whatis unix

7/28/2019 Whatis UNIX

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What is Unix ?

The UNIX operating system is a set of programs that act as a link between the computer and the user.

The computer programs that allocate the system resources and coordinate all the details of the computer's internals is called theoperating system or kernel.

Users communicate with the kernel through a program known as the shell. The shell is a command line interpreter; it translatescommands entered by the user and converts them into a language that is understood by the kernel.

Unix was originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis

Ritchie, Douglas McIlroy, and Joe Ossanna.

There are various Unix variants available in the market. Solaris Unix, AIX, UP Unix and BSD are few examples. Linux is

also a flavour of Unix which is freely available.

Several people can use a UNIX computer at the same time; hence UNIX is called a multiuser system.

A user can also run multiple programs at the same time; hence UNIX is called multitasking.

Unix Architecture:

Here is a basic block diagram of a UNIX system:

The main concept that unites all versions of UNIX is the following four basics:

Kernel: The kernel is the heart of the operating system. It interacts with hardware and most of the tasks like memorymanagement, tash scheduling and file management.


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Shell: The shell is the utility that processes your requests. When you type in a command at your terminal, the shell

interprets the command and calls the program that you want. The shell uses standard syntax for all commands. C Shell,

Bourne Shell and Korn Shell are most famous shells which are available with most of the Unix variants.

Commands and Utilities: There are various command and utilities which you would use in your day to day activities.

cp, mv, cat and grep etc. are few examples of commands and utilities. There are over 250 standard commands plus

numerous others provided through 3rd party software. All the commands come along with various optional options.

Files and Directories: All data in UNIX is organized into files. All files are organized into directories. These directories

are organized into a tree-like structure called the filesystem.

System Bootup:

If you have a computer which has UNIX operating system installed on it, then you simply need to turn on its power to make it live.

As soon as you turn on the power, system starts booting up and finally it prompts you to log into the system, which is an activity tolog into the system and use it for your day to day activities.

Whatis UNIX

UNIX is a so called operating system for workstations and has mainly ben developped at

Berkley University (BDS-UNIX). One of the striking difference between UNIX ans many

other operating systems is, that it has been a multiuser and multiprocess environment from

the very beginning. Multiprocess / multiuser means that different tasks can be run on the

same system by different people with different priviliges at the same time.

The system used by SGI is a derivate from the original UNIX, a so called dialect, named

IRIX.

Filesystem

The following commands are all crucial for navigating in the structure of the filetree. Unlike

other systems, in UNIX a filesystem cannot only incorporate files and directorys that are

stored locally on the hard drive. It can also merge (shared) drives on other systems (hosts) or

external devices like tape drives or printers, keyboards etc.

Navigation

ls ls [-RadLCxmlnogrtucpFbqisf1AM] [names]

Lists the content of a directory

ls -ls List in short form

ls -la All files (including .-files)

ls ./mydir List content of mydir

pwd pwd

Print working directory name

Pwd

cd cd [ directory ]


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Change working directory

cd Change to home directory

cd ~ Change to home directory

cd .. Change to parent directory

cd /usr/lib Change to absolute path /usr/lib

cd doc/letters Change to relative path doc/letters

du du [-sarklmL] [name . . .]

Summarize disk usage

du -k Show disk usage in kblocks (1024 bytes)

du -ks * Summarize all directorys (*) and show as kblocks

df df [-b][-f][-i][-k][-l][file-system ...]

Report number of free disk blocks. Shows usage of disks. Usefull to find out if

there is enough room to save / copy a project or file.df -k Display disk usage in kblocks

mount mount

mount [ -h host ] [ -fnrv ]

mount [ -cfnrv ] [ -o options ] fsname | dir

Mount and dismount file systems. Shows mounted disks. External disks are

preceeded by their hostname.

mount Show mounted disks

Modifying

cp cp [ -irp ] file1 [file2 ...] target

Copy files or directory structures

cp file1 file2 Creates a copy of file1, named file2

cp old* archive/ Creates a copy of all files starting

with "old" into the directory archive

cp -R letters archive/. Creates a copy of all files under

"letters" and puts them into "archive"

ln ln [ -sif ] file1 [file2 ...] target

Link files. Links are like pointers that "point" to the original location of the file.

