Whether it is user intention or just accident it may happen, that a single user can eat up all available system resources such as RAM memory or disk space. Depends on the nature of you Linux system you may want to limit your users to only what they might actually need.

Let's start with something like a fork bomb:

:(){ :|:& };:

The line above can almost instantly consume all resources since it creates recursive function all to it self as it forks unlimited children processes. One does not even need a root privileges to crash your Linux system. What about to limit user by a number of process he/she can spawn:

NOTE: All limits are applied to a current bash shell session only. To make a permanent change system wide use /etc/profile .

$ ulimit -u 10
$ :(){ :|:& };:
bash: fork: retry: Resource temporarily unavailable

This takes care of the fork bomb problem. But what about disk space? Linux command ulimit can limit users to create files bigger than a certain size:

$ ulimit -f 100
$ cat /dev/zero > file
File size limit exceeded (core dumped)
$ ls -lh file
-rw-rw-r--. 1 linux commands 100K Feb 21 18:27 file 

Some extreme examples:

With ulimit it is also possible to limit a maximum amount of virtual memory available to the process:

ulimit -v 1000
[lilo@localhost ~]$ ls
ls: error while loading shared libraries: libc.so.6: failed to map segment from shared object: Cannot allocate memory

Limit a user by number of opened files ( file descriptors )

$ ulimit -n 0
$ echo ulimit > command
bash: command: Too many open files

To check all your current limits use -a option:

$ ulimit -a