Bash Functions

A fork bomb is really just a bash function, below is an example of a bash function

helloworld() {
echo hello world
};

A fork bomb would be

:(){
 :|:&
};:

Now to explain.
:(){ - Creates the function
:|: - Next it call itself using recursion and pipes the output to another call of the function
& - Puts the function call in the background so child cannot die
}; - Terminate the function
: - Call (run) the function

If you would like a more human readable fork bomb it would be as follows

forkbomb(){
forkbomb | forkbomb &
}; forkbomb

Preventing Fork Bombs

We can prevent users from running fork bombs by limiting the amount of processes they are allowed to run.
We can achive this using /etc/security/limits.conf

To get started open /etc/security/limits.conf

nano /etc/security/limits.conf

In my example I want to limit the user john to 300 processes and any users in the group of students to 250 processes. For this I would put the following into my config file

john hard nproc 300
@students hard nproc 250

Please keep in mind that KDE and Gnome desktop system can lanuch many processes.

echo 'export HISTTIMEFORMAT="%h/%d - %H:%M:%S "' >> ~/.bashrc

Then reload your .bashrc

source ~/.bashrc

Now when you run history you should get a output similar to the one below

   502  Aug/12 - 13:48:31 tail -f /home/duffy/log/access.log
  503  Aug/12 - 13:49:01 echo hello
  504  Aug/12 - 13:49:24 more /var/log/syslog

Some other bash history tips

1. The best way of finding something quickly in your bash history is simply by pressing ctrl+r and then typing out the start of the command it will auto complete it with any matches found in your bash history
2. If you don't want to save duplicate commands in your bash history simply add export HISTCONTROL=ignoreboth to your .bashrc

   echo 'export HISTCONTROL=ignoreboth' >> ~/.bashrc

3. If you want to change the lenght of history add export HISTSIZE=1000 to your .bashrc

   echo 'export HISTSIZE=1000' >> ~/.bashrc

nano ~/.bashrc

Then come to the bottom of that file and add your aliases like this:

alias rm = "rm -i"
alias lightyrestart = "/etc/init.d/lighttpd restart"
alias server_name="ssh -v -l USERNAME IP ADDRESS"

Then Reload your .bashrc file and your aliases should be enabled

source ~/.bashrc