Operating System Concepts.
# Linux Commands – 2 days
Operating System Concepts
- there are 4 steps to learning an OS:
step-1: user level (basic Linux commands)
step-2: admin level (shell script programming)
step-3: programmer level (introduction to system call programming by using system calls).
step-4: design level / internals
step-1: user level (basic linux commands):
What is Linux?
Q. Why there is a need of an OS? What is a Computer?
- Computer is a hardware/machine, which performs different tasks efficiently and accurately for user.
- there are 4 basic functions of computer:
1. data storage
2. data processing
3. data movement
4. control
- Computer hardware mainly contains : CPU/Processor,
Memory Devices & IO Devices.
- As any user cannot directly interacts with any computer hardware component, hence there is a need of interface between user and hardware, and to provide this interface is the job of an OS.
What is a software?
What is a Program?
- there are 3 types of programs
1. user programs: main.c, slll.java, program.cpp etc...
2. application programs: notepad.exe, ms office, google chrome, calculator, games etc...
3. system programs: programs which are part of an Os i.e. inbuilt programs of an OS.
e.g. kernel, scheduler, memory manager, loader, dispatcher, linker, interrupt handler etc....
Q. What is an OS?
- An OS is system software (i.e. collection of system programs) which acts as an interface between user and hardware.
Kernel => it is a core part/program of an OS which runs continuosly into the main memory and does basic minimal functionalities of an OS.
e.g.
Linux => vmlinuz
Windows => win32krnl.exe / ntoskrnl.exe
- Kernel is like heart of an OS.
- Basic Minimal Functionalities/Core Functions/Kernel
Functions: Process Management, Memory Management,
Hardware Abstraction, CPU Scheduling and File & IO Management.
- to compile a java program : $javac hello.java => terminal
- to execute a java program $java hello
- to compile c program $gcc -c program.c => program.o (object code)
- to link object code of a program with precompiled
object modules:
$gcc -o program.out program.o => program.out
- to execute program $./program.out => in Linux $.\a.exe => in windows
- Shell => shell is an application program which acts as a CLI(Command Line Interpreter), which accepts command from user in a text format, interpret it and pass it to the kernel for an execution.
-In Linux there are 2 types of Shell
1. CUI Shell : terminal e.g. bsh, bash, csh, ksh etc....
2. GUI Shell
- To install an OS onto the computer system means to store OS Software (i.e. collection of thousands of system programs, application programs and utility programs which are in a binary format) onto the HDD.
- Linux Kernel vmlinuz is exists in /boot dir in a binary format.
- While installing Linux we need to format disk by using extended filesystem i.e. we need to create a new filesystem on that partition named as ext filesystem.
- While installing Linux we need to create 2 partitions:
1. swap partition =>
it is a portion of HDD kept reserved by Linux OS while installtion will be used as a swap area.
- swap area -> portion of HDD used by an OS as an extension of the main memory in which inactive running programs can be kept temporarily => memory managemnt.
- conventionally the size of swap area should be doubles the size of main memory.
2. data partition/root partition ( / ) partition:
in which LINUX OS keeps/stores data in an organized manner or in a hierarchical manner.
- When any user loggedin into the system or opens terminal, system takes user bydefault into the user’s home dir.
- to open terminal =>
way-1: press window key -> enter terminal in search bar and press enter
way-2: cntrl+alt+t
Command Name : pwd
– print working directory / present working direcitory
- this command prints an absolute path of present working directory.
Command Name : whoami
- this command prints name of currently logged in user into the system on terminal.
Command Name : ls – list dir contents
- contents of dir file are names of sub dir’s and files in it.
$ls - ls command displays contents
$ls => bydefualt it displays contents of cur dir in a columnwise alphabatically sorted manner.
$ls -1 => it displays contents of cur dir per line one entry in alphabatically sorted manner.
$ls -s => total no. of data blocks allocated for file to store actual data into the disk.
usually size of physical data block = 512 bytes.
Size of logical data block = multiple of 512
512 bytes / 1024 bytes / 2048 bytes / 4096 bytes
$ls -l ditpath
=> display contents of dir in a long list format
In output of ls -l command: total n
n = sum of total no. of data blocks allocated for files
listed below.
- First char of first column indicates type of a file
- In Linux/UNIX based operating system, there are 7 types of files:
1. regular file ( - ) : all text files, source files, image files, audio files, video files, pdf files etc.... considered as a regular file.
