, note
how man displays a screenful of text and then waits with a prompt --More--
at the bottom of the screen.
What man is doing
is sending everything it wants to display to the screen through a program known
as a ``pager' The pager program is called more. When you see
--More-- (in inverse video) at the bottom of the screen, just press the
space-bar to see the next screenful. Press <CR> to scroll a line at a
time.
Have you found the flag
yet? The -s flag should display the size in kilobytes. You don't need
to continue paging once you have found the information you need. Press q
and more will exit.
Listing File Sizes
Now type ls -as.
You can stack flags together like this-this tells ls to list all files,
even hidden files, and list their sizes in kilobytes.
Logging
Off
When you are finished you should
be sure to logout! You need to be careful that you've typed logout correctly.
The Unix operating system is not forgiving of mis-typed commands. Mis-typing
logout as ``logotu', pressing return and then leaving without glancing at
the screen can leave your files at anyone's mercy.
Directory
and File Structure
When you list files in Unix,
it is very hard to tell what kind of files they are. The default behavior of the
ls program is to list the names of all the files in the current directory
without giving any additional information about whether they are text files, executable
files or directories! This is because the ``meaning' of the contents of each
file is imposed on it by how you use the file. To the operating system a file
is just a collection of bytes.
There is a program
file which will tell you information about a file (such as whether it contains
binary data) and make a good guess about what created the file and what kind
of file it is.
File
Names
Unlike other operating systems,
filenames are not broken into a name part and a type part. Names can be many characters
long and can contain most characters. Some characters such as * and ! have special
meaning to the shell. They should not be used in filenames. If you ever do need
to use such a symbol from the shell, they must be specified sneakily, by ``escaping'
them with a backslash, for example \\!.
Directories
Directories in Unix start at
the root directory ``/'. Files are ``fully specified' when you list each directory
branch needed to get to them.
/usr/local/lib/news
/home/pamela/src/file.c
The ``File System' Tree
Structure
Usually disks are ``partitioned'
into smaller sized sections called partitions If one partition of the disk fills
up the other partitions won't be affected.
Only certain large directory
points are partitions and the choice of these points can vary among system managers.
Partitions are like the larger branches of a tree. Partitions will contain many
smaller branches (directories) and leaves (files).
The df Program
To examine what disks and partitions
exist and are mounted, you can type the df command at the %
prompt. This should display partitions which have names like /dev/sd3g---3
for disk 3, g for partition g. It will also display the space used and available
in kilobytes and the ``mount point' or directory of the partition.
Disk Space Maintenance
It's important to keep track
of how much disk space you are using. The command du displays the disk
usage of the current directory and all of its subdirectories. It displays the
usage, in kilobytes, for each directory-including any subdirectories it contains-and
ends by displaying the total.
-
% du
- display disk usage of
current directory
-
% du -s
- display only total disk
usage
-
% du -s -k
- some versions of Unix
need -k to report kilobytes
Scratch Space
Users have home directories
for storing permanent files. At various busy times of the year there may be shortages
of disk space on the Unix Cluster. You should use the du command to
stay aware of how much space you are using and not exceed the system limits.
Your Login
Directory
A login directory can always
be specified with ~username (~ is commonly called ``twiddle,' derived
from proper term ``tilde.') If you needed to list files in someone else's login
directory, you could do so by issuing the command:
% ls
~username
substituting in their username.
You can do the same with your own directory if you've cd'd elsewhere.
Please note-many people would consider looking at their files an invasion of their
privacy; even if the files are not protected! Just as some people leave their
doors unlocked but do not expect random bypassers to walk in, other people leave
their files unprotected.
Subdirectories
If you have many files or multiple
things to work on, you probably want to create subdirectories in your login directory.
This allows you to place files which belong together in one distinct place.
Creating Subdirectories
The program to make a subdirectory
is mkdir. If you are in your login directory and wish to create a directory,
type the command:
% mkdir
directory-name
Once this directory has been
created you can copy or move files to it (with the cp or mv
programs) or you can cd to the directory and start creating files there.