They thus, use less diskspace.

ln -s /serve/sgi/tools ~/SGItools symbolic link to /serve/sgi

in your homedirectory

mv mv [ -if ] file1 [file2 ...] target

Moves files to another location or rename them.

mv file1 file2 Renames file1, to file2


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mv old* archive/ Moves all files starting with "old"

into the directory "archive"

rm rm [-f] [-i] file

Remove filesrm file1 file2 Removes file1 and file2

rm old* Removes all files starting with "old"

rm -r unused/* Removes all files recursively that

are under "unused"

mkdir mkdir [-m mode] [-p] dirname

Make directories

mkdir newthings Creates a directory named "newthings"

rmdir rmdir [-p] [-s] dirname

Removes an empty directory. If the directory is not empty, it will not be

removed.

rmdir oldstuff Removes a directory named "oldstuff"

Permissions

chmod chmod [-R] mode file

chmod [-R] [ugoa]{+|-|=}[rwxstl] file

Change the permissions of a file or directory. To do so, you have to be the

owner of the file or directory. The permissions are coded into a 6-digit booleanstring.

Permissions are coded into a 6-digit boolean string with the

values 1,2,4 according to their byte-positions:

rwxrwxrwx 1200 guest testfile

^^^ Owner rights: read write

execute

^^^ Group rights: read write

execute

^^^ Others: read write

execute

^^^^ Group ID of the file/dir^^^^^ Owner ID of the file/dir

^^^^^^^^^ Name of the file

To compose a full permission number, add the according bits to

a

3-digit number according to owner-group-other:

rwxrwxrwx

421421421

chmod 644 owner:rw group:r other:r (typical dir)

chmod 755 owner.rwx group:rx other:rx (typical file)

File archiving / compression


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tar tar key [directory] [tapefile] [blocksize] [ name ... ]

Tape archiver. Writes a single archive-file from a set of files. Tar is often used

to either backup data or move complete directory-structures around. Tar

usually adresses a tape device, can thoug, be rerouted with the f option to

write to a file.

tar cvf arch.tar docs/* Creates a file named arch.tar with

anything under docs/

tar xvf move.tar Explodes transport.tar and creates

all the directories that have been

packed into the tar file.

compress compress [ -f ] [ -v ] [ -c ] [ -b bits ] [ name ... ]

Compress data. Results will be stored into a file with the suffix .Z

compress oldfile.txt Will result in oldfile.txt.Z

compress * Will compress every file in the

directory

uncompress uncompress [ -f ] [ -v ] [ -c ] [ name ... ]

Expand data with the .Z suffix

uncompress oldfile.txt.Z Will result in oldfile.txt

uncompress * Will decompress every .Z file

in the directory

gzip gzip [ -acdfhlLnNrtvV19 ] [-S suffix] [ name ... ]

gunzip [ -acfhlLnNrtvV ] [-S suffix] [ name ... ]Compress or expand files. Same as 'compress' but with a more efficient

algorithm.

gzip oldfile.txt Compress oldfile.txt to oldfile.txt.gz

gzip -d oldfile.txt.gz Decompress oldfile.txt.gz to

oldfile.txt

Helpers, alias and communication commands

The commands below will help you to identify your command shell environment. You can

lookup the current user, find out about what command will be executed when you type

something and last, lookup the versions of installed programms. Helpers

who who [-uTlHqpdbrtas] [ file ]

who am i

Who is on the system, who owns the current shell?

who Show all logged users (optional -u)

who am i Displays the user of the current shell

man man [-cdwWtpr] [-M path] [-T macropackage] [section] titleman [-M path] -k keyword


