I want rm all the files in the directory, not starting with the letter I or N - what is the easiest way to do this in bash?
You can do the following:
rm [^IN]*
[^IN] is a pattern that matches any character except I or N - this syntax is described in the Matching Pattern section of the bash manual.
[^IN]
I
N
Another way:
find . -maxdepth 1 -type f -name "[^NI]*" -delete
Obviously, this option is worse;)