Reverse command of cat is tac, head is tail, cut is paste...

cat filename shows the content of the file.
But "tac"? What it does?
It displayes the file in reverse order.
So funny!! Why they had implemented so??

Try "head & tail" command and "cut & paste" command.
For details see man pages!!

Accessing a file whose first character of the filename is "-"

A file with name "-file.txt" can not be accessed by the commands like "cat -file.txt" or "vi -file.txt".

But can be accessed using "cat ./-file.txt" or "vi ./-file.txt"
So if anynoe does not know the meaning of "./" will not be able to access the file!!
Its really funny!!!

Adding an admin account in Linux/Fedora

Multiple "Admin" account is possible in Linux.

First create an account & set the password.
Commnad: useradd -g users admin1 [Create a user with name admin1 in users group].

Now edit the corresponding line in /etc/passwd file.
Put zero in third field(user id) and zero in forth field(group id).
Save the file

Now log on to the system with this new account.
You will get "#" prompt!!

Limit number of telnet connection from a client

You can specify how many telnet connections are allowed to a telnet server from a host.
This can be done using the Firewall tool "iptables" and its module "connlimit".

The configuration for doing this is:

iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 3 -j REJECT

The above command will allow only 3 concurrent telnet connection from a host.