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 rmmyfile (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 rmmyfile 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).
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.
Common Commands
ls list the content of your current
working directory
ls -a list hidden files + normal files
ls -l list file details in long format
(-rwxrw-r-- 1 ee51ab beng95 2450 Sept29 11:52 file1)
•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.
mkdir make directory
cd directory change
to named directory
cd change to home-directory
cd ~ change to home-directory
cd .. change to parent directory
cd . current directory
cp file1 file 2 copy file1 to file2
mv file1 file2 move file1 to file2 or rename file1
to file2
rm file remove file
rmdir directory remove directory (make sure
directory is empty)
clear clear screen
cat file display the content on the screen at
once
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).
less file display the contents of a file onto the screen a page
at a time
head file writes the first ten lines of a file to the screen
head -5 file writes
the first five lines of a file to the screen
tail file writes the last ten lines of a file to
the screen
tail -5 file writes the last five lines of a file to the screen
grep “key” file It searches files for specified
words or patterns
grep -i ignore upper/lower case distinction
grep -v display those lines that do NOT match
grep -n precede each matching line with the line number
grep -c print only the total count of matched lines
wc file print the number of newlines, words,
and bytes in files
wc -l print the newlines count
wc -L print the length of the longest line
wc -w print the word counts
Redirecting the output
> We use the > symbol to redirect
the output of a command (ex:- cat > list 1)
>> The form >> appends standard
output to a file. (ex:- to add more names in list1 cat >> list1)
Redirecting the input
< We use the > symbol to redirect
the input of a command (ex:- cat > list 1)
Sort list it will sort the list alphabetically or numerically
cmd1 | cmd2 pipe
the output of command1 to the input of command2
wildcard * (any), % (pattern) , ? (exact word)
mancmd online description including options
whatiscmd description
of the command, but omits any information about options
apropos keyword When you are not sure of the exact
name of a command
chmod change permission on a file (Only
the owner of a file can use chmod to change the permissions of a file). The
options of chmod are as follows (ex - chmoda+rwbiglist)
|
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
|
quota -v To check your current quota and how much of it you have
used, type
df
. reports on the space left on the
file system
du
-s * No. of KBs used by each subdirectory
( -s flag will display only a summary (total size) and the * means all files
and directories)
gzip
file compress file
tar
file unzip file (extract the command)
zcat file read gzipped files without needing to uncompress them
first. (ex:- zcat science.txt.gz | less )
diff
file1 file2 compare the content of two files and
displays the differences. (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.
Ex:-
find . -name "*.txt" -print
find
. -size +1M -ls
Process
(A process is an executing program identified by a unique PID (process
identifier).)
ps 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.
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 & 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.
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
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.
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 !!!
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
