The permission denied error (code 13) warns you that your request cannot be carried out by the system, since your user/group IDs do not give you the required privileges. For instance, consider this file in my home directory :
$ ls -l test.txt
-rw-r----- 1 me mygroup 0 Jul 1 03:54 test.txt
rw-r----- part states that the owner (
me) can perform read/write operations on the file, while members of the
mygroup group can read it. Others can't do anything.
In order to read this file, you'll type :
$ cat test.txt
Yet, if you're not
me, and don't belong to the
mygroup group, you'll get a beautiful
Permission denied (13) error.
If you want to know more about the UNIX permissions system, have a look here, here or here. Note that this system applies to all programs (commands) on your system. Any program trying to read
test.txt must be running as
me, or someone in the
mygroup group in order to succeed (otherwise, the same error will occur).
When performing administration tasks such as package management or system configuration, it is usually necessary to have
root privileges. These tasks require access to files owned by
root, or the execution of kernel-tasks that can only be carried out with super-user privileges. On Ubuntu, you can run a command as
$ sudo mycommand
However, this requires you to be registered as a sudoer, which will probably be the case if you're the first or only user on that machine. More about sudo.