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.
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 myfile (which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm on myfile. When the process rm myfile has finished running, the shell then returns the UNIX prompt % to the user, indicating that it is waiting for further commands.
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).
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 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
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.
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 myfile (which has the effect of removing the file myfile). The shell searches the filestore for the file containing the program rm, and then requests the kernel, through system calls, to execute the program rm on myfile. When the process rm myfile has finished running, the shell then returns the UNIX prompt % to the user, indicating that it is waiting for further commands.
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).
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 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
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.
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 / )
Unix File Structure
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.
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
The ls command lists the contents of your current working directory.
Unix Terminal - running the ls command
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
As you can see, ls -a lists files that are normally hidden.
Unix Terminal - running the ls command
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. (See later in this tutorial)
1.2 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 unixstuff in your current working directory type
To see the directory you have just created, type
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
Type ls to see the contents (which should be empty)
Exercise 1a
Make another directory inside the unixstuff directory called backups
1.4 The directories . and ..
Still in the unixstuff directory, type
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
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, as we shall see later in the tutorial.
The parent directory (..)
(..) means the parent of the current directory, so typing
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
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 " / " .
The full path to the file report.doc is "/home/its/ug1/ee51vn/report.doc"
Unix File structure
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)
1.6 More about home directories and pathnames
Understanding pathnames
First type cd to get back to your home-directory, then type
to list the conents of your unixstuff directory.
Now type
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
~ (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
will list the contents of your unixstuff directory, no matter where you currently are in the file system.
What do you think
would list?
What do you think
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
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.
Then at the UNIX prompt, type,
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.)
Exercise 2a
Create a backup of your science.txt file by copying it to a file called science.bak
2.2 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 unixstuff directory (can you remember how?). Then, inside the unixstuff directory, type
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
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 tempstuff using mkdir , then remove it using the rmdir command.
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
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:
As you can see, the file is longer than 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
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
Then type
What difference did the -5 do to the head command?
tail
The tail command writes the last ten lines of a file to the screen.
Clear the screen and type
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
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 (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
As you can see, grep has printed out each line containg the word science.
Or has it ????
Try typing
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
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
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
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
wc (word count)
A handy little utility is the wc command, short for word count. To do a word count on science.txt, type
To find out how many lines the file has, type
Summary
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
3.1 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 specifing a file to read
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 specifing 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.
3.2 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
Then type in the names of some fruit. Press [Return] after each one.
pear
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
Exercise 3a
Using the above method, create another file called list2 containing the following fruit: orange, plum, mango, grapefruit. Read the contents of list2
3.2.1 Appending to a file
The form >> appends standard output to a file. So to add more items to the file list1, type
Then type in the names of more fruit
peach
grape
orange
^D (Control D to stop)
To read the contents of the file, type
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
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
3.3 Redirecting the Input
We use the < symbol to redirect the input of a command.
The command sort alphabetically or numerically sorts a list. Type
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
and the sorted list will be output to the screen.
To output the sorted list to a file, type,
Use cat to read the contents of the file slist
3.4 Pipes
To see who is on the system with you, type
One method to get a sorted list of names is to type,
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
will give the same result as above, but quicker and cleaner.
To find out how many users are logged on, type
Exercise 3b
Using pipes, display all lines of list1 and list2 containing the letter 'p', and sort the result.
Summary
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
4.1 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 unixstuff directory, type
This will list all files in the current directory starting with list....
Try typing
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
4.2 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.
4.3 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 behaviour 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
Alternatively
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,
will give you the commands with keyword in their manual page header. For example, try typing
Summary
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
5.1 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.
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
This will leave the other permissions unaffected.
To give read and write permissions on the file biglist to all,
Exercise 5a
Try changing access permissions on the file science.txt and on the directory backups
Use ls -l to check that the permissions have changed.
5.3 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
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.
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
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
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
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
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
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
For example, to restart sleep 1000, type
Typing fg with no job number foregrounds the last suspended process.
5.5 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
^C
To kill a suspended or background process, type
For example, run
% jobs
If it is job number 4, type
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
% 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
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
Note: It is not possible to kill off other users' processes !!!
