Accidentally deleted the "/usr/share" folder

The system does not start. Did it by using rm - R accidentally. How can I recover the system?

You need to reinstall all applications which have files in /usr/share.

1. Boot system from Recovery Mode with root prompt.

2. Remount rootfs to read-write mode with mount -o rw,remount /.

3. Raise up ethernet interface and get IP address from DHCP server: dhclient enp0s3 (check correct interface name in ip a or ifconfig -a).

4. Specify DNS server by

   rm /etc/resolv.conf
   echo "nameserver 192.168.1.1" > /etc/resolv.conf
   

   (change 192.168.1.1 to yours gateway/router)

5. You need to fix the dpkg package as it misses the /usr/share/dpkg/cputable file (otherwise you will face the "E: error reading the cpu table" on any apt/apt-get command)

   wget http://mirrors.kernel.org/ubuntu/pool/main/d/dpkg/dpkg_1.19.0.5ubuntu2.1_amd64.deb
   dpkg -i dpkg_1.19.0.5ubuntu2.1_amd64.deb
   

   or installing it from cache:

   dpkg -i /var/cache/apt/archives/dpkg_*.deb
   

6. update package cache:

   apt-get update
   

7. reinstall essential componenets

   apt-get install --reinstall debconf linux-base
   mkdir /usr/share/python
   apt-get install --reinstall python2.7-minimal python3-minimal
   

8. and then use one-liner below:

   apt-get install --reinstall $(dpkg -S /usr/share/ | sed 's/,//g' | sed 's/: \/usr\/share//g')
   

   Above:

   • dpkg -S /usr/share/ shows the list of comma-separated packages
   • sed 's/,//g' - removes commas
   • sed 's/: \/usr\/share//g' - removes : /usr/share in the end

   This part may fail with messages about some packages. For example on my VM I have had a problem with bsdmainutils, so I reinstalled them with:

   dpkg -i /var/cache/apt/archives/bsdmainutils_*.deb
   

   and then reran one-liner above.

9. fix broken packages

   dpkg --configure -a
   apt-get install -f
   

10. finally fix /etc/resolv.conf link by

    rm /etc/resolv.conf
    ln -s /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
    

11. reboot.

Note: the resulting fixed system do not show any problems while running sudo apt-get check or sudo debsums --all --changed.

Short answer: you can't and you have to reinstall the apps, as suggested by a previous answer.

However, before that, I would mount the drive in another computer and test undelete utilities which are available for ext3/ext4. A quick search in Google points to these ones:

extundelete: http://extundelete.sourceforge.net/

easeus: https://www.easeus.com/resource/drive/ext3.htm

TestDisk: https://www.cgsecurity.org/wiki/TestDisk

How to guide: http://translatedby.com/you/howto-recover-deleted-files-on-an-ext3-file-system/original/

etc.