UNIX Tutorial for Beginners

UNIX Tutorial for Beginner

Here you go with the tutorial.

1. Introduction to the UNIX operating system

a.   What is UNIX?

UNIX is an operating system which was first developed in the 1960s, and has been under constant development ever since. By operating system, we mean the suite of programs which make the computer work. It is a stable, multi-user, multi-tasking system for servers, desktops and laptops.

UNIX systems also have a graphical user interface (GUI) similar to Microsoft Windows which provides an easy to use environment. However, knowledge of UNIX is required for operations which aren't covered by a graphical program, or for when there is no windows interface available, for example, in a telnet session.

b.  Files and Processes

Everything in UNIX is either a file or a process.

A process is an executing program identified by a unique PID (process identifier).

A file is a collection of data. They are created by users using text editors, running compilers etc.

Examples of files:

·         a document (report, essay etc.)

·         the text of a program written in some high-level programming language

·         instructions comprehensible directly to the machine and incomprehensible to a casual user, for example, a collection of binary digits (an executable or binary file);

·         a directory, containing information about its contents, which may be a mixture of other directories (subdirectories) and ordinary files.

c.   The Directory Structure

All the files are grouped together in the directory structure. The file-system is arranged in a hierarchical structure, like an inverted tree. The top of the hierarchy is traditionally called root (written as a slash / )

In the diagram above, we see that the home directory of the undergraduate student "ee51vn" contains two sub-directories (docs and pics) and a file called report.doc.

The full path to the file report.doc is "/home/its/ug1/ee51vn/report.doc"

2. Section One

a.   Listing files and directories

ls (list)

When you first login, your current working directory is your home directory. Your home directory has the same name as your user-name, for example, ee91ab, and it is where your personal files and subdirectories are saved.

To find out what is in your home directory, type

% ls

The ls command (lowercase L and lowercase S) lists the contents of your current working directory.

There may be no files visible in your home directory, in which case, the UNIX prompt will be returned. Alternatively, there may already be some files inserted by the System Administrator when your account was created.

ls does not, in fact, cause all the files in your home directory to be listed, but only those ones whose name does not begin with a dot (.) Files beginning with a dot (.) are known as hidden files and usually contain important program configuration information. They are hidden because you should not change them unless you are very familiar with UNIX!!!

To list all files in your home directory including those whose names begin with a dot, type

% ls -a

As you can see, ls -a lists files that are normally hidden.

ls is an example of a command which can take options: -a is an example of an option. The options change the behavior of the command.

b.  Making Directories

mkdir (make directory)

We will now make a subdirectory in your home directory to hold the files you will be creating and using in the course of this tutorial. To make a subdirectory called Unix stuff in your current working directory type

% mkdir unixfolder

To see the directory you have just created, type

% ls

c.   Changing to a different Directory

cd (change directory)

The command cd directory means change the current working directory to 'directory'. The current working directory may be thought of as the directory you are in, i.e. your current position in the file-system tree.

To change to the directory you have just made, type

% cd unixfolder

Type ls to see the contents (which should be empty)

Exercise 1a

Make another directory inside the unixfolder directory called backups

d.  The directories. and..

Still in the unixfolder directory, type

% ls -a

As you can see, in the unixfolder directory (and in all other directories), there are two special directories called (.) and (..)

The current directory (.)

In UNIX, (.) means the current directory, so typing

% cd .

NOTE: there is a space between cd and the dot

means stay where you are (the unixfolder directory).

This may not seem very useful at first, but using (.) as the name of the current directory will save a lot of typing, as we shall see later in the tutorial.

The parent directory (..)

(..) Means the parent of the current directory, so typing

% cd ..

will take you one directory up the hierarchy (back to your home directory). Try it now.

Note: typing cd with no argument always returns you to your home directory. This is very useful if you are lost in the file system.

e.   Pathnames

pwd (print working directory)

Pathnames enable you to work out where you are in relation to the whole file-system. For example, to find out the absolute pathname of your home-directory, type cd to get back to your home-directory and then type

% pwd

The full pathname will look something like this -

/home/its/ug1/ee51vn

which means that ee51vn (your home directory) is in the sub-directory ug1 (the group directory),which in turn is located in the its sub-directory, which is in the home sub-directory, which is in the top-level root directory called " / " .