Copy a file from the current
directory into the new subdirectory by typing:
-
cp filename
directory-name/new-filename
- copy file, give it a
new name
-
cp filename
directory-name
- copy file, filename
will be the same as original
Or cd into the new
directory and move the file from elsewhere:
% cd directory-name
% cp ../filename .
copies the file from the directory
above giving it the same filename: ``.' means ``the current directory'
Specifying
Files
There are two ways you can
specify files. Fully, in which case the name of the file includes all of the root
directories and starts with ``/', or relatively, in which case the filename starts
with the name of a subdirectory or consists solely of its own name.
When Charlotte Lennox (username
lennox) created her directory arabella, all of the following
sets of commands could be used to display the same file:
% more lennox/arabella/chapter1
or
% cd lennox
% more arabella/chapter1
or
% cd lennox/arabella
% more chapter1
The full file specification,
beginning with a ``/' is very system dependent. On oceanography machines, all
user directories are in the /usra partition.
/usra/lennox/arabella/chapter1
Protecting
Files and Directories
When created, all files have
an owner and group associated with them. The owner is the same as the username
of the person who created the files and the group is the name of the creator's
default login group, such as users, system etc. Most users do not belong
to a shared group on our systems. If the creator of the file belongs to more than
one group (you can display the groups to which you belong with the groups
command) then the creator can change the group of the file between these groups.
Otherwise, only the root account can change the group of a file.
Only the root account can
change the ownership of a file.
Displaying owner, group
and protection
The command ls -lg
filename will list the long directory list entry (which includes owner
and protection bits) and the group of a file.
The display looks something
like:
protection owner group filename
-rw-r----- hamilton ug munster_village
The Protection Bits
The first position (which is
not set) specifies what type of file this is. If it were set, it would probably
be a d (for directory) or l (for link). The next nine positions
are divided into three sets of binary numbers and determine protection to three
different sets of people.
u g o
rw- r-- ---
6 4 0
The file has ``mode' 640.
The first bits, set to ``r + w' (4+2) in our example, specify the protection
for the user who owns the files (u). The user who owns the file can read or write
(which includes delete) the file.
The next trio of bits,
set to 4, or ``r,' in our example, specify access to the file for other users
in the same group as the group of the file. In this case the group is ug-all
members of the ug group can read the file (print it out, copy it, or display
it using more).
Finally, all other users
are given no access to the file.
The one form of access
which no one is given, even the owner, is ``x' (for execute). This is because
the file is not a program to be executed-it is probably a text file which would
have no meaning to the computer. The x would appear in the 3rd position and
have a value of 1.
Changing the Group and
the Protection Bits
The group of a file can be
changed with the chgrp command. Again, you can only change the group
of a file to a group to which you belong. You would type as follows:
% chgrp
groupname filename
You can change the protection
mode of a file with the chmod command. This can be done relatively or
absolutely. The file in the example above had the mode 640. If you wanted to make
the file readable to all other users, you could type:
% chmod 644 filename
or
% chmod +4 filename (since the current mode of the file was 640)
For more information see the
man page for chmod.
Default Protections: Setting
the umask
All files get assigned an initial
protection. To set the default initial protection you must set the value of the
variable umask. umask must be defined once per login (usually
in the .cshrc file). Common umask values include 022, giving read and
directory search but not write permission to the group and others and 077 giving
no access to group or other users for all new files you create.
The Unix
Shell Syntax
As mentioned earlier, user
commands are parsed by the shell they run. There are many shells other than the
the C shell which allow different types of shortcuts. We will only discuss the
C shell here, but some alternate shells include the Bourne shell ( /bin/sh),
the Bourne-Again Shell ( bash), zsh and tcsh (a C
shell variant).
The Path
One of the most important elements
of the shell is the path. Whenever you type something at the % prompt,
the C shell first checks to see if this is an ``alias' you have defined, and
if not, searches all the directories in your path to determine the program to
run.
The path is just a list
of directories, searched in order. Your default .cshrc will have a
path defined for you. If you want other directories (such as a directory of
your own programs) to be searched for commands, add them to your path by editing
your .cshrc file. This list of directories is stored in the PATH environment
variable. We will discuss how to manipulate enviroment variables later.
Flags and Parameters
Most commands expect or allow
parameters (usually files or directories for the command to operate on) and many
provide option flags. A ``flag' as we saw before, is a character or string with
a - before it-like the -s we used with the ls command.
Some commands, such as
cp and mv require file parameters. Not surprisingly,
cp and mv (the copy and move commands) each require two! One
for the original file and one for the new file or location.
