There are a couple of issues here, presented in descending order of importance
You're already logged in as root
, so you don't need to use sudo
(or its equivalents). You're already root! All you need to do is run gedit /etc/default/grub
.
However, if this is a server, you probably don't have a GUI running at all. That means that graphical applications like gedit
won't be able to run and you need a command line editor instead. The simplest of those is nano
:
nano /etc/default/grub
The best of them is emacs
although some poor misguided people1 insist on using a thing called vim
.
If you are running a GUI on your server, this could be happening for a variety of reasons, depending on how you're connecting to it. Please have a look at my answer here, I go through most of them.
1 This should not be taken seriously. That's a long running gag in the *nix community, vim
is an excellent piece of software, I just happen to be in the emacs
camp. We get pretty worked up about which is best but, in all honesty, they're both great.
vim
:P