1. Command not found – Perhaps the most common of all, this is mostly the result of typos. You must spell commands perfectly in order to execute them. One mistype, and you will draw this error. You may also get the error if the program you want to run is not installed. Also, if the command is not in your path, you may need to execute it from within its directory (i.e. /usr/sbin/command)
2. Permission denied – Linux is configured to assign permissions to each user. You can only access directories and files within your permission scope. If you access something outside of that scope, you will get this error. The administrator or root user should have access to everything. If you still get “permission denied” even as root, the file may have 0 permissions and will need to be modified with the chmod command.
3. No route to host – Something is wrong with your network settings if you get this error. It means that you cannot connect the host (usually via SSH) that you are trying to reach. You might try running traceroute to track down the problem.
4. Connection refused – This error is different from “no route to host” in that the blame is usually on the server you are trying to reach (hopefully not your own). Either that server is down, does not have the protocol (such as SSH) that you are trying to use, or is blocking connections on that port.