ls Command

• The ls is the most widely used command in unix or linux.
• The ls command is a command-line utility for listing the contents of a directory or directories given to it via standard input.

cat command

cat command is use to read the contents of files or displaying them, combining copies of  files and creating file .

See the use of cat command

Command	Details
cat [options] [filenames] [-] [filenames]	Syntax
cat file1	cat command is use to read the contents of files
cat file1 > file2	">" this is output redirection operator, cat file1 output is written to file2
cat >file1	To Write the data into file 1
cat > file2	To Write the data into file 2
cat file1 file2	concatenate copies of the contents of the two files file1 and file2
cat file1 file2 >file3	file1 and file2 data redirect to file3
cat >> file4	">>" the append operator , append the data in file4

touch command

In Unix and Linux touch command use for creating empty file,updating access time and modification time.

Command	Desc
syntax	touch [option] file_name(s)
touch file1 file2 file3	This command would create three new empty files.
touch -am file3	access(-a) and modification(-m) times to the current time
touch -r file4 file5	[-r] is use for reference , to use the times of fille4 for file5
touch -d '1 May 2005 10:22' file8	To change the last access time of file8 to 10:22 a.m. May 1, 2005
touch -d '14 May' file9	[-d] Partial date change Only the date need be provided.

Alias Command in Unix and Lunix

• Alias is a shortcut to reference a command. 
• It can be used to avoid typing long commands that are frequently used.
• For common patterns it can reduce keystrokes and improve efficiency. 

One best example why we will use alias ?
A user want to delete a file with confirm delete option for that we type
rm -i file.txt
remove file.txt? y
But some time we forgot to write -i , to avoid this such issue we use alias.

Command	Desc
Syntax Of alias	alias [-p] [name="value"]
Example:
alias	It will show you list of alish which is present in system
alias ls="ls -al"	lists the names of the files and directories within the current directory
alias df="df -h"	size, amount of space used, amount of space available (-h) in MB and GB
alias rm="rm -i"	increasing the safety of the system by making commands interactive. It is use for remove files
alias cp="cp -i"	increasing the safety of the system by making commands interactive .It is use for copy files
Removing Aliases
Syntax	unalias [-a] name(s)
Example:	unalias rm

Unix and Linux Basic Commands

Below are details of Basic Commands

Command	Description
date	date command is used to print or set the system date and time. Syntax: date options
cal	cal displays a simple calendar. If arguments are not specified, the current month is displayed. Syntax: cal options
clear	clear the terminal screen
pwd	pwd command is used to print name of working directory
uname	print system information. Generally used to know the flavor and version of the operating system
who	show who is logged on
uptime	Tell how long the system has been running.
ls	If you simply execute ls on the command prompt, then it will display the files and directories in the current directory.You can pass a directory as an argument to ls command. In this case, the ls command prints all the files and directories in the specific directory you have passed.Syntax: ls directory
touch	The touch command simply creates an empty file. The below touch command creates a file_name.txt in the current directory.Syntax: touch file_name.txt
cat	The cat command is used to display the contents in a file. Syntax: cat file_name.txt
head	The head command can be used to print the specified number of lines from the starting of a file. Syntax: head -3 file_name.txt
tail	The tail command can be used to print the specified number of lines from the end of a file. Syntax: tail -3 file_name.txt
mkdir	The mkdir command is used to create the specified directory.
cd	The cd command can be used to change from one directory to another directory. You need to specify the target directory where you want to go.
mv	The mv command is used to rename the files and it also used for moving the files from one directory into another directory.
cp	The cp command is used to copy the content of source file into the target file. If the target file already have data, then it will be overwritten.
rmdir	remove or delete directories. Syntax: rmdir directory_name
wc	wc command can be used to find the number of line, words and characters in a file.
cut	cut remove sections from each line of files. Syntax: cat new_file.txt|cut -c1
man	If you want to know more about all above command then go to man comman Syntaxman command_name.

File Attributes and Permissions

File Attributes

We can observe 7 attribute fields listed for each file. 

