Which of these default users can I delete?

bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin
operator:x:11:0perator:/root:/sbin/nologin
rpm:x:37:37::/var/lib/rpm:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
avahi:x:70:70:Avahi daemon:/:/sbin/nologin
vcsa:x:69:69:virtual console memory owner:/dev:/sbin/nologin
haldaemon:x:68:68:HAL daemon:/:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
pcap:x:77:77::/var/arpwatch:/sbin/nologin

Why do you want to delete them? They are all important system users and deleting them may have undesirable effects on your system.

That partially depends on what services you are willing to do without, or compromized. Example, "sshd" could be removed if you really don't intend to have an ssh server running. It could be configured to use another account instead, but it is there for a reason. There are too many things done as "nobody" on too many systems.

Why would you want to delete them anyways? You could try locking them with "usermod -L" and see what breaks? ("usermod -U" to unlock) But even then I wouldn't delete them.

I would advise against removing any of these entries. "You know not what you do."

As you see, nearly all of these entries are /sbin/nologin. That's a dummy "shell" that simply prints an error-message that "you can't log in." (You can actually run this program at the command-line to see what it says.) Because of this fact, indeed, no one can ever "log in" using any of these IDs.

(The shutdown and sync "users" are operator-convenience tools that would allow system-operators to be able to do things by "logging on." Rarely used.)

So I could remove those 2?

The complete answer is: no.

Why would you want to delete system users?

It won't make your computer more secure, it won't make your computer faster --
You might just break someting if you don't know what you are doing.

So why bother?

Personally i think he should delete them one by one. His system will be totally trashed, but he will learn through experience, not to do it again.

Its all fine to say dont do it, but he doesnt learn the lesson unless he does it.

if ur attempting to make ur system faster there are better more sensible ways of doing it like using smaller programs, also removing unused programs also closing unused start up programs. there are stacks of tweaks but removing default user is not one of them, they are there for a reason.

Sorry, but I disagree. If that's the only way to learn, then why have a place like LQ at all? Are you saying all the people who volunteer their time and effort helping others here are useless??

You are right in that his system most likely will be trashed. That's why he considered asking first, and that's why so many friendly, caring people have bothered to respond with the answers that you see above. That's why we're here.