It would seem logical that
if ls by itself just lists the current directory then cp
filename should copy a file to the current directory. This is logical-but
wrong! Instead you must enter cp filename . where the ``.'
tells cp to place the file in the current directory. filename
in this case would be a long filename with a complete directory specification.
Not surprisingly ls
. and ls are almost the same.
Creating
Files
The cat Program
cat is one of most
versatile commands. The simplest use of cat:
% cat .cshrc
displays your .cshrc
file to the screen. Unix allows you to redirect output which would otherwise go
to the screen by using a > and a filename. You could copy your
.cshrc, for example, by typing:
% cat .cshrc
> temp
This would have the same effect
as:
% cp .cshrc
temp
More usefully cat
will append multiple files together.
% cat .cshrc
.login > temp
will place copies of your
.cshrc and .login into the same file. Warning! Be careful not to
cat a file onto an existing file! The command:
% cat .cshrc
> .cshrc
will destroy the file
.cshrc if it succeeds.
If you fail to give
cat a filename to operate on, cat expects you to type in a file from the
keyboard. You must end this with a <Ctrl>-D on a line by itself. <Ctrl>-D
is the end-of-file character.
By combining these two-leaving
off the name of a file to input to cat and telling cat to
direct its output to a file with > filename, you can create
files.
For example:
% cat > temp
;klajs;dfkjaskj
alskdj;kjdfskjdf
<Ctrl>-D
%
This will create a new file
temp, containing the lines of garbage shown above. Note that this creates
a new file-if you want to add things on to the end of an existing file you must
use cat slightly differently. Instead of > you'd use
>> which tells the shell to append any output to an already existing file.
If you wanted to add a line onto your .cshrc, you could type
% cat >> .cshrc
echo \"blah blah blah\"
<Ctrl>-D
%
This would append the line
echo \"blah blah blah\" onto your .cshrc. Using >
here would be a bad idea-it might obliterate your original .cshrc file.
Text
Editors
cat is fine for files
which are small and never need to have real changes made to them, but a full fledged
editor is necessary for typing in papers, programs and mail messages. Among the
editors available pico, vi and emacs.
Be careful! Not all Unix
editors keep backup copies of files when you edit them.
pico
pico is a simple,
friendly editor--the same editor as used in pine. Type pico filename
to start it and type man pico for more information about how to use
it.
vi
vi is an editor which
has a command mode and a typing mode. When you first startup vi (with
the command vi filename) it expects you to enter commands.
If you actually want to enter text into your file, you must type the insert command
i. When you need to switch back to command mode, hit the escape key,
usually in the upper left corner of your keyboard.
To move around you must
be in command mode. You can use the arrow keys or use j, k, h, l to
move down, up, left and right.
For more information type
man vi. There are two reference sheets containing lists of the many
vi commands available from C&C (located at Brooklyn and Pacific).
Emacs
Emacs is a large editing system.
Copies of the manual are for sale at the CCO Front Desk and copies of the two-page
reference sheet are available in the reference sheet rack across from the Front
Office.
To use emacs,
type:
% setup emacs
% emacs
Files as
Output and Log Files
Ordinarily there are two types
of output from commands: output to standard output (stdout) and to standard error
(stderr). The > and >> examples above directed only
standard output from programs into files. To send both the standard output and
error to a file when using the C shell, you should type >& :
% command
>& filename
Logging
Your Actions to a File
Sometimes you may wish to log
the output of a login session to a file so that you can show it to somebody or
print it out. You can do this with the script command. When you wish
to end the session logging, type exit.
When you start up you should
see a message saying script started, file is typescript and when you
finish the script, you should see the message script done. You may
want to edit the typescript file-visible ^M's get placed at the end of each
line because linebreaks require two control sequences for a terminal screen
but only one in a file.
Comparing
Files
The basic commands for comparing
files are:
-
cmp
- states whether or not
the files are the same
-
diff
- lists line-by-line differences
-
comm
- three column output
displays lines in file 1 only, file 2 only, and both files
See the man pages on these
for more information.
Searching
Through Files
The grep program
can be used to search a file for lines containing a certain string:
% grep string filename
% grep -i string filename (case insensitive match)
or not containing a certain
string:
% grep -v
string filename
See the man page for grep---it
has many useful options.
more and the
vi editor can also find strings in files. The command is the same
in both-type a /string when at the --More-- prompt or in
vi command mode. This will scroll through the file so that the line
with ``string' in it is placed at the top of the screen in more or
move the cursor to the string desired in vi. Although vi
is a text editor there is a version of vi, view, which lets
you read through files but does not allow you to change them.
