Experimentation is another good option, and goes a long way, but it can be scary. If you're at all worried about doing permanent damage, install a virtual machine (i.e. VirtualBox + the server version of Ubuntu or whatever OS you want). That way you can experiment away without fear of doing permanent damage. If it happens just trash the the VM and start again.
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https://peepcode.com/products/advanced-command-line
Experimentation is another good option, and goes a long way, but it can be scary. If you're at all worried about doing permanent damage, install a virtual machine (i.e. VirtualBox + the server version of Ubuntu or whatever OS you want). That way you can experiment away without fear of doing permanent damage. If it happens just trash the the VM and start again.