
• /boot can easily be restored using grub tools. See here.
• As this answer recommends, apt install --reinstall <package> is a great way to restore missing files in /bin, /lib and /lib64.
  • Some packages that required reinstallation: libaio1, mysql-server, openvpn, vsftpd

Note to self:
rm -rf folder /* is not the same as rm -rf folder/*