The System
and Dealing with Multiple Users
Most Unix commands which return
information about how much CPU-time you've used and how long you've been logged
in use the following meanings for the words ``job' and ``process.'
When you log in, you start
an interactive ``job' which lasts until you end it with the logout
command. Using a shell like C shell which has ``job-control' you can actually
start jobs in addition to your login job. But for the purposes of the most information
returning programs, job (as in the ``JCPU' column) refers to your
login session.
Processes, on the other
hand, are much shorter-lived. Almost every time you type a command a new process
is started. These processes stay ``attached' to your terminal displaying output
to the screen and, in some cases (interactive programs like text editors and
mailers) accepting input from your keyboard.
Some processes last a very
long time-for example the /bin/csh (C shell) process, which gets started
when you login, lasts until you logout.
Information
about Your Processes
You can get information about
your processes by typing the ps command.
PID TT STAT TIME COMMAND
9980 s9 S 0:06 -csh (csh)
12380 s9 R 0:01 ps
The processes executing above
are the C shell process and the ps command. Note that both commands
are attached to the same terminal (TT), have different process identification
numbers (PID), and have different amounts of CPU-time (TIME), accumulated.
Information
about Other People's Processes
who
The simplest and quickest information
you can get about other people is a list of which users are logged in and at which
``terminals' (terminal here is either a terminal device line or telnet or rlogin
session). The command to do this is who and it responds quickest of
all the commands discussed here because it simply examines a file which gets updated
everytime someone logs in or out.
Be careful though! This
file, utmp, can get out of date if someone's processes die unexpectedly
on the system. Any program which uses utmp to report information may
list users who are not really logged in!
w
The w command is
slower than the who command because it returns more information such
as details about what programs people are running. It also returns a line containing
the number of users and the system load average. The load average is the average
number of processes ready to be run by the CPU and is a rough way of estimating
how busy a system is.
w also uses the
utmp file mentioned above. It takes longer than who because
it then looks around and collects more information about the users it finds
in the utmp file.
ps
The ps command used
earlier to list your own processes can be used to list other users' processes
as well. who and w list logins-but not individual processes
on the system. They don't list any of the running operating system processes which
start when the computer is booted and which don't have logins.
Since ps doesn't
use utmp it is the program to use when you really want to find out
what processes you might have accidentally left on the system or if another
user is running any processes. Note that although ps might report
processes for a user, it might be because that user has left a ``background
job' executing. In this case you should see a ``?' in the TT field and the
user won't really be logged in.
To get this fuller listing,
give the flags -aux to ps. For more information on the uses
of ps, type man ps.
finger
The finger program
returns information about other users on the system who may or may not be logged
in. finger by itself returns yet another variation of the list of currently
logged in users. finger followed by a username or an e-mail -style address
will return information about one or more users, the last time they logged into
the system where you are fingering them, their full name, whether or not they
have unread mail and, finally, the contents of two files they may have created:
.plan and .project
For more information about
using finger or ways to provide information about yourself to others,
type man finger.
Sending
Messages and Files to Other Users
Electronic mail programs run
on almost all the computers at Caltech and usually have two parts: a user interface
which lets users read and send messages and a system mailer which talks to mailers
on other computers. This mailer receives outgoing messages from the user interface
programs and delivers incoming messages to the user mailbox (which the interface
program reads).
/usr/ucb/mail
There are many user interfaces
available on the Unix computers, all of which provide similar functionality. The
program supplied with most Unix computers is /usr/ucb/mail (or
Mail). To read messages type Mail, to send messages type:
% Mail
address
Mail has been changed to
mailx.
You should next see a
Subject: prompt. If you don't see a prompt, don't worry, just type in your
one line subject anyway and press return. You may start typing your message
(but you will be unable to correct errors on lines after you have pressed <CR>
to move to the next line) or you may may specify a file to include with
r filename.
You may invoke a text editor
like vi by typing v. If you wish regularly to use an editor
other than vi you should see the information later in this section
about enviroment variables.
There are many other commands
you may enter at this point-see the Mail man page for all of them.
