Thanks for the input here forumers.
I just started Linux myself, here are some useful ones:
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df -h
For revealing disk space and usage, pretty useful if for server admins.
Make sure check out the /var/logs, majority of the legacy server at my company has /var/logs of 2GB
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du -s
Check the file usage of your current folder
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cd -
Change directory to your previous directory. Say if you were at /home/foo/bar and then cd to /var/log/audit, you can use cd - to instantly return to /home/foo/bar/
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truncate -s (number) (size) (file)
Truncate a file size, useful if the Linux cannot even use rm -rf due to full disk space.
Sample: truncate -s 10k app.log
Hope it helps.