Exercise 1b

Use the commands cd, ls and pwd to explore the file system.

(Remember, if you get lost, type cd by itself to return to your home-directory)

f.    More about home directories and pathnames

Understanding pathnames

First type cd to get back to your home-directory, then type

% ls unixfolder

to list the contents of your unixfolder directory.

Now type

% ls backups

You will get a message like this -

backups: No such file or directory

The reason is, folder backups is not in your current working directory. To use a command on a file (or directory) not in the current working directory (the directory you are currently in), you must either cd to the correct directory, or specify its full pathname. To list the contents of your backups directory, you must type

% ls unixfolder/backups

~ (your home directory)

Home directories can also be referred to by the tilde ~ character. It can be used to specify paths starting at your home directory. So typing

% ls ~/unixfolder

will list the contents of your unixfolder directory, no matter where you currently are in the file system.

Exercise:

% ls ~

Would list?

% ls ~/..

Would list?

Summary

Command	Meaning
ls	list files and directories
ls –a	list all files and directories
mkdir	make a directory
cd directory	change to named directory
cd	change to home-directory
cd ~	change to home-directory
cd ..	change to parent directory
pwd	display the path of the current directory

3. Section Two

a.   Copying Files

cp (copy)

cp file1 file2 is the command which makes a copy of file1 in the current working directory and calls it file2

What we are going to do now, is to take a file stored in an open access area of the file system, and use the cp command to copy it to your unixfolder directory.

First, cd to your unixfolder directory.

% cd ~/unixfolder

Then at the UNIX prompt, type,

% cp /vol/examples/tutorial/science.txt .

Note: Don't forget the dot . at the end. Remember, in UNIX, the dot means the current directory.

The above command means copy the file science.txt to the current directory, keeping the name the same.

(Note: The directory /vol/examples/tutorial/ is an area to which everyone in the school has read and copy access. If you are from outside the University, you can grab a copy of the file here. Use 'File/Save As..' from the menu bar to save it into your unixfolder directory.)

Exercise 2a

Create a backup of your science.txt file by copying it to a file called science.bak

b.  Moving Files

mv (move)

mv file1 file2 moves (or renames) file1 to file2

To move a file from one place to another, use the mv command. This has the effect of moving rather than copying the file, so you end up with only one file rather than two.

It can also be used to rename a file, by moving the file to the same directory, but giving it a different name.

We are now going to move the file science.bak to your backup directory.

First, change directories to your unixfolder directory (can you remember how?). Then, inside the unixfolder directory, type

% mv science.bak backups/.

Type ls and ls backups to see if it has worked.

c.   Removing Files and directories

rm (remove), rmdir (remove directory)

To delete (remove) a file, use the rm command. As an example, we are going to create a copy of the science.txt file then delete it.

Inside your unixfolder directory, type

% cp science.txt tempfile.txt

% ls

% rm tempfile.txt

% ls

You can use the rmdir command to remove a directory (make sure it is empty first). Try to remove the backups directory. You will not be able to since UNIX will not let you remove a non-empty directory.

Exercise 2b

Create a directory called tempfolder using mkdir , then remove it using the rmdir command.

d.  Displaying the contents of a file on the screen

clear (clear screen)

Before you start the next section, you may like to clear the terminal window of the previous commands so the output of the following commands can be clearly understood.

At the prompt, type

% clear

This will clear all text and leave you with the % prompt at the top of the window.

cat (concatenate)

The command cat can be used to display the contents of a file on the screen. Type:

% cat science.txt

As you can see, the file is longer than the size of the window, so it scrolls past making it unreadable.

less

The command less writes the contents of a file onto the screen a page at a time. Type

% less science.txt

Press the [space-bar] if you want to see another page, and type [q] if you want to quit reading. As you can see, less is used in preference to cat for long files.

head

The head command writes the first ten lines of a file to the screen.

First clear the screen then type

% head science.txt

Then type

% head -5 science.txt

What difference did the -5, to the head command?

tail

The tail command writes the last ten lines of a file to the screen.

