Ubuntu has many fonts preinstalled but I don't need all of them. I deleted some fonts files at /usr/share/fonts/ a while ago but there are packages left. So I ran `sudo apt purge` to remove them, including the fonts-ubuntu package because I dislike it. I found that I couldn't enter system setting after doing that. Only at this time I realized I removed other packages accidently. Luckily I found a thread at Ask Ubuntu and found out that package is gnome-control-center.
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