Summary
Command Meaning
ls -lag list access rights for all files
chmod [options] file change access rights for named file
command & un 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
Other useful UNIX commands
quota
All students are allocated a certain amount of disk space on the file system for their personal files, usually about 100Mb. If you go over your quota, you are given 7 days to remove excess files.
To check your current quota and how much of it you have used, type
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
The du command outputs the number of kilobyes 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
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
and note the size of the file using ls -l . Then to compress science.txt, type
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.
zcat will read gzipped files without needing to uncompress them first.
If the text scrolls too fast for you, pipe the output though less .
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
Lines beginning with a < denotes file1, while lines beginning with a > denotes file2.
find
This searches 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 fies 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
To find files over 1Mb in size, and display the result as a long listing, type
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.
If you are using the C shell, you can use the exclamation character (!) to recall commands easily.
You can increase the size of the history buffer by typing
To further study about Unix follow these links
http://www.fsid.cvut.cz/cz/U201/linux.html
http://www.ee.surrey.ac.uk/Teaching/Unix/
http://www.linux.org/lessons/beginner/toc.html/
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.
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 / )
Unix File Structure
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.
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
Code:
% ls
Unix Terminal - running the ls command
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
Code:
% ls -a
Unix Terminal - running the ls command
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. (See later in this tutorial)
1.2 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 unixstuff in your current working directory type
Code:
% mkdir unixstuff
Code:
% ls
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
Code:
% cd unixstuff
Exercise 1a
Make another directory inside the unixstuff directory called backups
1.4 The directories . and ..
Still in the unixstuff directory, type
Code:
% ls -a
The current directory (.)
In UNIX, (.) means the current directory, so typing
Code:
% cd .
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, as we shall see later in the tutorial.
The parent directory (..)
(..) means the parent of the current directory, so typing
Code:
% cd ..
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
Code:
% pwd
/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 " / " .
The full path to the file report.doc is "/home/its/ug1/ee51vn/report.doc"
Unix File structure
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)
1.6 More about home directories and pathnames
Understanding pathnames
First type cd to get back to your home-directory, then type
Code:
% ls unixstuff
Now type
Code:
% ls backups
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
Code:
% ls unixstuff/backups
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
Code:
% ls ~/unixstuff
What do you think
Code:
% ls ~
What do you think
Code:
% ls ~/..
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
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.
Code:
% cd ~/unixstuff
Code:
% cp /vol/examples/tutorial/science.txt .
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.)
Exercise 2a
Create a backup of your science.txt file by copying it to a file called science.bak
2.2 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 unixstuff directory (can you remember how?). Then, inside the unixstuff directory, type
Code:
% mv science.bak backups/.
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
Code:
% cp science.txt tempfile.txt
% ls
% rm tempfile.txt
% ls
Exercise 2b
Create a directory called tempstuff using mkdir , then remove it using the rmdir command.
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
Code:
% clear
cat (concatenate)
The command cat can be used to display the contents of a file on the screen. Type:
Code:
% cat science.txt
less
The command less writes the contents of a file onto the screen a page at a time. Type
Code:
% less science.txt
head
The head command writes the first ten lines of a file to the screen.
First clear the screen then type
Code:
% head science.txt
Code:
% head -5 science.txt
tail
The tail command writes the last ten lines of a file to the screen.
Clear the screen and type
Code:
% tail science.txt
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
Code:
% less science.txt
/science
As you can see, less finds and highlights the keyword. Type [n] to search for the next occurrence of the word.
grep (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
Code:
% grep science science.txt
Or has it ????
Try typing
Code:
% grep Science science.txt
To ignore upper/lower case distinctions, use the -i option, i.e. type
Code:
% grep -i science science.txt
Code:
% grep -i 'spinning top' science.txt
-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
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
Code:
% grep -ivc science science.txt
A handy little utility is the wc command, short for word count. To do a word count on science.txt, type
Code:
% wc -w science.txt
Code:
% 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
3.1 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 specifing a file to read
Code:
% cat
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 specifing 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.