Clear the screen and type

% tail science.txt

e.   Searching the contents of a file

Simple searching using less

Using less, you can search though a text file for a keyword (pattern). For example, to search through science.txt for the word 'science', type

% less science.txt

then, still in less, type a forward slash [/] followed by the word to search

/science

As you can see, less finds and highlights the keyword. Type [n] to search for the next occurrence of the word.

grep (please don't ask why it is called grep)

grep is one of many standard UNIX utilities. It searches files for specified words or patterns. First clear the screen, then type

% grep science science.txt

As you can see, grep has printed out each line contains the word science.

Or has it????

Try typing

% grep Science science.txt

The grep command is case sensitive; it distinguishes between Science and science.

To ignore upper/lower case distinctions, use the -i option, i.e. type

% grep -i science science.txt

To search for a phrase or pattern, you must enclose it in single quotes (the apostrophe symbol). For example to search for spinning top, type

% grep -i 'spinning top' science.txt

Some of the other options of grep are:

-v displays those lines that do NOT match

-n precede each matching line with the line number

-c prints only the total count of matched lines

Try some of them and see the different results. Don't forget, you can use more than one option at a time. For example, the number of lines without the words science or Science is

% grep -ivc science science.txt

wc (word count)

A handy little utility is the wc command, short for word count. To do a word count on science.txt, type

% wc -w science.txt

To find out how many lines the file has, type

% wc -l science.txt

 

Command	Meaning
cp file1 file2	copy file1 and call it file2
mv file1 file2	move or rename file1 to file2
rm file	remove a file
rmdir directory	remove a directory
cat file	display a file
less file	display a file a page at a time
head file	display the first few lines of a file
tail file	display the last few lines of a file
grep 'keyword' file	search a file for keywords
wc file	count number of lines/words/characters in file

4. Section Three

a.   Redirection

Most processes initiated by UNIX commands write to the standard output (that is, they write to the terminal screen), and many take their input from the standard input (that is, they read it from the keyboard). There is also the standard error, where processes write their error messages, by default, to the terminal screen.

We have already seen one use of the cat command to write the contents of a file to the screen.

Now type cat without specifying a file to read

% cat

Then type a few words on the keyboard and press the [Return] key.

Finally hold the [Ctrl] key down and press [d] (written as ^D for short) to end the input.

What has happened?

If you run the cat command without specifying a file to read, it reads the standard input (the keyboard), and on receiving the 'end of file' (^D), copies it to the standard output (the screen).

In UNIX, we can redirect both the input and the output of commands.

b.  Redirecting the Output

We use the > symbol to redirect the output of a command. For example, to create a file called list1 containing a list of fruit, type 

% cat > list1

Then type in the names of some fruit. Press [Return] after each one.

mango

banana

apple

^D {this means press [Ctrl] and [d] to stop}

What happens is the cat command reads the standard input (the keyboard) and the > redirects the output, which normally goes to the screen, into a file called list1

To read the contents of the file, type

% cat list1

Exercise 3a

Using the above method, create another file called list2 containing the following fruit: orange, plum and mango. Read the contents of list2

Appending to a file

The form >> appends standard output to a file. So to add more items to the file list1, type

% cat >> list1

Then type in the names of more fruit

mango

apple

orange

^D (Control D to stop)

To read the contents of the file, type

% cat list1

You should now have two files. One contains six fruit, the other contains four fruit.

We will now use the cat command to join (concatenate) list1 and list2 into a new file called biglist. Type

% cat list1 list2 > biglist

What this is doing is reading the contents of list1 and list2 in turn, then outputing the text to the file biglist

To read the contents of the new file, type

% cat biglist

c.   Redirecting the Input

We use the < symbol to redirect the input of a command.

The command sort alphabetically or numerically sorts a list. Type

% sort

Then type in the names of some animals. Press [Return] after each one.

dog

cat

bird

ape

^D (control d to stop)

The output will be

ape

bird

cat

dog

Using < you can redirect the input to come from a file rather than the keyboard. For example, to sort the list of fruit, type

% sort < biglist

and the sorted list will be output to the screen.