2. directory file ( d ) -
dir file is a special type of file whose contents are names of its sub dir’s and files.
3. character special device file ( c )
4. block special device file( b )
5. linkable file ( l ) : it like short cut files in Windows, to access deeply located file more frequently linkable file can be used.
6. pipe file ( p )
7. socket file ( s )
- pipe file and socket file is used inter process communication.
- Everything treated as a file, from System point of view all devices treated as a file
- In Linux/UNIX based operating systems devices can be catagorised into two categories:
1. character devices => devices from which data gets transferes char by char i.e. byte by byte
e.g. keyboard, monitor, printer etc...
Linux/UNIX based OS treats all char devices as char special device files.
- When an execution of any program is started, an OS bydefault opens 3 files for it:
1. stdin => standard input buffer which is associated with standard input terminal/device keyboard i.e. console
input.
2. stdout => standard output buffer which is associated with standard output erminal/device monitor i.e. console
output
3. stderr => standard error buffer which is associated with standard output terminal/device monitor i.e. console
output.
2. block devices => devices from which data gets transferes block by block i.e. sector by sector
e.g. all storage devices
Linux/UNIX based OS treats all block devices as block special device files.
- next 3 chars of first col after type of file indicates access perms for user/owner of that file, then next 3 chars indcates access perms for group members and last 3 chars indicates access perms for others.
Command Name : clear – to clear the screen
- clear command clears the screen and takes cursor to the left top, and blinking cursor resides command prompt symbol ($), indicates shell is waiting for the user to
enter next command.
- to clear the screeen => short cut key [ cntrl + l ]
Command Name : top
- to display information about running programs/processes.
- top command is like a s task manager in Windows.
- press key q to quit top
- all linux commands are case sensitive
Command Name : uname
- to display system information
$uname -a => to displlay all system information
$uname -v => to print version of system
Command Name : cd – change dir
$cd
- this command is used for navigation throughout filesystem, i.e. by using cd command we can go from one location any other location.
- filesystem => it is a way to store data onto the disk/partition in an oragnized manner so that it can be accessed by the system efficiently.
- Shell metacharacters:
/ (root dir)
~(cur logged in user’s home dir
. (current dir)
.. ( parent dir)
$cd / => goto the root dir
$cd ~ => goto the user’s home dir
$cd . => goto the cur dir / remains in a cur dir
$cd .. => goto the parent dir of cur dir
$cd - => goto prev accessed dir
Mumbai => Pune => Satara => Karad => Kolhapur => Banglore
Command Name : mkdir
- command used to create a new dir / directories
$mkdir
$mkdir -p one/two/three
- -p option can be used with mkdir command to create a dir hierarchy.
- dir by the name one gets created as a parent dir in a mentioned dir, then sub dir by the name two gets created inside dir one, and sub dir by the name three gets created inside dir two.
Command Name : rmdir
- we can delete only empty dir by using rmdir command.
Command Name : rm – remove file/files
- rm command is to used to delete/remove file/s, it also used to remove/delete non-empty dir with -R option $rm -R dirpath
- it deletes first contents of all sub dir’s and files of the dir recursively in it and then it deletes dir.
+ physical data block size = 1 sector = 512 bytes
- at hardware level head can access data from 1 sector at a time i.e. 512 bytes of data at a time.
- it can be decided at the time of disk manufacturing. + logical data block size => it can decided at the time of formatting i.e. creating a new filesystem.
- it must be in a multiple of 512 bytes, filesystem can access data as per logical data block size.
[ a ]
[ b c d e f ] - grp
[ g ]
[ h ]
[ i ]
[ j ]
cur logged in user => d
-rwxrw-r-- file.txt
user/owner => d
group members => b c e f
others => a g h i j
second col in a ls -l command => link count => no. of links having for that file in a filesystem => no. of ways
which file can be accessed.
- bydefault link count of a regular file = 1
- bydefault link count of a dir file = 2
when we createa a new dir one link gets created into its parent dir and 1 hidden entry gets created inside itself by the name.
- In Linux filename starts with dot ( . ) is considered as a hidden file.
$ls -a => to display all contents of dir including hidder files.
-Process => PCB Process Control Block - pid
- File => FCB/iNode File Control Block – iNode number
iNode number is an unique identifier of a file in a filesystem.
