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.
Types of UNIX
There are many different versions of UNIX, although they share common similarities. The most popular varieties of UNIX are Sun Solaris, GNU/Linux, and MacOS X.
Here in the School, we use Solaris on our servers and workstations, and Fedora Linux on the servers and desktop PCs.
The UNIX operating system
The UNIX operating system is made up of three parts; the kernel, the shell and the programs.
The kernel
The kernel of UNIX is the hub of the operating system: it allocates time and memory to programs and handles the filestore and communications in response to system calls.
As an illustration of the way that the shell and the kernel work together, suppose a user types rm
The shell
The shell acts as an interface between the user and the kernel. When a user logs in, the login program checks the username and password, and then starts another program called the shell. The shell is a command line interpreter (CLI). It interprets the commands the user types in and arranges for them to be carried out. The commands are themselves programs: when they terminate, the shell gives the user another prompt (% on our systems).
The adept user can customise his/her own shell, and users can use different shells on the same machine. Staff and students in the school have the tcsh shell by default.
The tcsh shell has certain features to help the user inputting commands.
Filename Completion - By typing part of the name of a command, filename or directory and pressing the [Tab] key, the tcsh shell will complete the rest of the name automatically. If the shell finds more than one name beginning with those letters you have typed, it will beep, prompting you to type a few more letters before pressing the tab key again.
History - The shell keeps a list of the commands you have typed in. If you need to repeat a command, use the cursor keys to scroll up and down the list or type history for a list of previous commands.
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.
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"
Starting an UNIX terminal To open an UNIX terminal window, click on the "Terminal" icon from Applications/Accessories menus.
An UNIX Terminal window will then appear with a % prompt, waiting for you to start entering commands.
1.1 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
ls is an example of a command which can take options: -a is an example of an option. The options change the behaviour of the command. There are online manual pages that tell you which options a particular command can take, and how each option modifies the behaviour of the command.
1.2 Making Directories mkdir (make directory)
Now make a subdirectory in your home directory to hold the files you will be creating and using. To make a subdirectory called rabin in your current working directory type
% mkdir rabin . To see the directory you have just created, type
% ls
1.3 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 rabin
Type ls to see the contents (which should be empty)
1.4 The directories . and .. Still in the unixstuff directory, type
% ls -a
As you can see, in the unixstuff 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 unixstuff directory).
This may not seem very useful at first, but using (.) as the name of the current directory will save a lot of typing.
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.
1.5 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 " / " .
(Remember, if you get lost, type cd by itself to return to your home-directory)
1.6 More about home directories and pathnames Understanding pathnames
First type cd to get back to your home-directory, then type
% ls unixstuff
to list the conents of your unixstuff directory.
Now type
% ls backups
You will get a message like this -
backups: No such file or directory
The reason is, 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 unixstuff/backups if you have make backups directory under unixstuff.
~ (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 ~/unixstuff
will list the contents of your unixstuff directory, no matter where you currently are in the file system.
What do you think
% ls ~would list?
What do you think
% ls ~/..
would list?
2.1 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 unixstuff directory.
First, cd to your unixstuff directory.
% cd ~/unixstuff
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 unixstuff directory.)
2.2 Moving filesmv (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 unixstuff directory (can you remember how?). Then, inside the unixstuff directory, type
% mv science.bak backups/.
Type ls and ls backups to see if it has worked.
2.3 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 unixstuff 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.
2.4 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.
lessThe 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.
headThe 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 do to the head command?
tailThe tail command writes the last ten lines of a file to the screen.
Clear the screen and type
% tail science.txt Q. How can you view the last 15 lines of the file?
2.5 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
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 containg 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 display those lines that do NOT match
-n precede each matching line with the line number
-c print only the total count of matched lines
OR YOU CAN TYPE grep --help to go with more command regarding grep
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
2.6 File system security (access rights)
In your unixstuff 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.
2.7 File and directory access rights
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 execute 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).
5.2 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
2.8 Processes and Jobs
A process is an executing program identified by a unique PID (process identifier). To see information about your processes, with their associated PID and status, type
% ps
A process may be in the foreground, in the background, or be suspended. In general the shell does not return the UNIX prompt until the current process has finished executing.
Some processes take a long time to run and hold up the terminal. Backgrounding a long process has the effect that the UNIX prompt is returned immediately, and other tasks can be carried out while the original process continues executing.
2.9 Running background processes
To background a process, type an & at the end of the command line. For example, the command sleep waits a given number of seconds before continuing. Type
% sleep 10
This will wait 10 seconds before returning the command prompt %. Until the command prompt is returned, you can do nothing except wait.
To run sleep in the background, type
% sleep 10 &
[1] 6259
The & runs the job in the background and returns the prompt straight away, allowing you do run other programs while waiting for that one to finish.
The first line in the above example is typed in by the user; the next line, indicating job number and PID, is returned by the machine. The user is be notified of a job number (numbered from 1) enclosed in square brackets, together with a PID and is notified when a background process is finished. Backgrounding is useful for jobs which will take a long time to complete.
Backgrounding a current foreground process At the prompt, type
% sleep 1000
You can suspend the process running in the foreground by typing ^Z, i.e.hold down the [Ctrl] key and type [z]. Then to put it in the background, type
% bg
Note: do not background programs that require user interaction e.g. vi
5.4 Listing suspended and background processes
When a process is running, backgrounded or suspended, it will be entered onto a list along with a job number. To examine this list, type
% jobs
An example of a job list could be
[1] Suspended sleep 1000
[2] Running netscape
[3] Running matlab
To restart (foreground) a suspended processes, type
% fg %jobnumber
For example, to restart sleep 1000, type
% fg %1
Typing fg with no job number foregrounds the last suspended process.
2.9 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 !!!
The Summary of the commands that can be used to access the files and directories and to kill and suspend the process,backgrounding, foregrounding are as follows:
Command Meaning
ls -lag list access rights for all files
chmod [options] file change access rights for name 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 process
kill 26152 kill process number 26152
No comments:
Post a Comment