To output the sorted list to a file, type,

% sort < biglist > slist

Use cat to read the contents of the file slist

d.  Pipes

To see who is on the system with you, type

% who

One method to get a sorted list of names is to type,

% who > names.txt

% sort < names.txt

This is a bit slow and you have to remember to remove the temporary file called names when you have finished. What you really want to do is connect the output of the who command directly to the input of the sort command. This is exactly what pipes do. The symbol for a pipe is the vertical bar |

For example, typing

% who | sort

will give the same result as above, but quicker and cleaner.

To find out how many users are logged on, type

% who | wc –l

Command	Meaning
command > file	redirect standard output to a file
command >> file	append standard output to a file
command < file	redirect standard input from a file
command1 | command2	pipe the output of command1 to the input of command2
cat file1 file2 > file0	concatenate file1 and file2 to file0
sort	sort data
who	list users currently logged in

5. Section Four

a.   Wildcards

The * wildcard

The character * is called a wildcard, and will match against none or more character(s) in a file (or directory) name. For example, in your unixfolder directory, type

% ls list*

This will list all files in the current directory starting with list....

Try typing

% ls *list

This will list all files in the current directory ending with ....list

The ? wildcard

The character ? will match exactly one character.

So ?ouse will match files like house and mouse, but not grouse.

Try typing

% ls ?list

b.  Filename Conventions

We should note here that a directory is merely a special type of file. So the rules and conventions for naming files apply also to directories.

In naming files, characters with special meanings such as / * & % , should be avoided. Also, avoid using spaces within names. The safest way to name a file is to use only alphanumeric characters, that is, letters and numbers, together with _ (underscore) and . (dot).

Good filenames	Bad filenames
project.txt	project
my_big_program.c	my big program.c
fred_dave.doc	fred & dave.doc

File names conventionally start with a lower-case letter, and may end with a dot followed by a group of letters indicating the contents of the file. For example, all files consisting of C code may be named with the ending .c, for example, prog1.c . Then in order to list all files containing C code in your home directory, you need only type ls *.c in that directory.

c.   Getting Help

On-line Manuals

There are on-line manuals which gives information about most commands. The manual pages tell you which options a particular command can take, and how each option modifies the behavior of the command. Type man command to read the manual page for a particular command.

For example, to find out more about the wc (word count) command, type

% man wc

Alternatively

% whatis wc

gives a one-line description of the command, but omits any information about options etc.

Apropos

When you are not sure of the exact name of a command,

% apropos keyword

will give you the commands with keyword in their manual page header. For example, try typing

% apropos copy

Command	Meaning
*	match any number of characters
?	match one character
man command	read the online manual page for a command
whatis command	brief description of a command
apropos keyword	match commands with keyword in their man pages

6. Section Five

a.   File system security (access rights)

In your unixfolder directory, type

% ls -l (l for long listing!)

You will see that you now get lots of details about the contents of your directory, similar to the example below.

Each file (and directory) has associated access rights, which may be found by typing ls -l. Also, ls -lg gives additional information as to which group owns the file (beng95 in the following example):

-rwxrw-r-- 1 ee51ab beng95 2450 Sept29 11:52 file1

In the left-hand column is a 10 symbol string consisting of the symbols d, r, w, x, -, and, occasionally, s or S. If d is present, it will be at the left hand end of the string, and indicates a directory: otherwise - will be the starting symbol of the string.

The 9 remaining symbols indicate the permissions, or access rights, and are taken as three groups of 3.

·         The left group of 3 gives the file permissions for the user that owns the file (or directory) (ee51ab in the above example);

·         the middle group gives the permissions for the group of people to whom the file (or directory) belongs (eebeng95 in the above example);

·         the rightmost group gives the permissions for all others.

The symbols r, w, etc., have slightly different meanings depending on whether they refer to a simple file or to a directory.

Access rights on files.

·         r (or -), indicates read permission (or otherwise), that is, the presence or absence of permission to read and copy the file

·         w (or -), indicates write permission (or otherwise), that is, the permission (or otherwise) to change a file

·         x (or -), indicates execution permission (or otherwise), that is, the permission to execute a file, where appropriate

Access rights on directories.

·          r allows users to list files in the directory;

·          w means that users may delete files from the directory or move files into it;

·          x means the right to access files in the directory. This implies that you may read files in the directory provided you have read permission on the individual files.

So, in order to read a file, you must have executed permission on the directory containing that file, and hence on any directory containing that directory as a subdirectory, and so on, up the tree.