When you are finished typing in your message (if you have used v to
run a text editor, you should exit from it) press <Ctrl>-D on a line by
itself. Most likely you will now see a CC: prompt. If you wish to
send copies of your message to someone besides the recipient you would enter
the address or addresses (separated by ``,') and press return. Otherwise press
return without entering an address.
PINE
PINE is a full-screen interactive
mailer, developed at UW, that is very straightforward to use. To use it type
pine. More information
is available from the UW C&C web server.
Write
The write program
can be used to send messages to other users logged onto the system. It's not a
great way of having a conversation, but it's simple to use. Enter:
% write
username
and you can start writing lines
to the terminal of the person you want to send messages to. The person must be
logged in, and, if they are logged in more than once, you must specify the terminal
to write to-for example write melville ttyh1.
Talk
talk is a program
which allows two users to hold a conversation. Unlike write, it can
be used between different computers; and, unlike write, it divides the
screen so that the things you type appear in the top half and the things written
to you appear in the bottom half.
To talk to users
on the same computer:
% talk
username
To talk to users
on another computer use the address format of username@nodename:
% talk brunton@jarthur.claremont.edu
Addressing
Remote Nodes
talk can only be
used to other Internet nodes-computers which usually have ending names such as
.edu, .com, .org, .gov, or .mil. Not all computers with these names are attached
directly to the Internet--- finger and talk won't work with
computers which are only attached by mail gateways.
Shortcuts
If you use certain command
flags regularly ( -lga for ls) you can alias them
to shorter commands. You can use wildcard symbols to refer to files with very
long names. You can easily repeat commands you have already executed or modify
them slightly and re-execute them.
Aliases
As mentioned above, you can
alias longer commands to shorter strings. For example, ls -F
will list all the files in the current directory followed by a trailing symbol
which indicates if they are executable commands (a *) or directories (a /). If
you wanted this to be the default behavior of ls you could add the following command
to your .cshrc:
% alias ls
ls -F
To list the aliases which are
set for your current process, type:
% alias
without any parameters.
Wildcards
Wildcards are special symbols
which allow you to specify matches to letters or letter sequences as part of a
filename.
Some examples:
- *
- The basic wildcard character.
Beware rm *!!
-
ls *.dat
- lists all files
ending in .dat
-
ls r*
- lists all files
starting with r
- ?
- a one character wildcard.
-
ls ?.dat
- lists
5.dat,
u.dat, but not 70.dat
- []
- limits a character to
match one of the characters between the brakets
-
ls *.[ch]
- lists all
.h
and .c files
-
more [Rr][Ee][Aa][Dd][Mm][Ee]
-
mores
the files README, readme,ReadMe,
and Readme, among others
Directory
Specifications
You've already met the shortcut.
The two other important directory symbols are ``.' for the current directory
and ``..' for the previous (parent) directory.
% cd ..
moves you out of a subdirectory
into its parent directory.
Environment
Variables
Environment variables are pieces
of information used by the shell and by other programs. One very important one
is the PATH variable mentioned earlier. Other important variables you can set
include:
To see what environment variables
are set and what they are set to, type the command printenv. To set
a variable, use the setenv command as in the example below.
% setenv TERM vt100
% setenv EDITOR emacs
Many programs mention environment
variables you may want to set for them in their man pages. Look at the csh
man page for some of the standard ones.
History
Most shells allow ``command
line editing' of some form or another-editing one of the previous few lines you've
typed in and executing the changed line. You can set a history ``environment variable'
to determine how many previous command lines you will have access to with
set history=40
Repeating and Modifying the Previous Command
The simplest form of command line editing is to repeat the last command
entered or repeat the last command entered with more text appended.
If the last command you
typed was:
% ls agreen
Then you can repeat this command
by typing:
% !!
This will return a list of
files. If you saw a directory leavenworth in the list returned and you
wanted to list the files it contained, you could do so by typing:
% !!/leavenworth
If you mistype leavenworth
as leaveworth you can correct it with the following command:
% ^leave^leaven
This substitutes leaven for leave in the most recently executed command. Beware!
This substitutes for the first occurrence of leave only!
Repeating Commands From Further Back
in History
You can type history at any time to get a list of all the
commands remembered. This list is numbered and you can type ! number
to repeat the command associated with number. Alternately you can type
! and a couple of letters of the command to repeat the last line starting
with the characters you specify. !ls to repeat ls -lg
agreen, for example.
