Edit - please see this duplicate question.
I just chowned -R my /usr folder, which totally corrupted my computer (not sure how, but I lost sudo power eventually). This seems like a horrible newb mistake.
Are there other ones I should avoid?
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Fast people don't hurry
Don't use not so well known programs or commands untested in a hurry. If you don't have enough time, think about how many time you need if it is going wrong.
Use a lab for experiments, often
You should read the man page for new commands, and test things, which are unclear, in a testing directory, where you have normal files, hidden files, links, symbolic links, subdirs and everything. Where it is no drama if something get lost. You could even use a chroot environment or a VM-Installation to have a sandbox.
And you should have daily backups. If you're not using it seriously (important posts, photos, valuable work) you don't need daily backups, but if they are very valuable, you should do hourly backups.
A backup of your lab would be fine, and needn't be so much up to date.
Login as root, you should always use sudo if you want to run something as root. And if you for some reason login as root you should be really careful.
If you're working with disk partitions, make sure the device is what you think it is. The other day I accidentally formatted my external hard drive thinking it was my USB flash drive. My external drive is almost always
There are probably a lot of ways to check what a device really is, but one way is to just mount the partition and check the contents before you make an attempt to destroy it.
Another way is to run this:
This will show the labels on your devices and tell you which file they are in
Also there are a few typos you should watch out. This one has already bitten me. I wanted to delete all
FORKBOMB. That's one of the worst things you can do to your machine. Here's the link that explains it.
Also, in my opinion, never do anything as root unless absolutely necessary.