Some examples

-rwxrwxrwx	a file that everyone can read, write and execute (and delete).
-rw-------	a file that only the owner can read and write - no-one else can read or write and no-one has execution rights (e.g. your mailbox file).

 

b.  Changing access rights

chmod (changing a file mode)

Only the owner of a file can use chmod to change the permissions of a file. The options of chmod are as follows

Symbol	Meaning
u	user
g	group
o	other
a	all
r	read
w	write (and delete)
x	execute (and access directory)
+	add permission
-	take away permission

For example, to remove read write and execute permissions on the file biglist for the group and others, type

% chmod go-rwx biglist

This will leave the other permissions unaffected.

To give read and write permissions on the file biglist to all,

% chmod a+rw biglist

 

c.   Killing a process

kill (terminate or signal a process)

It is sometimes necessary to kill a process (for example, when an executing program is in an infinite loop)

To kill a job running in the foreground, type ^C (control c). For example, run

% sleep 100

^C

To kill a suspended or background process, type

% kill %jobnumber

For example, run

% sleep 100 &

% jobs

If it is job number 4, type

% kill %4

To check whether this has worked, examine the job list again to see if the process has been removed.

ps (process status)

Alternatively, processes can be killed by finding their process numbers (PIDs) and using kill PID_number

% sleep 1000 &

% ps

PID TT S TIME COMMAND

20077 pts/5 S 0:05 sleep 1000

21563 pts/5 T 0:00 netscape

21873 pts/5 S 0:25 nedit

To kill off the process sleep 1000, type

% kill 20077

and then type ps again to see if it has been removed from the list.

If a process refuses to be killed, uses the -9 option, i.e. type

% kill -9 20077

Note: It is not possible to kill off other users' processes!!!

Command	Meaning
ls -lag	list access rights for all files
chmod [options] file	change access rights for named file
command &	run command in background
^C	kill the job running in the foreground
^Z	suspend the job running in the foreground
bg	background the suspended job
jobs	list current jobs
fg %1	foreground job number 1
kill %1	kill job number 1
ps	list current processes
kill 26152	kill process number 26152

7. Section Six

a.   Other Useful UNIX commands

df

The df command reports on the space left on the file system. For example, to find out how much space is left on the fileserver, type

% df .

du

The du command outputs the number of kilobytes used by each subdirectory. Useful if you have gone over quota and you want to find out which directory has the most files. In your home-directory, type

% du -s *

The -s flag will display only a summary (total size) and the * means all files and directories.

gzip

This reduces the size of a file, thus freeing valuable disk space. For example, type

% ls -l science.txt

and note the size of the file using ls -l . Then to compress science.txt, type

% gzip science.txt

This will compress the file and place it in a file called science.txt.gz

To see the change in size, type ls -l again.

To expand the file, use the gunzip command.

% gunzip science.txt.gz

zcat

zcat will read gzipped files without needing to uncompressing them first.

% zcat science.txt.gz

If the text scrolls too fast for you, pipe the output though less .

% zcat science.txt.gz | less

file

file classifies the named files according to the type of data they contain, for example ascii (text), pictures, compressed data, etc.. To report on all files in your home directory, type

% file *

diff

This command compares the contents of two files and displays the differences. Suppose you have a file called file1 and you edit some part of it and save it as file2. To see the differences type

% diff file1 file2

Lines beginning with a < denotes file1, while lines beginning with a > denotes file2.

find

This search through the directories for files and directories with a given name, date, size, or any other attribute you care to specify. It is a simple command but with many options - you can read the manual by typing man find.

To search for all files with the extention .txt, starting at the current directory (.) and working through all sub-directories, then printing the name of the file to the screen, type

% find . -name "*.txt" -print

To find files over 1Mb in size, and display the result as a long listing, type

% find . -size +1M -ls

history

The C shell keeps an ordered list of all the commands that you have entered. Each command is given a number according to the order it was entered.

% history (show command history list)

If you are using the C shell, you can use the exclamation character (!) to recall commands easily.

% !! (recall last command)

% !-3 (recall third most recent command)

% !5 (recall 5th command in list)

% !grep (recall last command starting with grep)

You can increase the size of the history buffer by typing

% set history=100