3.2 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
Code:
% cat > list1
pear
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
Code:
% cat list1
Using the above method, create another file called list2 containing the following fruit: orange, plum, mango, grapefruit. Read the contents of list2
3.2.1 Appending to a file
The form >> appends standard output to a file. So to add more items to the file list1, type
Code:
% cat >> list1
peach
grape
orange
^D (Control D to stop)
To read the contents of the file, type
Code:
% cat list1
We will now use the cat command to join (concatenate) list1 and list2 into a new file called biglist. Type
Code:
% cat list1 list2 > biglist
To read the contents of the new file, type
Code:
% cat biglist
We use the < symbol to redirect the input of a command.
The command sort alphabetically or numerically sorts a list. Type
Code:
% sort
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
Code:
% sort < biglist
To output the sorted list to a file, type,
Code:
% sort < biglist > slist
3.4 Pipes
To see who is on the system with you, type
Code:
% who
Code:
% who > names.txt
% sort < names.txt
For example, typing
Code:
% who | sort
To find out how many users are logged on, type
Code:
% who | wc -l
Using pipes, display all lines of list1 and list2 containing the letter 'p', and sort the result.
Summary
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
4.1 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 unixstuff directory, type
Code:
% ls list*
Try typing
Code:
% ls *list
The ? wildcard
The character ? will match exactly one character.
So ?ouse will match files like house and mouse, but not grouse.
Try typing
Code:
% ls ?list
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.
4.3 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 behaviour 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
Code:
% man wc
Code:
% whatis wc
Apropos
When you are not sure of the exact name of a command,
Code:
% apropos keyword
Code:
% 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
5.1 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.
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
Code:
% chmod go-rwx biglist
To give read and write permissions on the file biglist to all,
Code:
% chmod a+rw biglist
Try changing access permissions on the file science.txt and on the directory backups
Use ls -l to check that the permissions have changed.
5.3 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
Code:
% ps
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.
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
Code:
% sleep 10
To run sleep in the background, type
Code:
% sleep 10 &
[1] 6259
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
Code:
% sleep 1000
Code:
% bg
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
Code:
% jobs
[1] Suspended sleep 1000
[2] Running netscape
[3] Running matlab
To restart (foreground) a suspended processes, type
Code:
% fg %jobnumber
Code:
% fg %1
5.5 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
Code:
% sleep 100
To kill a suspended or background process, type
Code:
% kill %jobnumber
Code:
% sleep 100 &
If it is job number 4, type
Code:
% kill %4
ps (process status)
Alternatively, processes can be killed by finding their process numbers (PIDs) and using kill PID_number
Code:
% sleep 1000 &
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
Code:
% kill 20077
If a process refuses to be killed, uses the -9 option, i.e. type
Code:
% kill -9 20077
Summary
Command Meaning
ls -lag list access rights for all files
chmod [options] file change access rights for named file
command & un 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
Other useful UNIX commands
quota
All students are allocated a certain amount of disk space on the file system for their personal files, usually about 100Mb. If you go over your quota, you are given 7 days to remove excess files.
To check your current quota and how much of it you have used, type
Code:
% quota -v
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
Code:
% df .
du
Code:
% du -s *
gzip
This reduces the size of a file, thus freeing valuable disk space. For example, type
Code:
% ls -l science.txt
Code:
% gzip science.txt
To see the change in size, type ls -l again.
To expand the file, use the gunzip command.
Code:
% gunzip science.txt.gz
zcat
Code:
% zcat science.txt.gz
Code:
% zcat science.txt.gz | less /[code]
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
[code]% file *
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
Code:
% diff file1 file2
find
This searches 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 fies 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
Code:
% find . -name "*.txt" -print
Code:
% find . -size +1M -ls
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.
Code:
% history (show command history list)
Code:
% !! (recall last command)
Code:
% !-3 (recall third most recent command)
Code:
% !5 (recall 5th command in list)
Code:
% !grep (recall last command starting with grep)
Code:
% set history=100
http://www.fsid.cvut.cz/cz/U201/linux.html
http://www.ee.surrey.ac.uk/Teaching/Unix/
http://www.linux.org/lessons/beginner/toc.html/
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