The attribute fields are

1. File name
2. Modification date
3. Size
4. Group:(associated group for the file)
5. Owner
6. Number of links(the number of other links associated with this file),
7. Permission modes: the permissions assigned to the file for the owner, the group and all   others.

File Permissions
The permission field for each file consists of 10 characters as described by the diagram below

Unix File System

The Unix file system has a hierarchical (tree-like) structure with its highest level 
directory called root ( /).
Immediately below the root level directory are several sub-directories, most of which contain system files. 

The following figure shows a typical organization of files in Unix system.

bin - short for binaries, this is the directory where many commonly used 
          executable commands reside
dev - contains device specific files
etc - contains system configuration files
home - contains user directories and files
lib - contains all library files
mnt - contains device files related to mounted devices
proc - contains files related to system processes
root - the root users' home directory (note this is different than /)
sbin - system binary files reside here. If there is no sbin directory on your system, these files most likely reside in etc
tmp - storage for temporary files which are periodically removed from the filesystem
usr - also contains executable commands

Unix File Types 

All files in the Unix file system can be loosely categorized into 3 types, specifically:

1. ordinary files
2. directory files
3. device files

Ordinary Files

Ordinary files hold data and executable programs. Executable programs are the commands

(ls) that you enter on the prompt. The data can be anything and there is no specific format

 enforced in the way the data is stored.

The regular files can be visualized as the leaves in the UNIX tree.

Directories

Directories are files that contain other files and sub-directories. Directories are used to

organize the data by keeping closely related files in the same place. The directories are 

just like the folders in windows operating system.

The kernel alone can write the directory file. When a file is added to or deleted from this directory, the kernel makes an entry. 
A directory file can be visualized as the branch of the UNIX tree.

Special Or Device Files

These files represent the physical devices. Files can also refer to computer hardware such as terminals and printers. These device files can also refer to tape and disk drives, CD-ROM players, modems, network interfaces, scanners, and any other piece of computer hardware. When a process writes to a special file, the data is sent to the physical device associated with it. Special files are not literally files, but are pointers that point to the device drivers located in the kernel. The protection applicable to files is also applicable to physical devices.

User Login Process

When the user is trying to logging into system then Kernel should know who is that user.

Kernel manage this user logging process by running below programs

1. getty    
2. loging 

       [getty is short for “get terminal”.]

• Kernel calls program ‘init’. ‘init’ calls program ‘getty’, which issues a login prompt in the monitor.

• User enters login and password.‘getty’ calls program ‘login’ which scans file /etc/passwd to match username and password. 

• After validation, control passes to session startup program /bin/sh .User gets a shell prompt. 

• If the user fails to provide valid password, the login program returns the control back to the getty program.

What is UNIX ?

UNIX is a operating system which is multi user (multiple user can access Unix system at a same
 time ) and  multitasking (One user can run multiple program). Operating System is an interface
 between hardware and applications software's.

Unix Architecture

1.Shell

• Human interface point for Unix
• Program layer – provides an environment for the user to enter commands to get desired results.
• Korn Shell, Bourne Shell, C Shell are various shells used by Unix users

2. Kernel

• Heart of The Unix OS.
• Collection of C programs directly communicating with hardware
• Part of Unix system loaded into memory when Unix is booted

        Manages:-

• System resources
• Allocates time between user and processes
• Decides process priorities

3.  Hardware

 Unix Boot Process Setps

1. BIOS    : Basic I/O System Executes MBR.
2. MBR : Master Boot Record Executes GRUB.
3. GRUB  : Grand Unified Boot loader executes Kernel.
4. Kernel : Kernal executes  sbin/Init.
5. Init          : Init executes run level programs .
6. Runlevel : Runlevel program executes from /etc/rc .d/rc*.d/rc*.

Unix

Basic Concepts Of  Unix

⇨What is Unix?
⇨User Login Process
⇨Unix File System
⇨File Attributes and Permissions

Most Frequently Used UNIX / Linux Commands

⇨Basic Commands
⇨alias
⇨touch
⇨cat
⇨ls