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.
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.
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"
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.
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
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
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.
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)
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
|
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
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.
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.
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
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
|
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.
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
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
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
|
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
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.
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
|
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).
|
|
|
|
|
|
|
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
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
|
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
