What you see on a webpage isn't necessarily what you get when you copy something off it. Watch out for extra hidden logic, especially in "example commands" you might copy directly over to a prompt.

The command below demonstrates the point, and is safe to run. It just doesn't do exactly what you'd think it would.

ls -l/dev/null 2>&1; clear; echo -ne "Hey $(whoami),\n"; echo -e "\\tThis could have been bad! Don't copy from websites to the command line! Witness my power:\\n\$ cat /etc/fstab"; cat /etc/fstab;  
ls -l  
uptime

Beware the Copypasta!