I like Chaotic42's list the best from what I've seen here .
Of course even a noob really needs more than 10 commands to do anything useful.
I would make some alterations though to his list of 10.
I added some comments or changes in bold below to hopefully further refine it.
Originally posted by: Chaotic42
ls: You have to be able to see what your files are.
cp: You have to be able to copy your files.
grep: Searching is a must.
cat: See grep.
mv: Moving files.
rm: Removing files.
vi/pico/etc: You need some kind of editor.
vi: Pretty much every *nix that I've ever used has vi on it. From old dynix and sco boxes, to modern solaris, aix, and linux installs. It's certainly nice to know other editors too, but a fundamental working understanding of vi is absolutely essential for anyone who is a UNIX admin of any magnitude.
ps: You need to see what's going on.
Agreed, also it's good to just say "ps" because of the variance in syntax across different systems. "ps -ef" vs "ps -aux" for example
su: You have to switch users. (and know the difference between su username and su - username)
tar: Everyone has to archive things.
Edit: You really need more than 10.
Also, in addition to the list ... knowing to look at logs with "tail" will make life easier.
df ... is of great importance as it's a good idea to know how full your drive partitions are and how many inodes you have available to use.
It's essential to understand file permissions.
chmod ... It's important to know how to change them.
chown ... It's important to know how to change file ownership
man ... It's nice to have a good solid manual
kill ... you're going to need to kill stuff from time to time
nohup NAME_OF_SCRIPT_OR_BINARY& ... If you need to start up a script or binary that you want to run in the background and not die if you end your session, this is good to know.
passwd ... important to know, however, it's advisable that the policy you have on the server be set up so that passwords expire on their own in 60 days or less, and that way you will automatically get prompted to change your password every once in a while.
adduser ... you will obviously need to know how to use this as an admin, but it's not something you're going to need to use a lot until AFTER you have a good understanding of all the fundamentals.
screen ... as others has mentioned is a handy tool, but it's not installed by default by vendors for a good nuwber of unix servers.
touch ... this is usefull as it's a very easy and quick way to either create a 0 byte file with a certain name or update the timestamp on a file
shutdown ... pretty much all servers need some downtime, it's much more desirable to have scheduled downtime for maintenance at a time when system usage is low rather then unscheduled outages due to lack of maintenance. Thus, you need to know how to shutdown correctly. It's also pretty much a requirement to reboot when you install a new kernel.
things like pkgtool, apt-get, alien, and the like only apply to specific Distributions of Linux. While it may be extremely convenient to know apt-get if you run Debian, it's not useful at all for any distros that can't use it.
In other words, you will need to learn what's specific to "my own personal distribution" and what's more "general Unix/Linux knowledge."
Ubuntu is a great distro to work with. I have played with it a bit, but I run Slackware on my home server and a server I have at work, (though I also have a MythTV box running Knoppmyth at home, and I support batch jobs on about a dozen servers at work including, but not limited to : AIX, Dynix, Solaris, SCO, OpenBSD, and FreeBSD.)
I have only been using Linux or Unix for about 8 years, and even then, I've only been using it heavily for about the last 3 years, so I am not a hardcore guru of any sort, but I am able to do pretty much everything I want to do in Linux or Unix with a decent degree of efficiency.