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Print entries from the on-line reference manuals; find manual entries by

keyword

man cp Will show the manpage for the copy ('cp')

command

man -k tape Will search for all helppages, in which thekeyword 'tape' appears

man man ... if ever you have forgotten how to use 'man'

apropos apropos keyword

Locate commands by keyword lookup

apropos netscape Returns: 'The premier World Wide Web

browser'

Command interpretationalias INTERNAL SHELL COMMAND

Creates an alias for a string. Useful if you want to create your own shell

commands. 'alias' will simply list all your defined aliases.

alias ll 'ls -la' Will create a command 'll' that executes

'ls -la' when you punch 'll'

alias zipall 'gzip *' 'zipall' will gzip all files in the

current directory

which which [-a] [-f] [name...]

Locate a program file including aliases and path (csh only!) Searches thecommand aliases and your binaries-path for the command that would have been

executed if you had entered the requested string. Sometimes, you think that a

command should be executed, but it is not. Then it is often useful to check your

aliases with 'which'.

which ll Will tell you that 'll' is aliased to 'ls -la'

(if defined like above)

which acad12 tells you that /serve/sgi/acad12.sh will

execute

Processesps ps [ options ]

Report process status

ps List all active processes

ps -al List all processes in long format

ps -aledf List all processes in long format with

additional information

top top [ -i interval ] [ -D fullpathname ]

Display processes having highest CPU usage. If your workstation seems to be

very slow it is often due to a running process. You can identify it using 'top' andclose the application that causes the load. If the process is not yours, you will


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have to ask either the owner of a superuser to shut down that process for you.

top Updates every 5 seconds

top -i1 Updates every second

kill kill [-signal] pidTerminate a process by default. To do so, you need the PID (process ID). You

can get that, using either the 'ps' or the 'top' command.

kill 42310 Kills the process with the number 42310 if

yours

kill -INT 42310 Sends the process 42310 the INT signal (ctrl-

c)

Communication

login login [ -d device ] [ name [ environ ... ]]

Sign on. Changes the owner of the current shell or logs you into a systemlogin

rlogin rlogin rhost [ -l username ] [ -ec ] [ -8 ] [ -L ]

rlogin username@rhost [ -ec ] [ -8 ] [ -L ]

Remote login. Calls a remote server and logs you in under the specified

username

rlogin spielberg Connect to spielberg as current

user

rlogin spielberg -l scully Connect to 'spielberg' as user

'scully'rlogin scully@spielberg Same as above

ftp ftp [ -v ] [ -d ] [ -i ] [ -n ] [ -g ] [ host ]

Internet file transfer program. One of the oldes ways of moving data from one

server to another. Ftp also allows you to create and remove directorys remotely.

Be careful when using mput/mget. Files with the same name will be

overwritten without asking!

ftp spielberg Opens a ftp connection to spielberg

For ftp-internal commands, type 'help' at the prompt.The most useful are:

user scully Set the remote user to 'scully'

get document1 Fetches the file document1 to the local dir

mget doc* Fetches all files named doc* to the local dir

ls lists the remote dir

cd changes the remote dir

lcd changes the local dir

bin changes the transfer protocol from ascii

to binary and back

telnet telnet [-d] [-n tracefile] [-l user | -a] [-e escape-char] [host [port]]

User interface to the TELNET protocol. Opens a telnet connection to a given


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host

telnet spielberg Opens a telnet connection to spielberg

finger finger [-bfhilmpqsw] [name...]

User information lookup program. Check if a user is logged in on the remotehost. Also used to find out if someone has read email recently.

finger scully@spielberg See if user 'scully' is logged

in on host 'spielberg'

talk talk person [ ttyname ]

Talk to another user. Establish a talk connection between two machines. To do

so, you need to know if the requested user is currently logged in on the other

side (see 'finger')

talk scully@spielberg Try to open a connection to user

scully on spielberg

talk scully Open a connection to user scully

on current host

File processing

Since UNIX systems base heavily on plain text files for configuration, there are many

powerful tools that allow you to process such files. Since processing such files often requires

executing many commands after each other, there is something called 'piping' that allows youto direct the output of a command into the input of the next command. You can thus execute

command chains, without saving the intermediate results to a file.