The
.login and .cshrc Files
The .cshrc file is
run whenever a C shell process is started. Then, if this is a login process, the
.login file is executed. If you are using a NeXT console with a program
such as Terminal, you can usually choose whether you want each new window to execute
the .login file by making a change to your Preferences in the Terminal
program's Preferences menu. By default the .login will get executed.
If you are using a Sun
console and you have the default setup, any xterm windows which you start up
will not execute the .login.
Job Control
It is very easy to do many
things at once with the Unix operating system. Since programs and commands execute
as independent processes you can run them in the ``background' and continue on
in the foreground with more important tasks or tasks which require keyboard entry.
For example, you could
set a program running in the background while you edit a file in the foreground.
The fg and
bg Commands
When you type <Ctrl>-Z
whatever you were doing will pause. If you want the job to go away without finishing,
then you should kill it with the command kill %. If you don't want it
paused but want it to continue in the foreground-that is, if you want it to be
the primary process to which all the characters you type get delivered-type
fg. If you want it to continue processing in the background while you work
on something else, type bg.
You should not use
bg on things which accept input such as text editors or on things which
display copious output like more or ps.
What to Do When You've Suspended Multiple Jobs
If you've got several processes stopped-perhaps you are editing two
files or you have multiple telnet or rlogin sessions
to remote computers-you'll need some way of telling fg which
job you want brought to the foreground.
By default fg
will return you to the process you most recently suspended. If you wanted to
switch processes you would have to identify it by its job number. This number
can be displayed with the jobs command. For example:
% jobs
[1] Stopped vi .login
[2] + Stopped rn
[3] Running cc -O -g test.c
%
The most recently suspended
job is marked with a + symbol. If you wanted to return to job one instead, you
would type:
% fg %1
You can type %1 as
a shortcut.
Starting
Jobs in the Background
Some jobs should start in the
background and stay there-long running compilations or programs, for example.
In this case you can direct them to the background when you start them rather
than after they have already begun. To start a job in the background rather than
the foreground, append an & symbol to the end of your command.
You should always run background
processes at a lower priority by using the nice command. Non-interactive
jobs are usually very good at getting all the resources they need. Running them
at a lower priority doesn't hurt them much-but it really helps the interactive
users-people running programs that display to terminal screens or that require
input from the keyboard.
If you need to run CPU-intensive
background jobs, learn about how to control the priority of your jobs by reading
the manual pages (man nice and man renice).
Suspend, z and <Ctrl>-Z
Some programs provide you with
special ways of suspending them. If you started another shell by using the
csh command, you would have to use the suspend command to suspend
it.
If you wish to suspend
a telnet or rlogin session you must first get past the current
login to get the attention of the telnet or rlogin program.
Use (immediately after
pressing a return) to get rlogin's attention. <Ctrl>-Z will
suspend an rlogin session.
Use <Ctrl>-] to get
telnet's attention <Ctrl>-]z will suspend a telnet session.
Watch out, though, if you are connected from a PC with through Kermit! <Ctrl>-]
is Kermit's default escape sequence. You'll need to type <Ctrl>-]
z or define Kermit's escape sequence to something else such as <Ctrl>-K.
Some
Common and Useful Unix Commands For Files
cp
The cp command allows
you to create a new file from an existing file. The command line format is:
% cp
input-file-spec output-file-spec
where input-file-spec
and output-file-spec are valid Unix file specifications. The file specifications
indicate the file(s) to copy from and the file or directory to copy to (output).
Any part of the filename may be replaced by a wildcard symbol (*) and you may
specify either a filename or a directory for the output-file-spec. If
you do not specify a directory, you should be careful that any wildcard used in
the input-file-spec does not cause more than one file to get copied.
% cp new.c old.c
% cp new.* OLD (where OLD is a directory)
ls
command allows the user to
get a list of files in the current default directory. The command line format
is:
% ls
file-spec-list
where file-spec-list
is an optional parameter of zero or more Unix file specifications (separated by
spaces). The file specification supplied (if any) indicates which directory is
to be listed and the files within the directory to list.
Info-x : Info-x Tutorials and Documents : Tutorials
Topic: Unix Tutorial
Topic: Unix Tutorial
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