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TUCoPS :: Unix :: General :: bestunix.txt

Best UNIX Commands & Tips

             ***  My  Favourite  UNIX  Commands  ***
          *** A List Of Some OF The Most Useful UNIX **
     *** Hacking Commands, and Some Hints On Their Usage ***
         ***  Written By ZeeBee Australia Inc. 1990 ***

     Ok UNIX freaks, here is a little list of helpful UNIX 
commands and procedures which can be of much assistance in 
gaining the fullest potential from any UNIX system.  


     It is fun and often usefull to create a file that is owned 
by someone else.  On most systems with slack security ie 99% of 
all UNIX systems, this is quite easily done.  The chown command 
will change any of your files to make someone else the owner. 
Format is as follows:

chown ownername filelist

 Where ownername is the new owner, and filelist is the list of 
files to change.  You must own the file which your are goin to 
change, unless you are a superuser....then u can change ANYTHING!
     chgrp is a similar command which will change the group 
ownership on a file.  If you are going to do both a chown and a 
chgrp on a file, then make sure you do the chgrp first!  Once the 
file is owned by someone else, you cant change nything about it!


     Sometimes just seeing who is on the system is a challenge in 
itself.  The best way is to write your own version of who in C, 
but if you can't do that then this may be of some help to you:

     who   followed by on or more of the following flags:

          -b Displays time sys as last booted.
          -H Precedes output with header.
          -l Lists lines waiting for users to logon.
          -q displays number of users logged on.
          -t displays time sys clock was last changed.
          -T displays the state field (a + indicates it is 
possible to send to terminal, a - means u cannot)
          -u Give a complete listing of those logged on.

   **who -HTu is about the best choice for the average user**

##by the way, the list of users logged on is kept in the file 
/etc/utmp.  If you want to write your own personalised version of 
who in C, you now know where to look!###


     When a users state field (see -T flag option for who 
command) says that a user has their message function on, this 
actually means that it is possible to get stuff onto their 
     Basically, every terminal on the system has a file 
corresponding to it.  These files can be found in the /dev 
directory.  You can to anything to these files, so long as you 
have access -eg you can read them, and write to them, but you 
will notice that they never change in size.  They are called 
character specific files, and are really the link between the 
system and the terminals.  Whatever you put in these files will 
go staright to the terminal it corresponds to.
     Unfortunately, on most systems, when the user logs in, the 
"mesg n" command is issued which turns off write access to that 
terminal, BUT- if you can start cating to that terminal before 
system issues the mesg n command, then you will continue to be 
able to get stuff up on that terminal! This has many varied uses.

     Check out the terminal, or terminal software being used.  
Often you will be able to remotely program another users 
terminal, simply by 'cating' a string to a users screen.  You 
might be able to set up a buffer, capturing all that is typed, or 
you may be able to send the terminal into a frenzy- (sometimes a 
user will walk away without realizing that they are sill 
effectively logged on, leaving you with access to their 
account!).  Some terminal types also have this great command 
called transmit screen. It transmits everything on the screen, 
just as if the user had typed it !
     So just say I wanted to log off a user, then I would send a 
clear screen command (usually ctrl l), followed by "exit" 
followed by a carriage return, followed by the transmit screen 
code.  Using ths technique you can wipe peoples directories or 
anything.  My favourite is to set open access on all their files 
and directories so I can peruse them for deletion etc at my own 


     If you ever briefly get access to another persons account 
eg. they leave the room to go to toilet or whatever, then simply 
type the following:

chmod 777 $HOME
chmod 777 $MAIL

     Then clear the screen so they dont see what you just typed.

     Now you can go look at their directory, and their mail, and 
you can even put mail in their mail file. (just use the same 
format as any mail that is already there!). Next time they log in 
the system will automatically inform them they have new mail!


     Another way to send fake mail to people is to use the mail 
server.  This method produces mail that is slightly different to 
normal, so anyone who uses UNIX a bit may be suspiscious when 
they receive it, but it will fool the average dumb fuck user!

type telnet

the following prompt will appear:


now type :

open localhost 25

some crap will come up about the mail type:

mail from: xxxxxx     Put any name you want.

some more bullshit will come up. Now type:

rcpt to: xxxxxx  Put the name of the person to receive mail here.

now type:


now you can type the letter...end it with a "."
type quit to exit once you are done.


     Heres one for any experimenters out there...
It is possible to create files which simply cannot be deleted 
from the standard shell.  To do this you will have to physically 
have to use a sequence of control characters which cannot be 
typed from the shell.  Try things like Ctrl-h (this is the 
code for the delete key).  Just a file with the name Ctrl-h would 
not be deleteable from the shell, unless you used wildcards. So, 
make it a nice long series of characters, so that to delete the 
file, the user has no choice but to individually copy all his 
files elsewhere, then delete everything in his directory, and 
then copy all his files back.....this is one of my 
favourites..gets em every time!
     The following script file is an example which will create a 
file with the name Ctrl-h.  You MUST tyoe this file in using the 
vi editor or similar.
*****If you are not very good with vi, type "man vi" and print the 
help even contains stuff that I find useful now and 

type the following in vi...

echo'' > 'a^h'

  *** get the ^h (this really means ctrl-h) from vi type:

Ctrl v
Ctrl h 

  The Ctrl v instrcts vi to take the next character as a ascii 
character, and not to interpret it.
     change the access on the file you just created and now 
execute it.  It will create a file which looks like it is called 
a, but try to delete it !..use wildcards if you really want to 
delete it. 


Watch for more in the ZeeBee UNIX (ab)usage series.

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