Below, you will find the simple commands first, and the chaining of them into complex

command sequences in the 'redirection & piping' section that follows.

Simple commands

echo echo [ arg ]

Echo arguments. Prints arguments to the shell.

echo hi Will print "hi" into your shell

grep grep [options] limited regular expression [file . . .]

Search a file for a pattern

grep 'ando' archi.txt Return every line of archi.txt in

which 'ando' appears

grep 'ando' archi/cv*.txt Search in all cv* files under

'archi'

for the string 'ando'

cat cat [-u] [-s] [-v [-t] [-e]] file


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Concatenate and print files. Often used to pipe input into another process.

cat letter3.txt Print file letter3.txt

cat letter*.txt Print all files that match letter*.txt

cut cut -clist [file ...]cut -flist [-dchar] [-s] [file . . .]

Cut out selected fields or characters of each line of a file. Often used by piping

output from another process.

cut -c1-5 data.txt Get first 5 characters of every line

cut -d/ -f1,4-6 data.txt Get fields 1,4,5,6 delimited by '/'

sort sort [-cmu][-ooutput][-ykmem][-zrecsz][-dfiMnr][-btx][+pos1[-pos2]][-Ttdir]

[files]

Sort and/or merge files

sort archi.txt Sort the file archi.txt alphabetically

diff diff [-lrs] [-Sname] [-cefhn] [-xpat] [-biwt] dir1 dir2

diff [-cefhn] [-biwt] file1 file2

Differential file and directory comparator

diff oldfile newfile Searches the two files and seeks for

differences line by line

more more [-cdflrsuw] [-lines] [+linenumber] [+/pattern] [ filename ... ]

Browse or page through a text file

more longfile.txt Prints your file screen by screen

Piping and redirecting

Redirection:

The first thing you can do when interlinking processes, is that you

redirect input or output from / to processes.

Redirected output:

ls -la > listing.txt List dir and output to listing.txt

sort units.txt > sorted.txt Sort units.txt and output to

'sorted.txt'

Redirected input:

cat < data.txt Read the file data.txt as input for

'cat'

mail < order.txt Read the input for 'mail' from the

file

'order.txt'. The file order.txt

should

in this case carry all the commandsyou


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would normally type when calling

'mail'

Piping:

Another thing is piping data streams through a multitude of commands.

sort chaos.txt |grep 'apple' Sorts chaos.txt and seeks for lines

with 'apple'

ps -edalf |grep media |cut -c14-20 List all processes, find lines with

'media' and cut the second fieldDevices

A last, interesting feature about UNIX, is that in the complex filetree,

not only files and diretorys, but also input and output devices can be

found. Namely things like 'mouse', 'keybd' a.o. are all available in a

directory called /dev. There are two interesting devices that might be

useful: the 'null'-device and any ttyq-device.

The 'null' device (/dev/null) acts like a 'black hole'. Anything that is

sent to that device is trashed immediately.

convert my.gif my.tiff > /dev/null

In this case, any message that would normally appear on the screen when

convert is doing its work, is trashed to nowhere (/dev/null)

ttyq-devices are opened, whenever a shell is initialized. You can thus

send information to a shell, using the /ttyqXX devices:

ls -la > /dev/ttyq0

will list the content of a director not in your current shell, but in

the system console (if present)

In the manpages, there will often be references to 'stdout' and 'stdin'.

These are the standard-input and output devices of your system. Namely,

stdin is the keyboard, while stdout points to the monitor.

Last but not least, also processes have their presence in the filesystem.

They can be found under /proc

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