1. What, if anything, is wrong with each of the following commands?
a) ls -l-a
b) cat file1, file2
c) ls - s /etc
2. What is the command to calculate the size of a folder?
3. What's the command that will display all .txt files, including their individual permission.
4. How do you terminate all running processes at once ?
5. How can you find out how much memory Linux is using? (if you can, give us more than one examples)