No, given your description, you did not lose any of your files (except
/etc/passwd, which you deleted, but which you had backed up).
Boot an Ubuntu live CD/DVD or live USB flash drive. Select
Try Ubuntu (not
Install Ubuntu). When the desktop loads, open a Nautilus (file browser) window. You can find the partition for your Ubuntu system under
Devices. Click on it to mount it.
Now you can restore the backup. You have to do it as root, however. Here's one pretty easy way to do it.
Open a Terminal window (Ctrl+Alt+T). In the Terminal, type
cd and type the Space key, but don't press Enter yet.
In Nautilus, find the
etc directory inside the installed Ubuntu system. (This is not the same as the
etc directory for the live CD. The
etc directory inside the installed Ubuntu system is in the partition you just mounted.)
etc directory into the Terminal, which will paste its full path into the Terminal, completing the
cd ... command.
Press Enter to run the command. Now you're in the directory that contained your
passwd file. I am assuming that your backup file,
passwd_bkp, is also located here.
Run this command:
sudo cp passwd_bkp passwd
That restores the
passwd file from your backup, so now you should be able to reboot, remove the CD/DVD or USB flash drive, and boot back into your installed Ubuntu system. Your installed Ubuntu system should work again.
For the future, you should be aware that it is not recommended to manually edit
/etc/gshadow. Instead, you should use the utilities provided as part of Ubuntu to make any changes you need to users and groups on your system. You're probably aware that you can edit this in System Settings or with
users-admin. But there are also very powerful command-line utilities for this, which are still much safer and easier than manually editing the configuration files yourself. Here's documentation on the most relevant such utilities in Ubuntu:
You can change your username with some of those utilities. This answer explains one way in detail. However, you should be aware (as is currently touched on by a comment there) that some applications assume your username remains the same. So changing your username might cause some problems.